What Is Lead Welding on a Roof?

Roofers use two quite different ways of shaping and joining lead, and the words can confuse customers.

What is lead dressing?

Dressing is shaping lead by hand with a dresser, a flat wooden or plastic tool, and a mallet. The sheet is persuaded round corners, over tile profiles and against walls without heat. Joints are made by folding the edges into each other, as in a welt, or by overlapping one piece over the next. A bossed corner, worked with wooden tools until the sheet takes the shape of the angle, is another version of the same skill.

What is lead burning?

Burning, or welding, uses a hot flame to melt the lead at the join. A thin filler rod of lead is added to the molten edge, and the join is smoothed as it cools. The result is a seam of the same metal as the sheet either side, with no gap and no lap. It takes practice, steady hands and the right gear, since lead melts at a low temperature and a moment's carelessness can burn a hole straight through the sheet.

Welding has its place, and in the right spot it is the neat answer. It is used where a dressed seam would be awkward or weak.

Which joins suit welding?

Corners of a lead-lined box gutter, the inside angle of a parapet gutter, a secret gutter that must be made watertight in one piece and some roll ends are the classic examples. In those places the lead is lining a channel and a leak would run straight into the building. A welded seam gives a continuous surface with no lap for water to creep through. A corner at the foot of a box gutter is a place where standing water would pull a poor lap open, so a continuous seam earns its keep there.

It also suits repairs to old welded work and the making of lead rainwater heads and similar features off the roof. On a roof where the lead covers a bay or a dormer, a roofer might weld a corner so the joint disappears and the surface stays flat. Our page on the lead bay window roof shows where lead meets awkward geometry.

The reason most flashings are not welded is the same reason welded joints cause problems: lead moves. It grows when it is warm and shrinks when it is cold, and a joint has to cope with that every day.

Why not weld everything?

A welded seam is rigid. If a long run is welded end to end, there is nowhere for the stretch to go, and the sheet or the weld will crack in time. This is the same problem that splits over-long lengths, which we describe in our answer on why lead flashing cracks. A lapped or welted joint, by contrast, lets neighbouring pieces slide against one another and take up the movement.

So the good practice is to weld short sections where a joint is needed in the angle of a gutter, and to use laps and rolls to cut the run into pieces that can move. A flashing welded into a long straight line is a warning sign, because it may be hiding a fault waiting to appear. Welding is also unsuited to lead that has become old and contaminated, since the filler does not bond well to dirty or oxidised metal.

If a quote says lead burning or welded joints, ask what is being welded and why. There are good reasons, and an honest roofer will explain them without fuss.

Is it safe to weld next to timber?

The risk is real. A flame and timber decking are a bad mix, and the work needs fire precautions, such as shields beneath the join and a plan for checking the area afterwards. Ask how the roofer manages that, and who will do the burning, since it is a skill that takes practice and not every roofer on a site has it. A tidy, careful site is part of the job, and you are told each day what is happening on the roof.

Also ask whether a mechanical joint, such as a welt or a lap, would do the same job. Often it will. Where the lead is a lining in a box gutter, welded corners are usual. Where it is a flashing along a wall, they usually are not. See types of lead flashing for each position, and lead work for the whole service. Our survey comes first, with photographs of the lead you have now, and a single written total follows it.