Greenstone Slate Guide to Roof Installations
| General Information | |||
| Components | |||
|
|
|||
| Design Considerations | |||
Slate roof systems may
be classified into three
general categories:
• Standard
• Graduated
• Textural
Standard Slate
Roof Systems
Standard slate roof
systems are those roof systems composed of standard commercial
slate that is approximately 1/4 inch to 3/8 inch (6 mm
to 9 mm) thick. The most common have uniform lengths and widths,
with square tails or butts. Standard commercial slate may
be used to form a variety of designs on a roof, and are suitable
for many buildings where a long service-life steep-slope roof
covering is desired.
If desired, butts or
corners of each slate may be trimmed to a diamond, hexagonal,
or other pattern for specific portions or for all the roof
system. Variety in the pattern of standard slate roof systems
is sometimes attained by using random widths. A staggered
butt line can be achieved by using longer length slates randomly.
Graduated Slate Roof
Systems
Graduated slate roof
systems feature variations in thickness, size and exposure.
In graduated slate roof systems, the thickest, widest and
longest slates are placed along eaves. As slate courses progress
upslope to ridges, slates of gradually diminishing
size and/or thickness are used, creating a “graduated”
effect. This effect is intended to make the eave-to-ridge
distance look longer and give the roof a
more massive appearance. Slate for roofs of this type can
be obtained in any combination of thicknesses from 3Ú16 inch
(4 mm) to 11Ú2 inches (38 mm). Thicker slates are sometimes
specified. Graduated slate roof systems may be installed in
a wide variety
of patterns.
Because slate shingles
come in a wide variety of
sizes, thicknesses, textures and colors, a wide range of appearance
can be created. When a range of thicknesses are mixed throughout
a roof system,
the roof system is some-times referred to as a textured or
textual roof system. Textural slate roof systems are composed
of slate that has a rougher texture than standard slate. This
slate type also has uneven tails or butts.
Active roofing slate
quarries in North America exist in New York, Pennsylvania,
Vermont, Virginia, and the Canadian provinces of Quebec and
Newfoundland. The main area of slate production in Pennsylvania
is the Lehigh district. The active Vermont quarry district
lies in Bennington and Rutland counties and extends into Washington
County in New York. Virginia slate operations are now conducted
only in Buckingham County at Arvonia.
Imported slate is available
from Spain, Wales, China, Brazil and South Africa.
There is concern with
the durability of roofing slate from some quarries. Slate
can be tested to recognized standards if the source of a slate
at a quarry provides a consistent quality. Tests include those
in ASTM C 406, “Standard Specification for Roofing Slate,”
especially the modulus of rupture test. British Stan-dard
680 has an acid resistance test for carbonate impurities especially
calcite. A new European Standard for petrographic examinations
can describe the mineralogy and fabric of the stone and identify
most problematic features of building stones.
Specialized tools for
slate roofing include the slate hammer, stake, ripper, roof
scaffold bracket, and roof ridge or hook ladder. A slate hammer
has many uses. The hammer head is used for driving nails,
the claw for pulling nails, the point for punching slate and
the knife edge on the shaft for cutting slate.
A slate stake is used
with a slate hammer to cut slate in the field. It is a T-shaped
tool that can be stuck into a roof deck or scaffold plank
and provides a solid support at the underside of slate along
the line where a cut or hole punch is to be made. It is also
used as a straight edge to mark slate before cutting or trimming
slate at valleys and other flashing areas.
A slate ripper is used
to take out broken slate by cutting or pulling the concealed
nails. The upper portion of a ripper is designed to hook around
nails and loosen or cut them by force. The handle is designed
to be struck with a hammer to assist in cutting or loosening
a slate nail after the ripper has been positioned around or
against the nail.
A roof scaffold bracket
is used to secure toe boards. Roof scaffold brackets are adjustable
and hold wood plank toe boards parallel to the ground. A toe
board is a wood plank that provides a platform for a worker
and holds material and tools.
A roof ridge or hook
ladder is lightweight and has a hook that goes over a roof
ridge. It enables a worker to climb on a slate roof system
without putting full weight on individual pieces of slate.
Weight is transferred along the length of a ladder.
Material
Description
Slate rock originates
from sedimentary particles of clay and silt. Silt and other
particles were originally washed down into streams, which
transported the particles into ancient seas. As quantities
of these particles were deposited over time, successive parallel
layers of the material accumulated. These accumulations of
sediment, built-up over thousands of years to varying thicknesses,
and are called beds.
The pressure of successive
overlying layers compressed and consolidated the lower underlying
sedimentary layers, which formed shale. Years of these geological
forces caused great pressure on the underlying shale layers
and, as pressures rose, heat and force loads combined to chemically
change original clay materials into harder materials such
as chlorite, mica and quartz. Many years of these combined
geological forces transformed relatively weak shale rock into
hard slate.
The original layering of materials gives slate natural cleavage planes. These natural cleavage planes are what allows slate to be easily split into relatively thin layers. Slate also possesses a natural grain that usually occurs at right angles to cleavage. Roofing slates are com-monly split so that the length of the slate runs in the direction of the natural grain.
The surface texture
of slate, after it is split for commercial use, depends on
the quality and characteristics of the rock from which it
was quarried. Many slates split to a smooth, practically even,
uniform surface, and others split to a surface that is rough
and uneven.
As a result, a wide range of surface textures are available.
Some slate contains
narrow “ribbons of rock” that are different in
chemical composition and color from the principal slate. Slate
that has been trimmed so that
the ribbons are eliminated is known as clear slate. Slate
that contains acceptable ribbons is sold as ribbon stock.
The color of slate is
determined by its chemical and mineral composition. Because
these factors differ in various regions it is possible to
obtain roofing slate in a variety of colors and shades. Various
shades of the same slate may be used to provide interesting
color mottling or shaded patterns when applied across a roof,
or when interspersed randomly. The diversity of color makes
slate a sought-after material for the creation of certain
architectural or aesthetic conditions. When multiple colors
are used, a roof system is sometimes called polychromatic.
Exposure to weather
causes all slate to change slightly in color and composition.
The degree of color change varies. Slate that exhibits minimal
color change is classified as non-weathering. Slate that exhibits
a more obvious color change is known as weathering. Weathering
slate offers the designer another variation in roof aesthetics.
Weathering and semi-weathering gray and green slates contain
some slates that can weather to buff or tan as the mineral
particles in the slate oxidize, providing a varied color pattern
throughout a roof system.
