The short answer
Lead welding, or lead burning, fuses the edges of two lead sheets with a gas flame, often adding a thin strip of lead as filler, so the joint becomes continuous metal. It is used for repairs, for awkward junctions on flat roofs and for tight details where a folded seam will not fit. It takes training, because lead melts quickly.
How the technique works
Most leadwork joins sheets mechanically, by folding rather than fusing. Edges are folded over one another in a roll, a drip or a welt, and the fold sheds water. Welding does something different. The roofer heats the edges until the surface melts, feeds in a rod or strip of lead as filler where needed, and lets the metal run together and cool. What remains is a seam of solid lead, with no overlap for water to creep through.
The flame is usually a gas torch with a fine tip, and the roofer controls the heat by distance and speed. Lead has a low melting point and conducts heat quickly, so the margin between a clean fuse and a burnt-through hole is small. The surfaces also have to be scraped bright first, since the dull oxide skin will not fuse. This is why the work calls for training and practice, and why a general builder with a torch is not a lead welder.
The same method is the reason for the older name, burning, which refers to the flame rather than anything being scorched.
Preparing the joint
Good welds are made long before the flame is lit. The two edges are scraped back to bright metal, brought together with a small, even gap, and held so that nothing shifts while the lead is molten. Dirt, grease or damp in the gap will spit or leave a pin-prick through the seam, which is the very fault the weld was meant to cure. Timber below is shielded from the heat, and a bucket of water or an extinguisher stays within reach.
Where welded joints are used
Welding earns its place in a few situations. The first is repair: a split in an otherwise sound sheet can be closed by fusing it, which keeps the original metal in place. Our answer on patching a lead flat roof explains how that choice fits into a larger repair decision.
The second is a junction where folds will not work. A corner where a box gutter turns, or where lead meets an outlet or a pipe, may need a joint that follows an awkward shape. Welding lets the corner be formed as one piece of metal, with no fold to leak. The third is a lining that has to resist standing water, where the joints are working hard.
Where it is not used
On roofs with a steep pitch, rolls and welts are the traditional answer, and flashings against walls are normally held by mechanical fixings and wedges rather than welded. Welding is a tool for particular joints, not a general replacement for the folded details that give lead its movement. A roofer who reaches for the torch on every seam is usually working around a weak layout.
Movement, thickness and why a weld can crack
Lead swells in summer warmth and shrinks again in frost. A welded joint is stiff, since it has no fold to take up the movement. If a long welded seam is made across a sheet that wants to move, the stress finds the weld, and cracks can form later. That is why what the roof is built like counts for as much as the welder's skill: bays are kept to workable lengths, and welds sit where movement is small.
Thickness also decides how well lead takes a weld. Heavier codes are more forgiving than lighter ones. Leadworx lists Code 4 at 1.80 mm and Code 5 at 2.24 mm, with Code 4 the usual choice for general flashing. Very thin old lead can be hard-worked and brittle, and may not take a fresh weld without thinning or burning through.
A weld is only as good as the metal and the roof it sits on. Where the sheet around a repair is tired, the survey records it, because a weld that holds for a season and cracks next to the repair has not done its job.
Welding against fixing mechanically
Both methods have a place, and a sensible roofer will use each where it suits. The comparison below shows the broad differences.
| Joint | Strength | Allows movement | Typical use |
|---|---|---|---|
| Welded seam | Continuous metal | Little | Repairs, corners, linings |
| Roll or welt | Folded overlap | Good | Bays, steep joints |
| Wedged flashing | Held in a joint | Moderate | Abutments, chimneys |
The flashing row connects to another of our answers, on how lead flashing is held in place. If you are wondering whether the work could be done yourself, the honest answer for welding is no: see whether you can replace lead flashing yourself for what owners can sensibly handle. A hot flame near dry timber and old roof insulation is also a fire risk, which is another reason it is left to trained hands with the right precautions on site.
What to expect from us
The first visit covers the metal, the fall and the deck, and ends with a view on whether a weld, a patch or a fresh piece of sheet makes sense. Where welding is proposed, its cost sits on a line of its own in the itemised quote, next to the alternative, so the two can be compared.
Heat is treated with respect on every roof: combustibles are cleared away first, and the roof void is checked once the flame is out. Rubbish leaves with us at the end, and a written guarantee of 10 years covers the workmanship. The range of leadwork we carry out is set out on the lead roofing page, and you can phone 01453 933613, send a WhatsApp photograph or use the enquiry form.