Producer H - Product 8

This product was manufactured by sifting a “snow” of paper fibers, Portland cement and fumed silica onto a wet moving belt, spraying water, compressing and shaping the wet snow, cutting, punching nail holes, drying and autoclaving. Final operations included washing the efflorescence off the surface and spraying on a clear acrylic coating to provide a shiny finish.

The first public report of this product’s poor performance was in 1995. We found cracked and broken slates during our many field investigations (see Photo 4). These slates were designed to be hung (see Photo 5) on incompletely driven nails (as are natural slates) in plywood decks. A high percentage of the cracks in the slates were over the nail heads (see Photo 6) of the shakes. Blotchy color variation marred some roofs.

We observed small cracks in the surfaces of products waiting to be installed. These cracks contained some of the acrylic coating applied at the factory. Therefore, these cracks are a result of the manufacturing processes and are independent of exposure to the elements. On one job, shards were falling to the ground before the work was completed.

Laboratory testing showed a significant decrease in flexural strength, deflection at break and handleability index in the samples that had been exposed to the weather when they were compared with the unexposed samples. The deflection at break was less than the thickness of the nail heads typically used to install these products, explaining the high frequency of cracks in the product over the nail heads.

Producer I - Products 9 and 10

Product 9 is a fiber-cement shake similar but not identical to many others. We believe it is cast in molds, compressed, dried and top-coated. The producer provided a 50-year warranty. Within a relatively short time, the producer introduced Product 10 and offered a 30-year warranty.

Unlike other fiber-cement shakes, the top surfaces of these are dense and relatively intact. We saw the exposed shakes cup upward on every roof (see Photo 7). Almost any that were lying flat were broken (see Photo 8). These broken shakes channeled rain into the roof system and onto the secondary water barrier (underlayment), eroding it so water eventually leaked into the building. Warped shakes were prone to break under foot traffic, making rooftop maintenance impractical because any traffic would add to the number of broken shakes.

Producer I wrote that Product 9 was composed of 80 percent Portland cement and 20 percent wood fiber. We found 30 to 37 percent ignition loss in Product 9 and 24 to 26 percent in Product 10. The higher percentage ignition losses relative to the reported wood fiber content indicates the wood fiber component absorbed.

Producer J - Product 11

Product 11 was unique in that it was prepared in layers to make up the tapered shake. The bottom layer was a 0.003- inch-thick polyethylene terephthalate membrane with random directional fibers and a plant fiber- and aggregate-filled Portland cement matrix incorporating a 0.013-inch-thick isotactic polypropylene mesh with a square ½-inch pattern topped with a dense cementitious pigmented material.

We were unable to recover a whole sample from the field—the shakes almost disintegrated as they were removed from the roof. The top surfaces were powdery and broken. These shakes, at 170-psi flexural strength, had the lowest flexural strength of the group of manufactured slates and shakes tested.

What It Means

Our conclusions about the performances of the many fiber-cement slate and shake products are based on our extensive product investigation and testing. Wood-, paper- or perlite-reinforced Portland cement slate and shake products are not suitable for use as long-term roof coverings where they are expected to get wet. The fundamental weakness such products exhibit is a lack of durability (cracking and disintegration) and dimensional instability (warping and cupping) in the presence of water.

The fundamental materials science error common to all but one of these products was the use of a moisture-sensitive and dimensionally unstable material (wood fibers) within a brittle medium (Portland cement mortar). The Portland cement mortar lacked the tensile properties to resist the swelling of the wood fibers, so the material broke down during wettingdrying cycles.

During the 10 years covered by this report, there have been many instances when even casual testing, such as a simple water spray or submersion test and product mockup testing, would have shown the dangers inherent in these formulations.

The facts remain:

• There are no test methods or programs that accurately predict roof system durability.
• Only a roof system’s historical durability in a similar environment is an appropriate performance indicator.
• Long-term warranties may be the worst indicator of durability. As a further and more important lesson, we have learned the industry must advance research capabilities extensively and recognize fallibility of inadequate testing before introducing new materials into the roofing marketplace.