Letters

Slate association takes issue with installation article

As executive director of the National Slate Association, I was deeply concerned about "The installation checklist," May issue, by Dave DeRogatis, director of composites for Ply Gem Industries Inc., Cary, N.C.

ASTM International is responsible for providing critical testing for many industries, including roofing. Roofing specifications are based on ASTM International testing of materials and, to give the testing meaning, the materials are specifically defined. Most testing concerns structural integrity and material durability with the goal of public information and safety.

ASTM C119, "Standard Terminology Related to Dimension Stone," defines slate as "a metaphorical rock … composed mostly of micas, chlorite and quartz … distinguished by slaty cleavage … which allows the latter to be split into sheets."

I mention this because DeRogatis writes "slate materials have different burning speeds." Slate was welcomed into North America as a roofing material during the late 1700s in part because it is inflammable. It had already been in use for more than 1,000 years in other parts of the world because rock does not burn. The fire tests for slate are contained in ASTM E108, "Standard Test Methods for Fire Tests of Roof Coverings," not ASTM E84, "Standard Test Method for Surface Burning Characteristics of Building Materials," as DeRogatis references.

Wind uplift for slate might be tested according to UL 1897, "Standard for Uplift Tests for Roof Covering Systems," as referenced in the article, but UL 580, "Standard for Tests for Uplift Resistance of Roof Assemblies," and ASTM C1570, "Standard Test Method for Wind Resistance of Concrete and Clay Roof Tiles (Air Permeability Method)," might also be good choices.

DeRogatis references Florida Building Codes' Testing Application Standard (TAS) No. 100, "Test Procedure for Wind and Wind Driven Rain Resistance of Discontinuous Roof Systems" and No. 107, "Test Procedure for Wind Resistance Testing of Non-Rigid, Discontinuous Roof System Assemblies." Not to split hairs, but slate is rigid. The only correct TAS test is TAS 100.

He references UL 2218, "Standard for Impact Resistance of Prepared Roof Covering Materials," for "impact resistance." Again, this is for "prepared discontinuous roof covering materials," meaning something manmade from a collection of materials and chemicals. You could argue slate was "prepared," but that was 500 million years ago by Mother Nature. The correct test method for impact resistance of slate would be FM 4473, "Specification Test Standard for Impact Resistance Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls."

Next up, the freeze-thaw test in ASTM D1037, "Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel Materials." Nope. Slate has a >0.25 percent water absorption rate. No need for a freeze-thaw test.

He could have cut to the chase and referenced ASTM C406, "Standard Specification for Roofing Slate"; ASTM C120, "Standard Test Methods for Flexure Testing of Structural and Roofing Slate"; ASTM C121, "Standard Test Method for Water Absorption of Slate"; and ASTM C217, "Standard Test Method for Weather Resistance of Slate." These specifications and test methods have been the gold standard of the slate industry for more than 55 years.

DeRogatis warns us: "What if the roof structure or building collapses under the weight? … Real slate products can weigh more than 1,000 pounds per square foot." I would love to see that slate! That would have to be, well, a stalagmite. Slate ranges from 500 to 1,400 pounds per square depending on the thickness and size of slate specified. DeRogatis goes on to say that in comparison "engineered slate: weighs only 375 pounds per square foot." I doubt that.

I would like to say DeRogatis mixed up his information (accidently, I am sure) or didn't fact check. But I read with disbelief a thoroughly disingenuous conflation of slate and "synthetic slate." The distinguishing qualities of slate were inferred to be those of synthetic slate; the shortcomings of slate are described as inapplicable to synthetic slate.

DeRogatis did a huge disservice to the roofing industry and ASTM International. The industry already has a trail of failed "lifetime" synthetic products sold as slate look-alikes with meaningless warranties. We deserve clear information. What, in fact, are the "slate materials" made of? There was no mention in the entire article. ASTM International standards are vital to the roofing industry and consumer safety and should not be misrepresented.

Anyone interested in slate installation can go to the National Slate Association's website, www.slateassociation.org, and access its design and installation manual.

Tim Underhill
National Slate Association

Professional Roofing responds: Thank you for your letter. We appreciate the feedback.