When texture and color
are essential considerations
in roof design, architects and owners should consider the
source of the slate, its potential for color change and the
effect of weathering on the slate to be used. Slate producers
know from experience the probable degree of color change for
various slate materials obtained from their quarries. Producers
may be consulted on selection of particular slate for a
specific project.
For the purpose of classifying
the basic natural colors of roofing slate currently available
in large quantities for general use, the following color nomenclature
may be used:
• black
• green
• gray
• gray green
• mottled
purple
• mottled
green
• purple
• red
In addition, slate distributors
use the following names and nomenclature for certain roofing
slate:
• blue-black
• blue-gray
• mottled
gray-black
• mottled
gray-green
• mottled
purple-green
• mottled
green-purple
• purple
variegated
• sea green
When selecting a slate
color from a distributor, the color should be preceded by
the word “unfading” or “weathering”
to designate the color stability or change that may be expected
for a particular type of slate. In addition to the colors
previously mentioned, slate distributors use different combinations
of names to identify the colors of a particular slate.
Slate of commercial
standard thickness weighs approximately 850 pounds per square
(41 kg/m2).
A square of slate on a roof system set at the standard 3 inch
(75mm) headlap will vary in weight from 650 pounds to 8,000
pounds (295 kg to 3,629 kg) depending on the thickness of
each slate.
Slate varies significantly
in weight because of the numerous thicknesses, and types
available from the different quarries. Table
1 (Click here) shows the approximate weights of roofing slates
of different thicknesses. The actual weight of slate products
may be from 10 per-cent above to 15 percent below the weights
shown.
A quarry or supplier
should be consulted for specific slate weight(s), length,
width dimensions and thickness. Slate is also available with
sawn or chamfered edge, depending on the slate producer.
In the United States,
slate is sold by the square. A square of roofing slate is
defined by the U.S. Depart-ment of Commerce, National
Bureau of Standards in Simplified Practice Recommendation
No. 14 as:
“… a sufficient
number of slate shingles of any size to cover 100 square feet
of plain roofing surface, when laid with the approved or customary
standard headlap of 3 inches (75 mm).” Slates for surfacing
flat roofs are usually laid tile-fashion, without lap, in
which case a square of slate would cover an area greater than
100 square feet.”
The quantity of slate
per square varies from 89 pieces of 26 by 14 inch (660 by
360 mm) slate, to 686 pieces of 10 inch by 6 inch (250 mm
by 150 mm) slate. These quantities include an allowance for
the 3 inch (75 mm) headlap.
It should also be noted
that for roofs of lower slope, where a 4 inch (100 mm) headlap
is used, an additional quantity of slate is required to cover
1 square of roof. For very steep roofs or vertical siding/cladding
applications, where a 2 inch (50 mm) overlap is sufficient,
fewer slates will be needed.
Table
2 (Click here) shows
length and width dimensions for stan-dard slate, the minimum
number of slates required per square and respective exposures
for the slates listed.
To satisfy the need
for slate classification, the National Slate Association established
various standards for commercial slate during the 1800’s.
The primary standard was based on visual grading of slate
by color, surface texture, straightness, thickness and weight.
Even though grading was visual and varied from region to region,
the standard helped ensure that slate products (that met the
standard) would contain certain physical characteristics essential
for quality slate roofing. For many years, roofing slates
were specified according to the National Slate Association’s
standard.
Since 1957,
the American Society for Testing and Materials (ASTM) has
established a consensus material standard for slate used for
roofing shingles. This material standard is designated as
ASTM C 406, “Standard Specification for Roofing Slate.”
ASTM C 406 addresses material characteristics, physical properties
and sampling procedures appropriate to the selection of slate
for use as roofing shingles.
ASTM C 406 lists three
grades of roofing slate depending on geographic location and
environmental exposure: Grade S1 slates are to have specific
properties that are said to provide for an expected service
life of more than 75 years; Grade S2 slates are to have an
expected service life of 40 to 75 years; and Grade S3 slates
are to have an expected service life of 20 to 40 years. Actual
service lives for quality S1 slate roofs range from 75 to
more than 300 years.
There are three ASTM
standard test methods that apply to roof slate and provide
the basis of ASTM C 406. These are:
• ASTM
C 120, “Standard Test Methods of Flexure Testing of
Slate (Modulus of Rupture, Modulus of Elasticity)”
• ASTM C
121, “Standard Test Method for Water Absorption of Slate”
• ASTM C
217, “Standard Test Method for Weather Resistance of
Slate”
These test method standards
provide the basis for the grade classifications in ASTM C
406 which establishes the physical requirements shown in Table
3. (Click here)
The grade of slate
used should be commensurate with the expected design life
of the building. NRCA recommends designers specify slate that
meets the requirements of ASTM C 406, Grade S1, for all geographic
regions and environmental exposure conditions in North America.
NRCA recommends the
use of underlayments with slate roof systems. Underlayment
is applied over a roof deck before the application of roofing
slate. Underlayment performs two primary functions: It provides
temporary weather protection until a roof covering is installed,
and it provides a secondary weatherproofing barrier should
moisture migrate below the slate roof covering.
In addition, underlayments
are generally necessary for the following reasons:
• to comply
with local building codes
• to help
prevent dust, dirt and insects from entering buildings
There are different
underlayment configurations that can be used for slate roof
systems. Generally, these configurations can be categorized
as follows:
• single
layer of underlayment
• single
layer of self-adhered membrane
• double
layer underlayment system
A single layer of underlayment
is the most common underlayment configuration for slate roof
systems.
Asphalt-saturated felt
underlayments are the most common underlayments used in steep-slope
roof systems and typically use organic reinforcing mats.
The weatherproofing
material used to manufacture roof underlayment felt is asphalt
flux. Asphalt flux is obtained from the fractional distillation
of petroleum that occurs toward the end of the petroleum refining
process. Asphalt flux is sometimes further refined by air
blowing to produce roofing-grade asphalt at the refinery or
at the product manufacturer’s facility.
Depending on the type
of underlayment, asphalt may be used in two processes: first
as a saturant and second as a coating.
Saturant-grade
asphalt
Asphalt used in the
saturation process is a “soft,” less viscous asphalt
than a coating asphalt and is used to impregnate organic reinforcing
mats. Saturant-grade asphalt has a lower melting point than
coating-grade asphalt. Common underlayments such as No. 15
and No. 30 are asphalt-saturated felts.
