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Grades of Slate

Grades of Slate

Grades of Slate Tile Roofing

Grades of Slate Tile Roofing

Slate grading is based on thickness . . .

Regardless of the grade, all of Greenstone’s roofing slate has a life expectancy of at least 100 years. Slate is “graded” according to the uniformity in thickness that each piece exhibits. As with any natural product, the more particular the physical specification, the more demanding the production requirements. Therefore, a slate’s grade has great bearing on the availability and overall cost of the material. Here is a list and description of the four slate grades.

Standard Grade “Selects”: These premium slates are a very uniform 1/4″ in thickness with little texture. Standard Grade roofing slate weighs from 680 to 780 lbs. per square.

Rough Texture Grade: Since natural slate is hand split from blocks of stone, some pieces may be thicker than 1/4″ or have natural characteristics that produce uneven surfaces. These slates are recognized as Rough Texture Grade. Rough Texture Grade is not to be confused with imperfections in the stone that would jeopardize the slates structural integrity and waterproofing capabilities. Each slate is free of weakening imperfections, but exhibits a more enhanced grain and variation in thickness. Rough texture grade slate is manufactured at a thickness between 1/4″ and 3/8″ and weighs between 800 and 1000 lbs. per square.

Architectural Grade: Slates designated as Architectural Grade are the same as Rough Texture Grade except that the slates are from 3/8″ – 1/2″ thick. Architectural Grade slate weighs from 1200 to 1600 lbs. per square.

Heavy Grade: Slates designated as Heavy Grade are the same as Rough Texture Grade except that the slates are 1/2″ – 3/4″ thick. Heavy Grade slate weighs from 1800 to 2200 lbs. per square.

Estate Grade: Slates designated as Estate Grade are the same as Rough Texture except that the slates are at least 3/4″ thick. Estate Grade slate weighs from 2500-3000 lbs. per square.

About variable slate widths. . .

In a Random Width roof, the widths of the slate vary. An excellent explanation of Random Width natural Vermont roofing slates in the following quote from The Slate Book by Brian Stearns, Alan Stearns and John Meyer:

“A variety of widths will generally be provided by the quarry. The largest percentage of widths shipped will be approximately 2/3 the length of the slate being used; the smallest percentage will be of the largest widths. However, when specifying Random Width this will vary and percentages cannot be specifically ordered. The use of Random Width slate will generally reduce the material cost of the roof. In appearance, the alternating vertical joints between shingles on a Random Width roof will not always line up with the vertical joints from the courses below. If you want to achieve a symmetrical appearance, you will want to avoid Random Widths. In some cases, the use of Random Widths will make an otherwise standard appearance look more textured or cobbled. Random Widths slate can be used with most of the roof types and with other variables, but creating patterns in a Random Width roof is not common.”

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