Grades of Slate
Greenstone Slate® is measured by eye to thickness and is then split by hand. It is truly the result of a craftsmen’s touch. However, as the result of the process, no two pieces will be exactly the same. Once split to a thickness and then trimmed to a dimension, the slate is “graded” according to the uniformity in thickness of each piece. There are no machines or calipers used in this “grading technique”. It is the characteristics of the natural stone and the trained experience of the craftsmen that separates Greenstone’s Vermont slate from all other products. There is truly no comparative roofing product! Greenstone Slate® comes in five thicknesses or grades that—just like color—contributes substantially to the look of the roof. It also determines the ultimate level of protection, including hail rating.
When it comes to value, Architectural Grade is the most selected, measuring between 3/8″ and 1/2″ thick. It offers the best mix of characteristics with rich textures for aesthetic appeal, a high level of durability—it has earned a Class 4 hail rating (check with your insurance company about hail coverage for slate roofs)—yet will not need the extensive additional structural support that heavier grades require. However, it weighs 1200 to 1600 lbs per square.
Heavy Grade slates exhibit strong stone surface texture, and is visually discernible from synthetics. Measuring 1/2″ to 3/4″ in thickness they bring a more robust texture to the roof, as well as additional strength. This Class 4 hail rated slate weighs 1800 to 2200 lbs per square.
With Rough Texture Grade characteristics and each slate at least 3/4″ thick, nothing looks quite like an Estate Grade Vermont slate roof. With its enhanced grain, it is an unmistakable slate roof with an expected usage of over 150 years. Impervious to hail and even bullets, these ultra-premium slates weigh 2500 to 3000 lbs per square.
Standard Grade Rough Texture
This slate is split at thicknesses between 1/4” to 3/8”. It has the surface appearance of Heavy Grade in a thinner slate. It is ideal for traditional projects with a goal to limit the weight from 900 to 1000 lbs per square. Standard Grade Rough Texture is also commonly used in conjunction with our lightweight installation systems, reducing the installed weight to under 6 lbs per sq ft. Standard Grade Rough Texture has a Class 3 hail rating.
Standard Grade Selects
These slates are a very uniform 1/4″ in thickness and have smooth surfaces. Select Grade roofing slate is most commonly used in repair projects. It weighs from 680 to 780 lbs per square.
Above is a graduated roof that combines several grades of slate creating the most texture in the lower areas of the roof. The installation includes stainless steel gutters and snow guards.