Coating-grade asphalt
If the felt underlayment
is a coated felt, a coating-grade asphalt is applied to the
felt after the saturation process. Coating-grade asphalt is
more viscous than saturant-grade asphalt. Mineral additives,
or “fillers,” are added to the coating-grade asphalt
to stabilize the bitumen and reduce its natural flow characteristics,
and increase fire resistance and weatherability, making it
more suitable as a coating material.
Underlayments are reinforced
with mats that are designed to “carry” or support
the asphalt. Reinforcing mats of different thicknesses are
used to produce underlayments of different weights. These
reinforcements, sometimes referred to as carriers, are most
commonly made out of organic fibers. However, some underlayments
are produced that use reinforcing mats made of glass fibers.
Organic Mat
Over the years, organic
mat has been produced from various combinations of cotton
rag, wood fiber and other cellulose fibers. Currently, wood
and other cellulose fibers are the types of reinforcements
most widely used in organic mats. Organic mats are then saturated
with a soft, saturant grade asphalt intended to fill voids
between fibers.
Glass Fiber Mats
Glass fiber mats are
composed of inorganic, glass fibers. The fibers may be continuous
or random and are bonded together with plastic binders or
resin. Additionally, these glass fiber mats may be further
reinforced with chopped glass fiber strands or with continuous
random or parallel glass fiber strands. The mats are then
coated with asphalt.
The following American
Society for Testing and Materials (ASTM) standards apply to
asphalt saturated organic felt underlayments for slates:
• ASTM
D 224, “Standard Specification for Smooth-Surfaced Asphalt
Roll Roofing (Organic Felt).” This standard addresses
material characteristics and physical properties and provides
four classifications: Type I – minimum net mass per
unit area of roofing 39.8 lb/100 ft2 (1943 g/m2); Type II
– minimum net mass per unit area of roofing 54.6 lb/100
ft2 (2666 g/m2); Type III – minimum net mass per unit
area of roofing 51.1 lb/100 ft2 (2495 g/m2); and Type IV –
minimum net mass per unit area of roofing 39.8 lb/100 ft2
(1943 g/m2).
• ASTM D
226, “Standard Specification for Asphalt-Saturated Organic
Felt Used in Roofing and Waterproofing. This standard addresses
material characteristics and physical properties and provides
two classifications: Type I, commonly called No. 15 asphalt
felt, and Type II, commonly called No. 30 asphalt felt.
NRCA recommends that
asphalt organic felt underlayments meet or exceed the minimum
physical property values listed in ASTM D 224 or D 226, Type
II. ASTM D 226 covers asphalt-saturated organic felts with
or without perforations. When used as underlayment, only non
perforated asphalt felts should be used.
Polymer-modified bitumen
sheet membrane products are now being used as underlayments
in some steep-slope roof systems. Some of these materials
are marketed for use specifically as underlayments
for steep-slope roof systems. Others are heavy,
non-porous sheets commonly used in low-slope
roof systems.
Bitumen used in this
type of underlayment is asphalt that has been modified with
polymers. The common polymers currently used are atactic polypropylene
(APP) and styrene butadiene styrene (SBS). These polymers
alter the basic physical characteristics of the asphalt and
provide enhanced weathering, aging and sealing characteristics.
Polymer-modified bitumen base sheets are generally reinforced
with either a glass fiber or polyester mat. Polymer-modified
bitumen products specifically marketed for use as underlayments
range in weight from 35 mils to 90 mils (0.7 mm to 2.3 mm)
thick. In some cases, self-adhering polymer-modified bitumen
membranes most often used as ice dam protection membranes,
are installed over roof decks as underlayments. Self-adhered
polymer-modified underlayments range from 20 mils to 60 mils
(0.5 mm to 1.5 mm) thick. However NRCA recommends the use
of a minimum thickness of 40 mils (1.0 mm).
Some of these materials
are more resistant to wrinkling and distortion upon exposure
or after installation and exhibit better watershedding properties
than traditional asphalt-saturated organic felts. As a result,
more of these types of underlayments are being used for projects
where higher degrees of underlayment performance are desired.
An ice dam protection
membrane is a specialized type of underlayment. This underlayment
provides additional protection from moisture intrusion along
eaves where ice dams can occur during winter.
An ice dam protection
membrane may consist of:
• a single layer
of self-adhering polymer bitumen membrane
• two plies of
No. 30 asphalt-saturated, non perforated felt where the first
ply sheet is fastened to a deck and the second ply sheet is
adhered to the first with roof cement or cold adhesive
• a combination
of one heavyweight coated base sheet and one ply of No. 30
asphalt-saturated non perforated felt where the heavyweight,
coated base sheet is fastened to a deck and the ply sheet
is set in roof cement or cold adhesive.
The most common product
used for ice dam protection membranes is a self-adhering,
polymer-modified bitumen membrane. The bitumen used in
this type of membrane is an asphalt that has been modified
with polymers, typically SBS.
These polymers alter the basic physical character-istics of
asphalt and provide enhanced weathering and sealing characteristics.
Self-adhering polymer-modified
bitumen membranes are generally reinforced with either glass
fiber or a thin layer of polyethylene on the top side. These
membranes should not be exposed for extended periods of time
before the application of the slate. Some polymer-modified
bitumen membrane products incorporate granule surfacings to
provide more slip-resistant surfaces for workers.
The product is manufactured
with an adhesive on the back side of the membrane to create
a self-adhering material. A release paper covers the adhesive
layer to prevent the membrane from sticking to itself after
it is wound into a roll.
The following ASTM standard
is applies to modified
bitumen ice dam protection membranes:
• ASTM
D 1970, “Standard Specifications for Self-Adhering Polymer
Modified Bituminous Sheet materials used as Steep Roofing
Underlayment for Ice Dam Protection.” This standard
addresses thickness, tear resistance, adhesion properties,
low-temperature flexibility and thermal stability as well
as other physical properties.
NRCA recommends that
self-adhering polymer-modified bitumen sheet membranes used
as ice dam protection membranes in slate roof systems be a
minimum of 40 mils (1.1 mm) thick and comply with ASTM D 1970.
Roof cements are commonly
used in the application
of slate roof systems. The base material used in the manufacture
of roofing cement is an air-blown asphalt. The asphalt is
thinned, or “cut-back,” with
a petroleum-based solvent to create a softened, workable mixture.
Some roofing cements contain mineral fibers as stabilizers.
