Flat Seam (Flat-Lock) Roofing

Flat seam, or flat-lock, roofing uses small rectangular sheets of metal, traditionally about 18 x 24 in (460 x 610 mm) in copper, with edges folded so that each pan hooks into its neighbours on all four sides. Because the joints lie flat, the roof has a smooth, tiled appearance. The method suits flat and low-slope roofs, very steep surfaces, and curved forms such as domes, barrel vaults and cupolas, and was widely used historically with tinplate, terne and copper.

How it is built

• Pan corners are clipped, then two adjacent edges are folded over and two folded under to form locks of about 3/4 in (19 mm).
• Pans lock together in both directions. Seams running down the slope are continuous; cross seams are staggered.
• Each pan is held by concealed cleats, 16 or 20 oz copper in copper roofs, engaged in the locks.
• Pans are arranged so water always flows from one pan onto the pan below.

Soldering and sealing

• On slopes of 3 in 12 or less, every seam is soldered. Edges are cleaned, fluxed and pre-tinned about 1.5 in (38 mm) wide before folding; after locking, seams are malleted down and sweated full of solder.
• From about 3 in 12 to 6 in 12, seams may be sealed with concealed sealant or butyl tape instead.
• Above 6 in 12, seams may be left dry.

Expansion

Large soldered flat seam roofs cannot move freely, so expansion battens or joints are placed at intervals of about 25 to 30 ft (7.6 to 9 m) in both directions.

Notes

Flux residues must be neutralised and rinsed after soldering. Only as many pans should be laid each day as can be soldered that day.