The short answer
Lead welding, also called lead burning, joins two pieces of lead by melting the edges with a gas flame, usually with a thin strip of lead added as filler, so the joint becomes one piece of metal. It is used for watertight corners, outlets, box and parapet gutter details and other places where a lap or roll cannot be formed. It needs clean, dry lead and skilled hands.
What lead welding is
Most joints in leadwork are mechanical. One sheet laps over another, or two edges are folded together into a seam called a welt, or a timber roll is covered by a cap of lead. Water is kept out by the overlap and the fall, and the metal allows each piece to move.
Lead welding is different. The leadworker heats the edges of the two pieces with a small, hot gas flame until they melt and flow together. Usually a narrow strip of lead is melted in as filler. When it cools, the two sheets are fused into a single piece with no overlap. The joint is made of lead and is as waterproof as the sheet around it.
The trade names it lead burning. The word is a little misleading, since the aim is to melt and fuse, not to burn. It is a skilled job. Too little heat gives a weak joint that looks sound and splits later. Too much heat thins the sheet or burns through it. Practice, clean metal and the right preparation matter far more than the equipment.
Welding is a joining method that is used within lead roofing, not a separate service. Our lead roofing page shows how it fits with flashings, gutters, flat roofs and valleys.
Where welding is used on a roof
Welding earns its place where the shape of the lead makes a lap or a roll difficult or unreliable.
- Corners and returns. Where a gutter turns a corner or meets a wall at an angle, a welded corner gives a continuous, watertight joint without a cut in the sheet.
- Outlets and hoppers. A lead outlet welded to the gutter lining forms one piece with it, rather than a joint that relies on a seal.
- Box and parapet gutters. A box gutter is a channel between two roof slopes or beside a wall, and a parapet gutter sits behind a parapet wall. Welded joints can be used at junctions in the lining and at upstands, where the lead turns up a wall.
- Repairs. A crack or hole in sound lead can sometimes be filled by building up lead into the gap, which can be a neater result than a patch.
- Complex junctions. Around a chimney, beside a dormer cheek or at the awkward meeting of three slopes, welded details avoid bulky folds.
Where it would not be used is in long straight joints across a flat roof, where a roll or a welt suits the material better because they let the sheet move, which we explain in our answer on why lead cracks.
What welding cannot do
A welded joint is rigid. It does not give, because it is solid lead. If it is placed where the sheet needs to expand and contract, it will take strain that a lapped joint would have absorbed. That strain can crack the lead beside the weld.
That is why welding is not a way to rescue a bay that is too long, or a gutter with no drips. If the cause of a split is movement, fusing the crack closed treats the symptom and leaves the cause. A welded repair in a bay with the wrong length or fixings will usually fail again next to the weld.
Welding also depends on the condition of the metal. Old lead that is oxidised, dirty or thin can be hard to fuse. The surface must be clean, with the grey carbonate scraped back to bright metal. Where the sheet has thinned across a wide area, welding a patch onto it does not help, and renewing the bay is a better answer. Our answer on repairing split lead explains the choice.
The final limit is safety. A gas flame is an ignition source, and roofs have timber decks, old felt and dry debris. A hot work method must be used with care, with the deck protected and a watch kept for smouldering after the work is finished.
Conditions for a good welded joint
A lead weld is only as good as the day it is made.
- Dry metal and a dry deck. Moisture in or under the lead turns to steam when heated and spoils the joint. We do not weld in the rain, and we are careful about damp mornings. Our answer on leadwork in winter covers the weather constraints.
- Clean lead. The surface is shaved or scraped to bright metal over the joint, and kept clean until the weld is made.
- The right code. Heavier sheet takes a weld better. A lead code is the numbered grade of sheet thickness, from 1.32 mm at code 3 to 3.55 mm at code 8.
- A protected working area. Wind pulls the flame about and cools the lead, so shelter is part of the set-up.
- Time to cool. The area is watched after the work and checked before the roof is left.
After welding, the lead is left to weather. The bright joint dulls to the same grey as the sheet around it as the patina forms.
Is lead welding safe, and how do you judge a weld?
Working with molten lead means fumes and dust that the leadworker must manage. Lead is a toxic metal and the usual hygiene applies: do not eat or smoke while handling it, wash hands before food, keep dust down and clear up properly. For the wider question of risk, see whether roof lead is dangerous, which covers what the risk means for the people who live under it.
As a customer, you will find it hard to judge a weld from the ground. Look at the photographs. A good weld is a smooth, even bead, flush with the sheet, with no pits, no holes and no burnt-through edges. A poor one is lumpy, thin in places and sometimes cracked along its length. Our photographs of the work on your roof let you check the joints before they weather.
When a quote mentions welding, ask where it is being used and why. A roofer should be able to explain why a welded joint rather than a lap or roll suits that spot. Lead bay roofs show the sort of detail where welds are common. The cost of a full gutter relining, with or without welded details, is covered in our answer on box gutter relining cost.
Before anyone climbs up, photographs of the gutter from a window or the ground help us judge whether the existing joints are welded, lapped or simply sealed over.
