Terne-Coated Stainless Steel (TCS)

Terne-coated stainless steel, often shortened to TCS, is a roofing and flashing metal made by coating stainless steel sheet with tin. It was developed as a durable, unpainted successor to traditional terne plate, which was carbon steel or iron coated with a lead-tin alloy and had to be painted to survive.

Coating history

Early terne coatings contained mostly lead. For environmental reasons lead was later dropped in favour of a tin-zinc alloy of roughly equal parts, and that product has since been discontinued too. Current terne-coated stainless uses a coating of about 99.9% tin, applied electrolytically (by galvanic deposition) rather than by hot-dipping. Proprietary names include TCS II and Roofinox.

Appearance and weathering

Fresh material is bright; the tin surface then dulls to a soft, uniform matte grey patina outdoors. Because the base metal is stainless, the roof does not need painting. Service lives of 100 years or more without painting are commonly cited.

Base grades

Grade 439 stainless is used as the substrate for ordinary inland exposures, and 316L, which resists chlorides better, for coastal sites.

Uses

TCS is formed and soldered much like traditional terne, so it is widely used to replace old terne and tin roofs on historic buildings, as well as for standing seam, flat seam and batten seam roofs, gutters and flashings on new work. Copper is the other common choice for replacing terne roofs.

Practical notes

As with all stainless products, the thermal movement of long pans must be allowed for with sliding cleats, and contact with less noble metals such as zinc, aluminium or galvanized steel should be avoided in wet locations.