Question answered
What is lead welding?
The short answer
Lead welding, also called lead burning, joins two pieces of lead by melting their edges and a little lead filler with a gas flame, so they fuse into one piece with no overlap or fixing in the joint. It is used on flat roofs, gutters and some detailed corners. The alternative is dressing: folding, rolling and overlapping the sheet so that water sheds. Each suits different details.
Two ways of joining lead on a roof look similar from the ground and behave quite differently. One fuses the metal, the other folds and overlaps it.
How a welded joint is made
Lead melts at a low temperature for a metal, which is what makes burning possible. The fitter cleans the two edges to bright metal, brings them together and runs a small flame along the joint while feeding in a thin rod of lead. The two sheets and the filler flow together and set as a single piece. The filler rod is lead too, so the finished seam is the same metal throughout.
Done well, the seam is flat, continuous and as thick as the lead around it. There is no gap for water to creep through and nothing in the joint that depends on a fixing or a seal.
It is a skill that takes practice. Too much heat burns through the sheet, and too little leaves a joint that looks sound and is not. A naked flame beside roof timbers is also a fire risk, so the work is planned, the area is checked and nobody leaves the roof straight after.
Dressed joints: laps, rolls and drips
Most lead on a pitched house is not welded at all. It is dressed, which means shaped by hand so the sheets overlap in the direction of the water flow. On a flat or gently sloping roof, joints run across the fall as drips, which are small steps, and along the fall as rolls, where the lead is wrapped over a timber core.
These details let the lead move. Lead expands and contracts with the weather, and a dressed joint can slide a little. It has been the traditional answer for generations, and on many listed buildings it is the one that the conservation officer will expect to see. Our page on lead on listed buildings covers that side.
Where the lead is laid in short bays between drips and rolls, there is simply no need to weld.
Where a weld earns its place
Welding is chosen where an overlap would be awkward or weak. The corner of a gutter lining, the junction where a box gutter meets an outlet, the edge of a lead-lined cistern or a hip on a small flat roof can all be easier to seal with a fused seam than with a lap.
A weld also avoids bulky laps where space is short, such as inside a narrow gutter, and it helps where the geometry is tight and the water has nowhere else to go. A lap that is only a few centimetres deep can be driven open by wind; a fused seam has no such weak point.
Welded seams show darker or brighter than the lead around them at first, and they weather to match. The early look is similar to the powdery stage any fresh lead goes through, and it evens out. Outlets, upstands and hips are typical places for a weld.
The risks of welding in the wrong place
A long fused seam stops the lead moving. If it runs the length of a large sheet without a break, the stress of expansion builds up and eventually the lead can crack beside the weld. That is the reason welds are made in controlled short lengths, not used as a cure for any joint.
Welding also adds heat to a roof structure. On old timber, dry thatch nearby or a building with little ventilation, dressed joints may be the safer choice. A good fitter will say so before lighting the torch.
Poor welds are not always visible from a distance. A pinhole, a thinned patch or a cold joint can pass for years and then let water through in a hard rain, which is why we photograph the seams as well as the surrounding lead on a survey. Ask who will do the welding, too. It is a separate skill from general roofing, and a fitter who burns lead regularly will be glad to show photographs of past seams. Our lead roofing pages explain the movement behind it.
Welded or dressed: how we decide
We choose the joint that suits the detail, the building and the fall, not the one that is quicker. For a protected building the working rule is to match what is already there. For a modern flat roof the question is how long the runs are and where the movement will go.
If a quote mentions welding, ask where it will be used and why. A good answer names the details, such as a gutter corner, and says what the dressed alternative would be. Our page on fitting lead flashing yourself explains why this is not a home project, and what lead wedges are covers the fixing used on dressed upstands.
You receive photographs of the seams and laps with a written quote that shows one total price.
Lead burning: two more questions
Is a welded lead roof stronger than one with dressed joints?
Not as a rule. A fused seam is watertight in itself, but it cannot move. Dressed laps, rolls and drips let the sheet expand and contract, so each approach is stronger in a different way, and which one wins depends on the detail in question.
Does a listed building have to keep dressed lead joints?
Often the expectation is to match what is there, and according to Adur & Worthing Councils, work that does not use matching materials can need listed building consent. Check with the council's conservation team before any change of method.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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