The Slate Roof Checklist

The Greenstone guide to optimizing your investment in a slate roof.

Not all slate is the same, but sorting out the differences in slate does not take long. It starts with understanding that slate is metamorphic stone that was formed millions of years ago. The differences in slate start with the geological conditions where it was formed. Fortunately, slate has been independently analyzed, performance standards established and categorized putting a clear performance rating on slate sourced from different areas of the world. Greenstone Slate® is sourced from 58 quarries in the Slate Valley of Vermont, one of the highest quality slate formations ever found.

✓ Be sure the slate you buy is properly rated.
Greenstone Slate® is rated and documented as ASTM S1 slate, the highest rating possible. You can visit ASTM, an international quality standards development organization to learn more about slate ratings.

✓ The slate rating should be determined by qualified testing laboratories
Greenstone Slate is rated S1 and is tested only by National Slate Association approved testing laboratories. Be sure to confirm the credentials of a quarrier’s independent testing facility.

✓ Consider slate thickness
The highest value slate is at least 3/8″ thick. It not only offers the durability that no insurance company could challenge, the texture is aesthetically miles ahead of thinner slates. Check with your insurance agent about slate to determine coverage and even possible discounts when some slate types are used.

✓ Smaller slates are even more rugged than large ones
While large slates can cost less to install, in a severe weather event such as as a hail storm with hail larger than 1.25″, smaller, thicker slates are the most durable.

✓ The weight of your roof
Stone is heavier than most other materials and care must be taken to make sure the building’s structure will support it. Structural reinforcement is sometimes needed. But a lightweight slate roof installation is often the solution, and we have two options using genuine slate that through new installation systems allow installing slate on almost any roof—great for cedar shake replacement! Click for more on reduced weight slate roofs.

✓ The best roofing material deserves the best installation materials
Considering the long life span of a slate roof, the hardware and accessories you select should be very durable. It is economically unwise to use anything but the most permanent items utilizing non-ferrous metals.

✓ Copper Nails and Flashing: We recommend copper nails for proper installation of slate. Copper’s ability to adjust to changes in temperature and its non-corrosive qualities make it an ideal solution for slate roofs. The slight savings of using non-copper nails will surely cost you more in repairs and will significantly shorten the life span of your roof. Flashing should be used at all intersections of vertical or projecting surfaces through the roof or against which the roof abuts, such as walls, parapets, dormers, sides of chimneys, etc. Copper is particularly suitable for all flashings (including valleys) as it is easily worked and shaped. It also adjusts itself to temperature stresses.

✓ Snow Guards: Snow guards are rooftop devices that allow snow and ice to drop off in small amounts or melt completely before falling to the ground. Snow guards are installed on sloped roofs to prevent damage.

✓ Experienced Installation Contractor: The better the installation techniques, the better the roof will protect the building and the longer the roof will last! If you have a preferred roofing contractor, we will work with them to answer questions and assist with any advice we can provide—including making sure they have access to our extensive online installation documentation.

✓ Finding a contractor: We will help you find an independent contractor to do the installation work. We know many contractors around the country and worldwide. Keep in mind that we cannot take responsibility for any contractor’s work. We only guarantee the slate. Inquire here.


We are ready to assist with selecting and specifying slate, advising on installation, or helping a homeowner find a contractor.

Call us at 800 619 4333 or click here to contact us.

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