Field Performance

On behalf of users, owners and installers, we investigated fiber-cement slate and shake products produced by several manufacturers during the 1990s. Following is a summary of some of our observations:

• Trouble with some fiber-cement products often started during unloading and stockpiling of the products. Some slate displayed cracks while other pieces curled. However, this was not the major problem.
• A short time after installation, the slate and shake products began to deteriorate. Contractors reported the products curling, developing cracks and often disintegrating completely. Deterioration was reported to progress quickly; in some instances, contractors reported any attempt to walk on a roof for repairs destroyed all tiles touched. In one case, we were engaged to investigate a 30,000-square-foot roof in such poor condition that the contractor could not find 12 undamaged tiles for laboratory testing.
• Our general observations revealed some discolored or faded products. Others curled without cracking but opened enough to allow water into buildings. In general, most fiber-cement materials quickly became unfit for roofing or became so unattractive owners were unhappy with the appearance. After we were retained to investigate failures of fiber-cement slate and shakes of different manufacturers, we realized there was an apparent common factor in their compositions. As a result, we undertook a testing program to find the reasons for the problems.
  
  

Investigating the problems

We investigated and tested 11 artificial shake and slate products manufactured by nine manufacturers. Our investigations involved hundreds of roofs in more than 20 states. Some roofing products failed before installation was completed. Few survived past their tenth year of exposure.

This chart summarizes our observations during field sampling of the 11 products investigated. The figure lists typical field observations recorded on weathered samples. A “yes” in any cell of the figure denotes a widespread condition, meaning that not just one or two or even a few shakes or slates show the condition listed, but there is an overall system failure. For example:

• Cracking of the top coating includes micro-cracking or the start of general deterioration.
• Top-surface exfoliating is frequently the next step in the deterioration process.
• Widespread through-cracking of slates or shakes allows water into a system; broken slates or shakes can act as a sliding board for workmen on the roof, and the shards can be missiles when they fall from a roof.
• Warped or cupped slates or shakes are often a prelude to cracking with or without rooftop traffic and admit more windblown rain than was intended by the system. Maintenance of a roof system and rooftop equipment is impractical if a roof cannot support foot traffic.
• Soft or “punky” top surfaces invite and encourage water penetration.
• Moss or grass growth are not necessarily detrimental to a roof system, but their presence indicates the long-term presence of water, which almost always damages roofing materials.
• Exfoliation of the layers is the principal failure mode of products made by pressing together matrix layers. The separation is caused by water intrusion and swelling of the lamina.

Water, and not just freeze-thaw action, is the fundamental source of these distresses on products. Many manufacturers attempt to avoid problems by limiting the application of fiber-cement products to geographical areas that see few freeze-thaw cycles. This was of some use, but the manufacturers failed to realize wet-dry cycles (without freeze-thaw) will similarly damage roofing materials.

Individual Products

Without naming companies, we provide discussions of individual producers and their products. The information includes statistics we obtained by performing laboratory testing during our investigations.

In our discussion of each product, we present values for a “handleability” index as follows: U = 0.5 PΔ/t where P is breaking load in pounds; Δ is the deflection at break; and t is the test specimen’s thickness.

The handleability index is a relative term that measures a material’s capability to be handled without breaking. We have found products with a high handleability index can be installed and serviced without experiencing damage.

Typical values of the handleability index for steep-slope roofing products are:

• Cedar shakes: 74 pounds
• Slate: 8 to 64 pounds depending on source and orientation to grain
• Asbestos-cement roofing products: 1.8 to 32 pounds depending on thickness

Based on slate’s successful performance as a steep-slope roofing product, we consider a roofing product should have a minimum handleability index of 8. However, other factors may affect product breakage, including size, shape and attachment method.