Some manufacturers
are now offering modified bitumen roofing cements.
There are two common
types of asphalt roofing cement: “flashing cement”
and “lap cement.” Flashing cements are commonly
used on vertical surfaces and are of a trowelable consistency.
Lap cements are
used more specifically for bonding asphaltic materials together,
and their consistency is characterized as either trowelable
or brushable.
Asphalt roof cements
are also available in different grades. The two most common
grades are referred to as winter or summer. The primary difference
between the winter and summer grades is their softening point
temperature; winter grades have a lower softening point temperature
than summer grades.
Common uses for asphalt
roof cement in slate roof systems are:
• to bond
two layers of felt underlayment together to form an ice dam
protection membrane along the eaves in lieu of a self-adhering
modified bitumen membrane
• as a bedding
cement for the purpose of sealing the base or flange of a
metal accessory to a roof
• to provide a
temporary seal around roof
penetrations or at walls before installing flashing components
• as a bedding
cement to secure and seal
hip/ridge units
The following ASTM standards
apply to asphalt roof cement used as a utility cement or flashing
cement:
• ASTM
D 2822, “Standard Specification for
Asphalt Roof Cement.” This standard addresses composition,
pliability, high-heat behavior and adhesion properties, as
well other physical
requirements. Type I is a cement composed of a low-softening
point asphalt, and Type II is composed of a high-softening
point asphalt.
These classifications are further categorized by
the intended application: Class I for dry surfaces or Class
II for damp, wet surfaces.
• ASTM D 4586,
“Standard Specification for Asphalt Roof Cement, Asbestos
Free.” This standard addresses composition, pliability
and high-heat behavior, as well other physical requirements.
The Type I and Type II classifications are the same as those
in ASTM D 2822; however, this standard does not differentiate
between wet or dry
surface usage.
The following ASTM standard
applies to asphalt roof cement used as a lap cement:
• ASTM
D 3019, “Standard Specification for Lap Cement Used
with Asphalt Roll Roofing, Non Fibered, Asbestos Fibered,
and Non Asbestos Fibered.” Three types of lap cement
are
described in this standard: Type I, brushing
consistency with no stabilizers: Type II, heavy
brushing or light troweling consistency with a quantity of
shortfibered asbestos stabilizers:
Type III, heavy brushing or light troweling
consistency with non-asbestos stabilizers.
A roof deck provides
the structural substrate over which a roof system is applied.
It also must be able to provide adequate withdrawal resistance
for fasteners used to attach the roof system. Because of the
typical longevity of slate roof systems, a roof deck material
of comparable long-term expected service life is important.
Slate roof systems require a structure capable of supporting
heavier loads than most other steep-slope roof systems. The
thickness and structural characteristics of wood sheathing/decking
should be increased with thicker layers or heavier slates.
Slate may be applied
over the following substrates:
• wood
plank
• wood board
• structural
wood panels
For new construction,
NRCA recommends designers specify roof deck slopes intended
for the application of steepslope materials at 4:12 (18 degrees)
or greater. Slate should be applied only over continuous or
closely spaced wood plank or board decking and not over a
spaced, or “skipped,” application of wood plank
or board decking.
The proper thickness
and species of wood decking required for a specific roof is
determined by design loads, including uplift, anticipated
for the roof assembly and the distance between the supporting
members. End joints of each piece of decking should be staggered.
The end joints, except for matched ends (e.g., tongue-and-groove)
should also be centered over supporting members.
Slate may be applied
over wood plank or board decking that is not warped, cupped
or bowed. The terms “plank” and “board”
are generally differentiated by thickness. The Western Wood
Products Association (WWPA) defines the term “plank”
as “A piece of lumber, from 2 but not including 5 inches
thick, generally used with wide face horizontal.” The
term “board” is defined by the WWPA as “…
a term generally applied to lumber when the nominal size is
1 inch thick and 2 or more inches wide.” Wood boards
used for decks should be a minimum of 6 inches (150 mm) nominal
width.
A wood plank or board
roof deck is composed of solid-sawn dimensional lumber. It
is normally supported by wood beams — often glue-laminated
timber or glulams — and/or solid lumber joists, purlins
or rafters. Wood planks or boards may be single or double
tongue-and-groove, straight-edged, ship-lapped, or grooved
for splines or longitudinal edges.
Structural wood panel
decking consists of either plywood or oriented strand board
(OSB).
Plywood Roof Decks
A plywood roof deck
is composed of panels made of thin wood layers called veneers
that are peeled from logs. The veneers are laid at right angles
to each other then glued together under heat and pressure.
This cross-lamination adds strength and stability to all-veneer
panels. Panels consist of a number of cross-laminated layers
that vary in number according to a panel’s thickness.
Oriented Strand
Board (OSB) Roof Decks
An OSB roof deck is
composed of panels made from layers of compressed, glued wood
strands. These strand layers are oriented at right angles
to one another before being glued and formed into panels.
NRCA is concerned with
potential fastener-holding problems and dimensional stability
because of the effects of moisture where OSB and other non-veneer
products are used as roof decking.
All plywood and wood-based
panel roof decks suitable for application should be made from
plywood or wood-based panels rated for structural use as roof
sheathing. Most building codes require a label on plywood
or wood-based panels ensuring that the plywood or wood-based
panel complies with the criteria set forth in U.S. Product
Standards (PS) PS
1-95, “Construction and Industrial Plywood”, for
all veneer plywood or PS 2-92, “Performance Standard
for Wood-Based Structural-Use Panels;” | or APA—The
Engineered Wood Association (APA) standard PRP-108, “Performance
Standards and Policies for Structural-Use Panels,” for
structural-use panels (e.g., oriented strand board, all-veneer
plywood).
Performance standards
PS 1-95 and PS 2-92, which were initiated by APA — The
Engineered Wood Association, formerly the American Plywood
Association, have been developed under the “Procedures
for the Development of Voluntary Product Standards”
of the U.S. Department of Commerce. Performance standard PRP-108
was developed by APA — The Engineered Wood Association.
NRCA recommends that
plywood or wood-based panels intended for use as roof sheathing
meet or exceed the requirements set forth by PS 1-95, PS 2-92
or PRP-108. Some roofing materials manufacturers will allow
application of roof products over other wood substrates.
When installing slate
roof systems, if plywood is used as the roof deck material,
5Ú8 inch (15 mm) CDX nominal thickness is the minimum
recommended
by NRCA.
