The short answer
Lead welding, known in the trade as lead burning, is a way of joining sheet lead by melting the two edges together with an oxygen and gas flame, usually with a thin strip of lead added as filler. The result is a single continuous piece with no seam to open. It is used on flat lead roofs, gutters and cisterns, and for repairs to splits. It takes skill, because the metal melts at a low temperature and can burn through in a second.
What lead welding actually is
Lead melts at a far lower temperature than most metals. A skilled leadworker can bring the edges of two sheets to the melting point with a small gas torch, feed a strip of lead into the molten pool and let it cool into a joint with the same strength as the sheet around it. There is no flux, no solder and no adhesive. The word "burning" comes from the old practice of using a flame to do this, though nothing is actually burned.
The joint is called a weld, though it is closer in nature to the joints made on plastic pipe than to steel welding. A well-made lead weld has a regular row of overlapping ripples, and the surface is cleaned before the flame goes anywhere near it, because lead oxide forms in moments and stops the metal from running together.
Where lead welding is used on roofs
Most roof lead is joined by folding, not by welding. Roll joints, drips and welts are the normal ways of connecting sheets on a flat lead roof or a gutter, and they let each sheet move as the weather changes. Welding is the exception. It is used where a fold would not shape the metal, such as a box gutter outlet, a corner where several sheets meet, or the end of a lead-lined parapet gutter.
It is also used for repairs. A split in a sheet of otherwise sound lead can be closed by cleaning the edges, bringing them together and welding them. That is a better repair than a patch of bitumen tape, which depends on sticking to the old surface and rarely lasts a winter. If the lead has failed along a wide area, however, a weld only closes one crack while others open nearby. Our page on why lead cracks explains why that happens.
The limits of a lead weld
Lead expands and contracts with temperature, more than many roofing metals. A long continuous welded sheet has nowhere to take up that movement, and the stress builds at the weld until something gives. That is why welds are kept short and why long runs are broken up with rolls and drips.
Welding also demands a clean, dry surface. Rain, condensation or the moisture trapped under old lead will make the flame spit, and the weld will be full of tiny holes. A roof that is damp underneath may need drying out first, or the weld can be done after a patch of timber is replaced. And because it uses an open flame next to timber boards and felt, it carries a fire risk. A responsible leadworker keeps a watch on the roof after finishing, because a smouldering board can take a while to show.
Welding compared with the other ways of joining lead
Lead can be joined in several ways, and the choice depends on where the joint sits and which way the water runs. A lap, where one sheet simply overlaps the next, relies on gravity and a generous overlap. A roll is a rounded timber batten wrapped in lead, which lets the sheets slide a little as they warm. A drip is a step in the deck that throws water onto the sheet below. A welt is a folded seam used for vertical or steeper work.
Welding differs from all of these because it is not a lap at all. It removes the joint and leaves one sheet. That is useful where water could sit, such as in a flat gutter with no fall to speak of, and where there is no room for a fold. It is also why a weld is judged on its neatness and fusion, and why a poor one can trap a hairline gap that wicks water by capillary action.
Safety and health
Lead is toxic if breathed in or swallowed. Heating it produces fume, so work is done outdoors with good ventilation, and washing hands before eating, drinking or smoking is a basic rule on any lead job. Old paint on the sheet and the surrounding timber should also be considered. You can read more in our answer on whether lead flashing is dangerous.
When we would use it
On a flat lead roof or a cast gutter with a split, a short weld may be the right repair. On a flashing that has simply lifted or perished, welding is the wrong tool and new lead is the answer. We will say which of those you have, and we show you a photo of the split before we pick up a torch. Our wider approach to lead is on the lead roofing and leadwork page.
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