Roofing slate quality is influenced by the quality of the stone and by the quality of the manufacturing process that makes that stone into a roofing slate.

The Stone

Not all metamorphosed ocean sediments are capable of protecting a building for 100+ years. It takes a special combination of chemistry and geologic metamorphosis to create a slate durable enough to withstand centuries of exposure on a roof.

The US slate industry has identified flexural strength, water absorption and weathering resistance as the three physical properties of slate most directly related to the life expectancy of slate on a roof. These properties are commonly evaluated using ASTM C406 Standard Specification for Roofing Slate, and based on the outcomes of three independent tests, the slate in question is assigned a grade. Each grade corresponds to an expected service life.

ASTM Test C120 measures the slate’s ability to withstand a load with out breaking. To be graded S-1, the samples must achieve a minimum breaking load of 575 pounds. The current test standard, C120-09, reflects changes made in the last decade to improve consistency in testing and isolate the effects of sample thickness. The National Slate Association has a great series of articles ( http://slateassociation.org/material-standards/) on the ASTM testing process and the evolution of the C120 methodology.

ASTM Test C121 measures the water absorption of the slate. Water absorbed into the pores of stone can accelerate chemical weathering and when subjected to temperature extremes, the freeze thaw cycles can physically deteriorate the slate. A lower percentage of water absorption is believed to indicate a longer service life. To receive a S-1 grade, a maximum of 0.25% water absorption is allowed.

ASTM Test C217 measures the weather resistance of slate by pouring sulfuric acid on the slate surface then scratching the slate and measuring how far into the slate the scratch penetrates. The idea is that the acid accelerates the natural chemical weathering of the slate, so a slate that resists the acid and scratching has better resistance to weathering and will presumably have a longer service life. The maximum depth of softening for an S-1 grade is 0.002 inches.

Compared to concrete or metal, slate is a much lesser known material and very few testing labs have experience performing these tests. Applying a very nuanced test methodology to an unfamiliar material can result in erratic results. To help instill confidence in ASTM test results, the National Slate Association vets testing labs for their experience and qualifications. A list of labs recognized for their knowledge of slate, experience testing slate and attention to test methodology is posted on the NSA website (http://slateassociation.org/members/slate-testing-labs).

Beyond the properties tested in the ASTM C406 specification, there are other attributes of the slate stone itself that can make for a better slate roofing experience.

The arrangement of minerals needs to allow for uniform cleavage so that the slate can be split into thin, planar pieces free of kinks or curvature. It should be free of detrimental inclusions such as the various forms of pyrite, which can oxidize and stain the roof or even worse rust away leaving holes in the slate. The stone should also have a low carbonate content to ensure that it won’t develop a white chalky appearance. It also needs to be workable with hand tools to allow customization on the roof, if it is too hard or brittle the slate piece can shatter like glass when an installer tries to work with it.

Of course, a proven track record is often the test of longevity. Slate from the Penrhyn quarry in Wales has been documented to last 200+ years, and many Vermont and New York slates last well over 100 years as evidenced by 'dated' roofs.

In general the Vermont and New York slates and selected imported slates (Canadian and Welsh to name two) meet the S1 standard, and we recommend using these slates exclusively. The New England Slate only sells S1 slates.

Manufacturing Process

After the stone itself, the other big component of slate quality is the manufacturing process. Good quality control during manufacturing can dramatically increase the efficiency of installation and improve the overall appearance of the final product. Some of the quality control measures we employ include:

• Consistency of thickness. Slate is split by hand using a hammer and chisel, so some variation in thickness is to be expected and helps give slate it unique natural appearance. Extreme variation in thickness though can make installation difficult or create excessive gaps between courses. An experienced roofer will transition gradually from thicker to thinner slates, but we strive to sort out unacceptably thick or thin pieces to facilitate installation.
• Nail hole position. As individual slate are split, one end often ends up slightly thicker than the other. We make every effort to put the nail holes in the thinner end. When the thicker end has the nail holes the course above has to ride up over the thick end, causing it kick up and not lay properly. By putting the hail holes in the thin end, the thicker end is exposed and the next row of slate lays nicely in line with adjacent pieces.
• Knots/Knurls/Ledges. These surface features of slate (knots stick out from the surface like a lump, while knurls are depressions in the slate’s surface) come in all sizes. Smaller features are innocuous and give slate its unique natural appearance. Larger features may be acceptable when present on the tab (exposed) end where they won’t come into contact with other slates. But large knots, especially those positioned to come in contact with other slates should be culled out. The knots or ledges can act as a pressure point and cause the piece being installed on top of the feature to break. This photo shows pieces with excessive texture- pieces that would be sorted out because they’d cause problems when the next row gets put down.
• Breakage. It may sound simple, but we take great care to pull out any broken or cracked pieces as we trim, stack or put nail holes in the slate. Each piece is sounded to expose cracks and broken or weak pieces are discarded or trimmed to smaller sizes.
• Squareness. Each piece of slate should have four 90-degree corners. Hasty trimming, or worn, improperly aligned trimming equipment can result in result in out-of-square slate, making installation difficult and degrading the appearance of the roof. We closely monitor our equipment to ensure nice square corners.
• Packaging. All of our roofing slate is securely enclosed in wooden crates or banded onto heavy-duty hardwood pallets. Partial rows are braced and banded to prevent movement and damage during shipping.

Slate is a natural product and part of its appeal is the inherent variation among pieces. But by making sure our product has a reasonable range of thickness, the nail holes are in the proper end, that there is minimal breakage out of the pallet, that the slate has square corners and that it is packaged to prevent damage during shipping, we believe our slate will allow a faster, more efficient installation and a more satisfactory finished roof.

On the rare occasions that a sub-standard product makes it onto one of our pallets and shows up at a jobsite, we stand behind our product 100% and work with you to achieve the best possible outcome. The satisfaction of our customers is extremely important to us. We enjoy building long term relationships with our customers, earning their trust one job at a time with good quality slate and excellent service.