All wood panel roof
decks should consist of panels rated for structural use as
roof sheathing. For
particularly demanding applications, such as
prefabricated panelized roof deck systems where cross-panel
strength and stiffness or shear properties are critical, designers
are recommended to use panels meeting the standard’s
requirements for “Structural I Rated Sheathing.”
Wood sheathing panels
should be spaced approximately 1Ú8 inch (3 mm) to allow
for expansion. End joints of wood sheathing panels that do
not occur over supporting members should be blocked to provide
adequate support for the sheathing panel ends.
NRCA does not recommend
the direct attachment of slate to gypsum; concrete plank;
cementitious wood fiber; or similar, non-wood materials.
Caution should be exercised
when roof decks are constructed of wood that has been treated
with an
oil-borne preservative. Many roofing materials man-ufacturers
recommend that wood roof decks be constructed with wood that
has been treated with a non-oil preservative pressure treatment,
or with
non-treated air- or kiln-dried lumber. For additional information
regarding preservative wood treatment, consult the American
Wood Preservers Association (AWPA).
Because of the deterioration
of some fire-retardant-treated wood panels caused by chemical
reaction, special care should be given to investigate the
use of fire-retardant-treated wood panel decks in the design
of a steep-slope roof system.
NRCA recognizes that
other types of structural decking are becoming more common
in steep-slope roof construction, specifically in the commercial
sector; however, NRCA recommends that slates be attached only
to structural wood panel decks or to wood plank or board decks
or batten systems unless a wire tie attachment system is used.
NRCA does not recommend the direct attachment of slate to
gypsum; concrete plank; cementitious wood fiber; or similar
non-wood materials. If deck types or attachment methods other
than these are used, NRCA suggests that a nailing substrate
consisting of structural wood panels, such as plywood, is
installed over the structural deck.
Examples of deck types
over which NRCA suggests a layer of plywood are:
• steel
roof decks
• poured
and precast, structural concrete decks
For ventilation purposes,
if necessary, and to allow acceptable clearance for proper
fastener penetration of a plywood nailing substrate, NRCA
suggests the use of wood battens or metal channels attached
over a structural deck to raise and separate the plywood panels
from the deck surface below. The design, placement, spacing,
height and attachment of wood battens or metal channels is
the responsibility of the designer. A complete roof assembly,
including the structural deck, plywood nailing substrate and
roof covering should be designed to meet local building code
requirements. Key factors to consider during the design phase
are fire resistance, structural loading
and wind resistance requirements of the applicable building
code.
When installing slate
roof systems, NRCA does not recommend the installation of
a nailing substrate over cementitious wood fiber deck panels,
poured and precast lightweight insulating concrete, or poured
and precast gypsum roof decks.
The possibility of
water migration through a slate roof covering should be carefully
considered. The watershedding capabilities of a primary roof
covering are closely related to the slope of a roof, dimension
of the overlap and headlap, distance between side joints in
neighboring courses location of fastener holes, surface conditions
of the slate and severity of weather conditions anticipated.
Slate roof coverings
are designed for use as
multilayered, watershedding roof systems. Slate roof coverings
rely on slope to shed water off a roof system’s surface.
Many slate roof systems
have outlived the
underlayment felts over which they were installed. Therefore,
where the long-term watershedding characteristics of the underlayment
are necessary to provide the weatherproof integrity of finished
roof systems, designers should carefully consider the type
and quality of underlayments to be specified.
An underlayment should
be comparable to the design service life of a primary steep-slope
roof covering and components. Consideration should be given
to good local practices, which draw upon experience with various
underlayment products and methods that have been developed
to address special conditions.
There are different
underlayment configurations that can be used for slate roof
systems. Generally, these configurations can be categorized
as follows:
• single-layer
underlayment
• single-layer
of self-adhered underlayment
• double-layer
underlayment
A “single-layer
underlayment” is one layer of underlayment fastened
to a deck before the application of the slate. In single-layer
applications, all underlayment felts or sheets should be lapped
a minimum of 2 inches (50 mm) over the preceding sheet. End
laps should be a minimum of 4 inches (100 mm). The underlayment
should be fastened approximately for the slope of the roof,
sufficient to hold the felts in place until the installation
of primary roof covering materials.
A “single layer
of self-adhered underlayment” is one layer of self-adhering
polymer-modified bitumen membrane applied over the roof deck.
Typically, this type of membrane is designed for use as an
ice dam protection membrane. Designers should note that these
types of membranes, when installed over an entire roof area,
tend to act as vapor retarders. Potential problems with ventilation,
moisture control and vapor-retarder placement should be considered
during the design phase.
A “double-layer
underlayment system” is two layers
of underlayment fastened to a roof deck before application
of slate. When a double-layer underlayment is required, felt
should be applied horizontally at a 19 inch (480 mm) overlap
and an 17 inch (430 mm) exposure. An underlayment should be
fastened sufficiently to hold the felts in place until primary
roof covering material is applied.
When a double-layer
underlayment configuration is used and where the underlayment
layers are identical materials, they are commonly installed
in a shingled fashion with a 19 inch (480 mm) overlap and
a 17 inch (430 mm) exposure. However, if one layer is used
to “dry-in” the building temporarily or underlayments
of two differing compositions are used, each layer may be
applied in a single layer configuration.
The following are recommendations
for underlayment and slate headlap with respect to roof slope:
• for roof slopes of 8:12 (34 degrees) and above, a minimum of one layer of No. 30 underlayment felt or one layer of polymer-modified bitumen underlayment is recommended.
Where weather conditions
are severe and hard
wind-driven rains are common, NRCA recommends specifying a
minimum of two layers of No. 30 asphalt-saturated felt or
one layer of a polymer-modified bitumen underlayment with
40 mil (1.0 mm) minimum thickness for use as underlayment
with a slate roof.
• for roof slopes of 4:12 (18 degrees) to 8:12 (34 degrees), a minimum of two layers of No. 30 underlayment or one layer of a polymer- modified bitumen underlayment are recommended for use as underlayment with standard sized slate as long as the slate is installed with a 3 inch (75 mm) minimum headlap.
NRCA does not recommend
the design of slate roofing on slopes less than 4:12 (18 degrees).
At slopes less than 4:12 (18 degrees), slate is a decorative
roof covering only, and a weatherproof membrane installed
under the slate is necessary.
Regardless of the type of underlayment required or roof slope, in locations where the average temperature for January is 30° F (-1° C) or less, NRCA suggests installation of an ice dam protection membrane.
