Stainless Steel Roofing

Stainless steel is iron alloyed with 10.5% or more chromium. The chromium forms a thin, self-healing passive film, so the metal does not need a zinc or paint coating to resist rust. In roofing it is used for roll-formed standing seam panels, traditional hand-seamed roofs, individual shingles, gutters and flashings, and very widely for fasteners and concealed cleats.

Grades and finishes

Different grades suit different exposures. For tin-coated (terne-coated) stainless roofing, ferritic grade 439 is used in normal environments and the molybdenum-bearing austenitic grade 316L near the coast, where chlorides are a concern. Tin-coated versions weather to a soft matte grey.

Behaviour on the roof

• Thermal movement: austenitic grades such as 304 expand about 17 x 10^-6 per degree C (about 9.6 x 10^-6 per degree F), noticeably more than carbon steel, so long panels need clips that allow sliding.
• Galvanic position: stainless is close to copper on the galvanic scale, so 300-series stainless fasteners are the usual choice with copper roofing, but stainless will accelerate corrosion of zinc, galvanized steel or aluminium in contact with it in wet conditions.

Use in restoration

Stainless is also the base metal of terne-coated stainless, the common replacement for historic terne roofs.