# Soldering Sheet Metal Roofing

> Soldering joins copper, terne-coated stainless, zinc and similar roof sheets with molten tin-based alloy to make watertight seams, especially on low slopes.

- URL: https://roof.wiki/soldering-sheet-metal-roofing/
- Topic: Installation & Methods
- Also known as: roof soldering; sweating seams; tinning
- Regions: Global
- First published: 2026-11-02 · Last reviewed: 2026-11-02
- Cite as: "Soldering Sheet Metal Roofing." roof.wiki. https://roof.wiki/soldering-sheet-metal-roofing/

## Key facts

- **Solder standard (US):** ASTM B32
- **Typical alloy:** 50/50 tin-lead or lead-free equivalent
- **Pre-tinning width:** About 1.5 in (38 mm)
- **Required on flat seam copper:** Slopes 3:12 or less

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.

## Sources

1. 8.5. Flat Seam Roofing — Copper Development Association. https://www.copper.org/applications/architecture/arch_dhb/arch-details/roofing/flat_seam_roofing.php
2. 14.3. Copper Roofing (architectural specifications) — Copper Development Association. https://archive.copper.org/applications/architecture/arch_dhb/specifications/arch_specs/sheet_roofing.php
3. Section 07 61 16 Sheet Metal Roofing (VA master specification) — Whole Building Design Guide / U.S. Department of Veterans Affairs. https://www.wbdg.org/FFC/VA/VANCAS/VANCA076116.pdf