An ice dam protection
membrane generally is a self-adhering polymer-modified bitumen
membrane.
NRCA recommends that these types self-adhering membranes be
a minimum of 40 mils (1.0 mm) thick and comply with ASTM D
1970.
Underlayment felts,
as well as polymer-modified bitumen base sheets, cemented
together with asphalt roof cement or a cold adhesive can also
be used as ice dam protection membranes.
An ice dam protection membrane should be applied starting at the eaves and extending upslope a minimum of 24 inches (610 mm) from the inside of the exterior wall line of a building.
Ice dam protection
membranes, by themselves, cannot be relied upon to keep leaks
from occurring. Careful consideration of roof ventilation,
insulation and project-specific detailing for particular climactic
conditions is vital. Also, self-adhering polymer-modified
bitumen underlayment must not be left exposed for long periods
of time. Self-adhering polymer-modified bitumen underlayments
should be covered with primary roofing material as soon as
practical to prevent premature degradation of the modified
bitumen material. See manufacturers’ individual product
requirements.
Designers should note
that these types of membranes, when used as underlayments
over an entire roof area, tend to act as vapor retarders.
Potential problems with ventilation, moisture control and
vapor-retarder placement should be considered during the design
phase.
Because of the long-term
service life of slate, serious consideration should be given
to the type of fasteners to be specified. Several fastener
types apply to certain projects, such as copper slating nails,
stainless steel, bronze or cut-brass roofing nails. NRCA suggests
the use of copper slating nails for slate roofs. Unprotected
black-iron and electroplated nails are not recom-mended. An
additional benefit or consideration for the use of copper
is that copper slating nails can be easily cut with a slate
ripper when repair work is needed.
Nail shank should not
be of larger diameter than the fastener holes in the particular
slate being used. Nails recommended for most standard-sized
slate roof systems are sharp-point, 3Ú8 inch (9 mm) large
flat head, copper-wire slating nails with a smooth, round,
barbed or otherwise deformed shank. Generally, thicker slates
require the use of larger diameter, longer nails. Consideration
should also be given to the slope of a roof, weight of slate,
wind loads anticipated, and roof sheathing specified all these
items relate to slate fastener selection for a specific project.
Fasteners should be
long enough to penetrate through all layers of roofing material
and achieve secure anchorage into a roof deck. Fasteners should
extend through the underside of plywood and penetrate at least
3Ú4 inch (19 mm) into wood board or plank decks. The required
length of slate fasteners varies according to the thickness
and exposure of the slate being used. For most 3Ú16 inch (5
mm) to 1Ú4 inch (6 mm) thick slate, laid with a 3 inch (75
mm) head lap over an underlayment, a 11Ú2 inches (38 mm) or
13Ú4 inches (44 mm) nail length is adequate.
Standard-sized slate
is fastened with two nails. All roofing slate should have
a minimum of two nails. However, slate that is subject to
high-wind conditions and/or 3Ú4 inch (19 mm) and thicker
should be fastened with four nails.
Holes are punched from
1Ú4 inch to 1Ú3 inch (6 mm to 8 mm) the length of the slate
from the upper end, and 11Ú4 inches to 2 inches (32 mm
to 50 mm) in from the edges. Where four holes are used, it
is typical to punch two additional holes approximately 2 inches
(50 mm) above the two regular holes.
When attaching slates, nails should not be driven tight against the slate as if to draw the slate tight to the deck. Slating nails should be driven so that a nail’s head just touches the surface of the slate so the slate hangs on the nail.
In high-wind areas,
a 1 inch (25 mm) dab of flashing-grade roof cement, roofer’s
cement or polyurethane sealant placed under the exposed part
of the slate near the leading edge can help secure it.
Other methods of attaching/affixing
slate in certain situations are with slating hooks and wire
tie systems.
The exposure of slate
is the portion of the slate shingle that is not covered by
the course above and is, therefore, the length of the slate
roofing unit exposed to the weather. The proper exposure for
a particular length of slate is obtained by deducting the
3 inch (76 mm) headlap from the total length of the slate
then dividing that number by two. For instance, the proper
exposure for a 24 inch (610 mm) slate is:
24 inches -
3 inches = 21 inches;
21 inches ÷ 2 = 10 1Ú2 inches
(600 mm - 75
mm = 525 mm ÷ 2 mm = 263 mm)
Table
4 (Click here) shows the proper exposures for various lengths of slate
if all are to be set with a 3 inch (76 mm) headlap.
Slate can be installed
to graduate by thickness and/or size. Thicker and/or longer
slates are laid at eaves graduating to the thinner or smallest
size at ridges. A typical graduation by thickness is 1Ú2 inch
(13 mm) to 3Ú8 inch (10 mm) to 1Ú4 inch (6 mm). A typical
grad-uation by size is 20 inches (500 mm) to 18 inches (450
mm) to 16 inches (400 mm) to 14 inches (350 mm).
When multiple colors
are used, the percentage of each color to be used throughout
the roof system should be specified. An example is 40 percent
unfading green, 40 percent weathering green and 20 percent
purple. With regard to weathering slates, some quarries can
reasonably predict the percent and intensity of color change
from the base color to weathered color.
Before the first course
of slate is installed, a row of starter slates is applied
along the eave of a roof system to serve as the starter course.
The starter course’s primary purpose is to shed water
that may migrate through the joints of the slates in the overlying
first course.
The lower edge of the
starter course should extend beyond the downslope perimeter
(eave) approximately 2 inches (50 mm) to assist in directing
runoff away from the fascia board and other underlying building
components. When gutters or eave troughs are used, the overhang
may be reduced to approximately 1 inch (25 mm) or less. Slates
should be installed to extend approximately 1 inch to 2 inches
(25 mm to 50 mm) beyond the rake edge.
Starter slates may be
applied face down. This allows the smooth backs of the starter
course and first course of slate contact each other.
An eave cant is necessary
to raise the butt edge of a starter and first course of slate
the same way the headlap or third layer of slate raises the
butt edge of all succeeding courses. The thickness of the
eave cant should be about the thickness of the eave slates.
A traditional eave cant is a wood lath 1Ú4 inch (6 mm)
thick and 2 inches (50 mm) wide. It is nailed to a
deck and covered with eave flashing metal and
underlayments. Beveled boards and raised metal
eave flashings are also used.
