Technical Article

Facts About Stone Coated Roofing

by Committee for Fire Safe Dwellings

  • The Class "A" rating on stone-coated steel is conditional.
  • Stone coated steel roofing installed over wood shake is rated as Class "C" if specific additional fire resistive materials are not installed.
  • To achieve a Class "A" rating over wood shake roofing requires a layer of ½ -inch gypsum board. (With some manufacturers it is not possible to have a Class "A" installation over wood shakes.)
  • The installation weight for Class "A" over shakes, including steel panels, old wood shakes, wood support system, and gypsum boards, is 730 pounds per square (a square equals 100 square feet of surface area). This is heavier than composition and light weight concrete tile roofing.
  • The installation weight for Class "C" over shakes, including the steel panels, old wood shakes and wood support system, is 560 pounds per square.
  • The warranty coverage on the stone surface coating is typically only 20 to 25 years, after which the homeowner has no coverage if the stone coating erodes down to bare metal.
  • Warranty coverage is typically prorated, meaning the homeowner will most likely be paying an ever increasing amount as time passes for any necessary repairs, even those covered under the warranty.

Questions you should ask (and answers you can use):

When is a Class "A" roof not Class "A"?

Manufacturers of stone-coated steel products fail to highlight the fact that their Class "A" rating is conditional upon the type of installation. Depending on the way it is installed, it can be a Class "A", "B", or "C" system. Achieving Class "A" in reroof applications over wood shakes is, in the case of some manufacturers, not possible, and with others, requires the extra cost and considerable weight of a ½ -inch gypsum board. Stone coated steel roof products were "grandfathered" into the building code as Class "A", however, they have failed standard burning brand tests for a Class "A" rating which were conducted by a certified testing laboratory.

When is a "lightweight" roof not lightweight?

Referring to stone coated steel roof panels as "lightweight," when used in a Class "A" reroof application over wood shakes, is very misleading. The steel roof panel manufacturers imply that their product, when installed as a Class "C" assembly, over an existing wood shake or shingle roof, weighs 140 to 150 pounds per square. It is very important to recognize that when the weight of the existing roof of 350 pounds per square or more and the weight of the wood support system (batten and counterbattens) of approximately 60 pounds per square are taken into account, the actual weight of the combined components is almost four times greater than quoted by the manufacturers.

When these products are installed as Class "A" assembly, the installation is the same as for a Class "C" assembly, except a layer of ½ - inch gypsum board has to be added at an approximate weight of 170 pounds per square. This brings the combined weight of all the components to 730 pounds per square, which is well above the generally accepted maximum weight of 600 pounds per square definition of lightweight roofing. At weights of 600 or more pounds per square, most communities require an engineering evaluation of the roof structure that could lead to the necessity for structural reinforcement. It should also be noted that some steel manufacturers can not achieve a Class "A" rating on their products that are installed over wood shakes regardless of weight or construction components.

Is a stone-coated steel roof installed over a wood shake or shingle roof fire safe?

In the late 1980's and early 1990's, the fire service experienced many residential fires involving this type of reroof system which resulted in extensive fire damage and occupant fire safety concerns. In an effort to address these issues, the California Fire Chiefs' Association (CFCA) and the Steel Roof Manufacturers' Association (SRMA) conducted several fire tests (Building Standards Committee Report 1993-4, FPO/CFCA) at the Weyerhaeuser Laboratory in Longview, Washington, on June 9 and 10, 1993. These tests showed that reroof systems of this type, when installed with a fireblock material, 1- 1/2 inch thick foil faced Manville Microlite fiberglass insulation installed directly on top of he wood shakes, greatly reduced the spread of fire through the reroof system. The CFCA accepted the test result using this type of fireblocking as a reasonable and prudent method to provide a fire resistive reroof system. However, this system of installation has a maximum fire resistive rating of Class "B".