The short answer
Lead welding joins sheet lead by gently melting the edges of two pieces with a gas flame, sometimes adding a thin strip of lead, so they fuse into one. No solder is used. It is used for watertight joints on flat roofs, gutters, parapets and bay roofs where a lap or a roll would be awkward or would trap water. It needs a skilled operator.
How the joint is made
Lead melts at a far lower temperature than most metals, and that is what makes welding practical on a roof. The roofer cleans both surfaces to bright metal, brings the edges together and runs a small, controlled flame along the joint until they soften and flow. A strip of lead of the same grade is added as filler where the seam needs building up.
The result is a continuous seam in the same metal as the sheet, with the same colour and the same expansion rate. There is no second material to age at a different speed. Because the process uses a naked flame, it is controlled carefully on any roof with timber below, and the deck, the surrounding felt and the nearby boarding are protected and checked during and after the work.
Why not solder?
Solder is a different alloy from the sheet, and over time the two metals react with each other and with moisture. Soldered joints in sheet lead tend to crack as the roof moves, because the solder is harder than the lead around it and does not flex in the same way. They are also more likely to stain.
A welded joint avoids both problems. The metal around the weld moves like the rest of the sheet, so the seam does not become a stiff line in an otherwise supple covering. That is why lead welding is associated with long-lasting leadwork rather than quick repairs. For the wider reasons sheet fails, our page on why lead cracks covers them.
Where welding earns its place
The common uses are details where a standard roll, drip or lap would be awkward. On a flat roof, welded seams can join sheets in places where water collects. In a parapet or box gutter, welds let the lining be formed into corners and stop-ends without a gap. Around a lead-covered bay, a welded joint can turn a corner neatly.
- Corners and stop-ends in lead-lined gutters.
- Joins where flat roof sheets meet a wall or a rooflight upstand.
- Repairs to old lead, where a crack is cut out and a new patch is fused into place.
- Exposed lead on a church or period building, where a visible lap would look wrong.
On a plain pitched roof, welding is rarely needed, because a well-made lap and a correctly sized flashing do the job. Our page on lead flashing wedges shows the more usual way of holding a flashing in a wall.
Limits, and when it is the wrong answer
Welding needs a clean, dry surface, a still enough day and a person who has learned the skill. It does not suit old, dirty or oxidised lead without cleaning, since contamination in the joint causes weak spots and pinholes. It also does not remove the need for movement allowances. A welded sheet still expands, and a long, rigidly held weld will crack if the bays are too big.
That is why the weld sits alongside good design instead of replacing it. Short bays, correct falls and proper fixings still do most of the work, and the same question of how each metal is joined comes up in lead or zinc.
Welding is also a hot process, and it is not a job for a householder. Timbers under the deck can smoulder long after the flame goes out. Our page on DIY lead flashing sets out what an owner can sensibly attempt.
Looking after a welded roof
A welded seam looks like a fine, slightly raised line in the sheet. Over time it weathers to the same grey as the lead around it. New lead, weld included, can show a pale carbonate film in its first months, which fades as the surface matures. If you want to control it, lead patination explains the oil treatment.
After heavy weather, look from the ground or a window for ripples, lifted edges or cracks along a seam. A hairline crack caught early can often be cut out and rewelded cheaply. Left alone, water travels along the deck and shows up as a stain much further away. For the broader family of work, see lead roofing.
Questions to put to any roofer
If a quote mentions welded joints, ask who will carry out the welding and where on the roof it falls. Ask what protects the timber and felt around the flame, and who checks the area afterwards. A careful roofer will answer without fuss and will not mind being asked.
Ask, too, whether the welds are really needed. A bay that could be formed with a roll or a drip is cheaper and just as sound. Welding is justified where there is no practical alternative, such as a stop-end in a lined gutter or a corner on a small flat roof.
Finally, ask for photographs of the finished seams before the scaffold is struck, so you can check them from the ground.
How we handle it
Our survey photographs show the condition of each joint, and the written quote follows as a single total. Welding goes into the plan only where the detail needs it, not as a way of making a job sound more specialised. The 10-year workmanship guarantee is in writing, and the waste goes with us when we leave.
Because open flame is involved, we tell you in advance which days and stages the welding falls on, so you know what to expect on site and can keep the area below clear.