Because steep-slope
roof systems are frequently interrupted by the intersection
of adjoining roof sections; adjacent walls; or penetrations,
such as chimneys and plumbing soil-pipe stacks, all of which
create opportunities for leakage, special provisions for weather
protection must be made at these locations. These components
used to control water entry are commonly called flashings.
Careful attention to flashing details is essential to successful
long-term roof performance regardless of the type of roof
construction.
Flashings in this section
are divided into the following categories:
• perimeter/edge
metal
• penetrations
• valleys
• vertical surfaces
Flashing metals should
be made from a material of thick enough gauge to achieve at
least the expected design life of the steep-slope roof covering
used with it.
Depending on the severity
of the climate, anticipated rainfall and freeze-thaw cycling,
the use of perimeter edge metal should be considered.
Where climate or roof
edge construction dictates the need for perimeter edge metal,
the type and minimum thickness of the metal should be commensurate
with the anticipated service life for the slate roof system.
NRCA suggests metal penetration flashings for slate roof systems
be fabricated from one of the following metal types and minimum
thicknesses.
• 24 gauge
(0.025 inch [0.64 mm] thick) prefinished galvanized steel
• 24 gauge (0.024
inch [0.61 mm] thick)
stainless steel
• 16 ounce (0.022
inch [0.56 mm] thick) copper
• 16 ounce (0.026
inch [0.66 mm] thick)
lead coated copper
In some regions, particularly
those with mild climates, other types of metal and/or metals
of lesser thickness than are shown above may be used successfully.
NRCA considers these applications to be area practices. Refer
to the Introduction for additional information about area
practices.
There are many other smaller penetrations that need to be flashed into slate roof systems, such as plumbing soil stacks, vents, exhaust fans, furnace of water heater flue pipes, electrical standpipes and others. This is typically accomplished with the use of some type of flat flange that extends around the penetration and is installed under the slate and underlayment on the upslope side of the flange and extends down on top of the slate at the downslope side of the flange. Attached and sealed to the flange is a cylinder, rectangular box or neoprene gasket that is used to seal around the penetration. The flange can be set into mastic for additional protection. These flashing components are often supplied by other trades but may be installed by a roofing contractor.
The type and minimum
thickness of the metal used for penetration flashings should
be commensurate with the anticipated service life for the
slate roof system. NRCA suggests metal penetration flashings
for slate roof systems be fabricated from one of the following
metal types and minimum thicknesses.
• 24 gauge
(0.024 inch [0.64 mm] thick)
prefinished galvanized steel
• 0.032 inch (0.81
mm) thick aluminum
• 0.032 inch (0.81
mm) thick prefinished aluminum
• 24 gauge (0.024
inch [0.61 mm] thick) stainless steel
• 16 ounce (0.022
inch [0.56 mm] thick) copper
• 16 ounce (0.026
inch [0.66 mm] thick) lead-
coated copper
• 4 pound (0.062
inch [1.57 mm] thick) lead.
In some regions, particularly
those with mild climates, other types of metal and/or metals
of lesser thickness than are shown above may be used successfully.
NRCA considers these applications to be area practices. Refer
to the Introduction for additional information about area
practices.
A valley is created
at the downslope intersection of two sloping roof planes.
Water runoff from the portions of the roof areas sloping into
the valley flows toward and along the valley. Because of the
volume of water and lower slope along a valley line, this
area is especially vulnerable to leakage. A clear, unobstructed
drainage way is desired in valleys, so the valley may carry
water away quickly and perform successfully for the life of
the roof system.
Where roofs of two equal
slopes join to form a valley, the slope of the valley is less
than that of the two adjacent fields of the roof. For example,
where two sloped roofs with slopes of 4:12 (18 degrees) intersect
at a valley, the actual valley slope is only about 3:12 (14
degrees).
With slate roof systems,
there are three basic types
of valleys:
• open
valleys
• closed or mitered
valleys
• rounded valleys
These three general
types of valleys are constructed only after the necessary
layer(s) of underlayment and any valley lining material specified
have been applied to a deck.
Valley underlayment
construction consists of a full-width, 36 inch (900 mm) sheet
of No. 30 underlayment felt or a polymer-modified bitumen
underlayment, base sheet or ice dam protection membrane. This
valley underlayment is centered in the valley. Valley underlayment
sheets should be secured with only enough fasteners to hold
them in place until the balance of valley materials are applied.
The courses of underlayment from the fields of two adjoining
roof areas are extended so that each course overlaps the valley
underlayment by at least 12 inches (300 mm). The valley is
then lined with the balance of the valley flashing and slate.
Another recognized installation method is weaving the intersecting
underlayment courses through the valley extending the underlayment
a minimum of 18 inches (460 mm) beyond the center line of
the valley on each side.
To prevent leakage
it is important with all types of valley construction to avoid
placing fasteners near the center of a valley. Generally,
slate fasteners should be kept back from the center of a valley
by a minimum of 8 inches (200 mm).
To avoid fastening
too close to the center of valley metal, slate may be secured
by wire-tie attachment. Wire-tie methods of attachment may
be used with closed and open types of valleys. Using wider
slate may provide an alternative to wire-tie attachment of
slates, because all wider slates allow fasteners to be installed
without penetrating valley flashing metal.
Open valleys are typically
lined with sheet metal. A metal valley is constructed by installing
lengths, typically 8 feet or 10 feet (2.4 m or 3 m) of corrosion-resistant
metal through the valley.
The slate and, with some area practices, the underlayment is lapped onto the flange on either side of the valley metal, leaving a clear space between the roofing material to channel runoff water down the valley.
The type and minimum
thickness of the metal used in an open valley should be commensurate
with the anticipated service life for the slate roof system.
NRCA suggests valley metal for slate roof systems be fabricated
from one of the following metal types and minimum thicknesses.
• 24 gauge
(0.025 inch [0.64 mm] thick) prefinished galvanized steel
• 24 gauge (0.024
inch [0.61 mm] thick) stainless steel
• 16 ounce (0.022
inch [0.56 mm] thick) copper
• 16 ounce (0.026
inch [0.66 mm] thick) lead-
coated copper
• 4 pound (0.062
inch [1.57 mm] thick) lead.
In some regions, particularly
those with mild climates, other types of metal and/or metals
of lesser thickness than are shown above may be used successfully.
NRCA considers these applications to be area practices. Refer
to the Introduction for additional information about area
practices.
