Use this 3 part SimpleSpecs™ master specification to specify custom-fabricated sheet metal roofing and roofing underlayment.
Custom sheet metal roofing is fabricated from various metals including galvanized steel, pre-coated galvanized steel, aluminum-zinc alloy coated steel, pre-coated aluminum-zinc alloy coated steel, stainless steel, copper, copper-clad stainless steel, and aluminum. Other metals are also available and can be added to this section.
Roofing is installed over solid sheathing and an underlayment rather than spaced sheathing or framing. It is nearly impossible to make sheet metal roofing absolutely watertight due to its high coefficient of thermal expansion.
The advantage of rubberized asphalt underlayment is its ability to self-seal around fastener penetrations and at seams as temperatures rise. Rubberized asphalt flashings used under sheet metal is subjected to extremely high temperatures and can reach flowable consistency. A high-temperature version is available from many manufacturers.
A slip sheet is required under sheet metal to prevent adhesion and allow the metal to expand and contract.
Roofing is susceptible to waviness in the metal as the metal expands and contracts, referred to as oil canning. Oil canning is primarily an aesthetic issue and is worse on wide, flat surfaces. Placing intermediate ribs between the primary ribs minimizes oil canning, as does detailing the roofing to allow for expansion and contraction. Roofing should be allowed to move. It should not be fastened in a manner that prevents movement. Increasing the gage of the metal has little effect on oil canning.
Refer to the following for assistance in selecting and detailing sheet metal roofing:
– Architectural Sheet Metal Manual by the Sheet Metal and Air Conditioning Contractors National Association (SMACNA), available line at smacna.org,
– Technical resources by the Copper Development Association (CDA), available at copper.org,
– Roofing Manual: Metal Panel and SFP Roof Systems by the National Roofing Contractors Association (NRCA), available at nrca.net.
Related SimpleSpecs™ master specification: