Soldering Sheet Metal Roofing

Soldered seams are used where a metal roof must be fully watertight even when water stands on it, as on flat seam roofs at slopes of 3 in 12 or less, built-in gutters, valleys and complex flashings. The process melts a tin-based alloy into a tightly folded joint so that it flows by capillary action and seals the seam.

Materials

• Solder: North American specifications cite ASTM B32, typically 50/50 tin-lead or a lead-free alloy of similar or greater strength. Alloy and flux must suit the metals being joined.
• Flux: cleans the metal and helps solder wet the surface. Acid fluxes are common for copper.
• Heat: traditional soldering coppers ("irons") are heated in a furnace or by torch. One copper specification calls for furnace-heated coppers of at least 5 lb (2.3 kg) per pair, or torch-heated heads of at least 3 lb (1.4 kg).

Process for flat seam copper

1. Clean and wire-brush the edges, apply flux, and pre-tin them about 1.5 in (38 mm) wide before folding.
2. Lock the seams and dress them flat with a mallet.
3. Sweat solder fully into the seams.
4. Neutralise flux residues with a baking soda or washing soda solution and rinse thoroughly.

Only as many pans as can be soldered the same day should be installed, and tinned edges should be protected overnight.

Limits

• Soldered roofs cannot move at their joints, so large areas need expansion joints, often every 25 to 30 ft (7.6 to 9 m).
• Copper heavier than 20 oz is usually lapped and riveted as well as soldered.
• Lead sheet is joined by lead welding (lead burning) rather than tin solder.