NRCA also suggests
that valley metal be formed with a “W”-shaped
splash diverter or rib in the center. A center rib can be
especially beneficial in valleys where adjoining roof areas
are of unequal slope because the rib helps prevent “wash
over” of runoff. A center rib should not be less than
1 inch (25 mm) high. For easier installation and controlling
thermal expansion and contraction, NRCA suggests valley metal
pieces used with slate roofing be no longer than 10 feet (3
m). NRCA recognizes that “V-”shaped valley metal
performs satisfactorily in certain environments but not when
a valley is formed by two different roof slopes.
NRCA recommends that
valley metal used with slate be a minimum of 18 inches (450
mm) wide. This means flanges on either side of the metal valley
center line are approximately 8 inches (200 mm) wide. Having
a flange width of approximately 8 inches (200 mm) allows the
slate to lap onto the flange at least 4 inches (100 mm).
Open valleys permit
clear, unobstructed drainage and they are advantageous in
locations where fallout from surrounding foliage settles on
the roof system and tends to accumulate in the valley. Valley
metal should be made from a material thick enough to achieve
at least the expected design life of the steep-slope roof
covering.
In some climates, particularly
those in areas prone to accumulations of snow and ice or with
regular freeze thaw cycling open valley construction can be
enhanced by one of the following procedures:
• lining
the valley with a self-adhering polymer-
modified bitumen underlayment material before application
of the metal valley
• stripping
in flanges on either side of the metal
valley with a 9 inch to 12 inch (230 mm to 300 mm) strip of
self-adhering polymer-modified
bitumen underlayment material. The self-adhering material
is adhered onto the valley metal flange and onto an underlying
width of similar
self-adhering membrane material
• attaching
valley flashing metal with cleats rather than through-fastening
• tapering
the valley so that it is wider at the low point than it is
at the high point
Tapering the valley
has the following advantages:
• allows
for increase in runoff water volume to be received at the
downslope end
• allows
any ice that may form within the valley to free itself when
melting and slide down and exit the valley rather than lodging
somewhere along the length of the valley
In a closed valley, slate on both sides are cut at an angle parallel to the center line of the valley and are butted together, forming a mitered joint. In areas of the United States where heavy accumulations of foliage fallout are anticipated or if moss can be expected to grow between the slate roofing joints, a closed valley can hamper runoff. Therefore, specifying a closed valley should be carefully considered to be sure it is appropriate for the particular project.
Closed valleys for slate may also be formed by installing the slate tight to the valley line and placing individual pieces of metal flashing under each course of slate along the valley centerline.
To form a rounded valley,
field slates are taper-cut and fanned or swept through the
valley and slates are used to create the valley surface. Individual
metal valley flashings are installed with each course of slate.
Note: Rounded valleys are the least common of the three slate
valley types. Of the three common slate
valley types, closed rounded valleys
are the most labor-intensive and intricate to construct.
There are four types
of metal flashings that are commonly used at locations where
a roof intersects a vertical wall:
• apron
flashing — a metal used at a head-wall transition, such
as the downslope side of chimney
• step flashing
— used at a side-wall transition, such as the side of
a dormer
• cricket or backer
flashing — used at the upslope side of a roof penetration,
such as a chimney.
• counterflashing
— secured to a vertical wall and used to cover and protect
the top edge of an apron, step, and cricket or backer flashing.
Figure
13 (Click here) shows a chimney
penetration and the use of all four flashing types. Generally,
before flashings are applied, an asphalt-saturated felt underlayment
should be applied to a roof deck around roof penetrations.
However, in moderate and severe climates, an ice dam protection
membrane can be installed around the bases of chimneys or
curbs. If appropriately specified and constructed, an ice
dam protection membrane can assist in keeping water from migrating
into a roof system at roof-to-wall intersections during times
of severe winter freeze-thaw cycling.
Apron flashings provide a weatherproofing transition material where a roof area intersects a head wall. Common locations for apron flashings are the front, downslope, side of a dormer or chimney; curbed roof penetrations; and clerestory transitions.
The type and minimum
thickness of the metal used for apron flashing should be commensurate
with the anticipated service life for the slate roof system.
NRCA suggests metal apron flashings for slate roof systems
be fabricated from one of the following metal types and minimum
thicknesses.
• 24 gauge (0.025
inch [0.64 mm] thick) prefinished galvanized steel
• 24 gauge (0.24
inch [0.61 mm] thick) stainless steel
• 16 ounce (0.022
inch [0.56 mm] thick) copper
• 16 ounce (0.026
inch [0.66 mm] thick) lead-coated copper
• 4 pound
(0.062 inch [1.57 mm] thick) lead.
In some regions, particularly
those with mild climates, other types of metal and/or metals
of lesser thickness than are shown above may be used successfully.
NRCA considers these applications to be area practices. Refer
to the Introduction for additional information about area
practices.
For slate roof systems,
when a roof area intersects a side wall, flashing is installed
at the end of each course of slate.
For most climactic regions, NRCA suggests using metal step flashing that is equal to the length of the slate by 8 inches (200 mm) wide so a minimum step flashing headlap is achieved and a 4 inch (100 mm) extension is obtained onto each underlying shingle and 4 inches (100 mm) up the vertical surface.
The type and minimum
thickness of the metal used for step flashing should be commensurate
with the anticipated service life for the slate roof system.
NRCA suggests metal step flashing for slate roof systems be
fabricated from one of the following metal types and minimum
thicknesses.
• 24 gauge (0.025
inch [0.64 mm] thick)
prefinished galvanized steel
• 24 gauge (0.024
inch [0.61 mm] thick)
stainless steel
• 16 ounce (0.022
inch [0.56 mm] thick) copper
• 16 ounce (0.026
inch [0.66 mm] thick)
lead-coated copper
• 4 pound (0.062
inch [1.57 mm] thick) lead.
In some regions, particularly
those with mild climates, other types of metal and/or metals
of lesser thickness than are shown above may be used successfully.
NRCA considers these applications to be area practices. Refer
to the Introduction for additional information about area
practices.
The length of step
flashing is generally the length
of slate.
Special attention needs to be paid at the first (bottom) step flashing where an eave intersects a continuous vertical surface to ensure water is diverted to the outside of the wall covering.
When a roof area intersects
the upslope side of a chimney or curbed roof penetration,
either a cricket or backer flashing should be installed at
this location. A cricket diverts water around the penetration,
and a backer flashing simply provides a weatherproofing transition
material where the roof intersects the back side of the penetration.
NRCA recommends that
designers specify crickets
at the upslope sides