Lead welding or bossing: what is the difference?

Bossing is the shaping of lead by hand. The leadworker lays a flat sheet over the shape to be covered, then works it with a mallet and a set of boxwood tools, a dresser, a bossing stick and a bending stick, until the metal takes the profile of the corner beneath. Nothing is heated. Lead is soft enough to be pushed, stretched a little and folded without cracking, provided it is worked steadily and never hit hard in one place.

You see bossing wherever a flat piece has to turn a corner without a seam: the outside corner of a dormer cheek, the bend of a bay window roof, the base of a pipe or the end of a roll. A well-bossed corner has no ripples, no thin patches and no tool marks, and it lies hard against its backing. A rushed one buckles, and a buckle is the first place that water finds. It is slow, quiet work, and the slowness is what you are paying for.

Lead welding, sometimes called lead burning, joins two pieces of lead by melting their edges together with a gas flame while a strip of lead is fed into the seam as a filler. When it cools, the joint is lead all the way through. There is no solder to fail and no sealant to dry out, so a sound weld behaves like the sheet around it.

Welding needs clean metal, a steady hand and the right heat. Too little and the filler sits on top without fusing. Too much and the sheet thins, sags or burns through. It is also hot work on or beside a building, which calls for care with timber decking, felt and dry debris below the working area. That is one reason some roofers avoid it where a bossed or folded joint will do the same job, and a reason to ask how any flame work on your roof will be controlled before it begins.

Bossing suits the corners and curves. Welding suits the places where two sheets must become one and a lapped joint would let water in. A typical example is the corner where a lead-lined box gutter meets an upstand, or a joint in a lead-covered bay where the profile makes a simple overlap impossible.

The ordinary run of flashing needs neither. A straight apron, a stepped flashing along a boundary wall or a cover flashing over a chimney is cut, folded and dressed, with laps where the lengths meet. Our guide to lead flashing shows how those lengths overlap. A good roofer will tell you which joints on your roof need a weld and which do not, and why, rather than welding everything to look thorough. More welds do not mean better work. Each one is a decision about where the metal is allowed to move, and a roofer who can explain that decision for every weld on your roof has usually thought the whole job through.

Lead grows a little in the heat and contracts in the cold, and a long, rigid sheet will tear itself apart at its weakest point sooner or later. That is why leadwork is divided into short lengths with rolls, drips and laps, so that each piece can shift independently. A weld that locks two large areas together can undo that freedom and become the very point where the metal gives way.

So a weld is judged by where it sits as well as how it looks. Ask where the movement goes. If the answer is that the sheet is held in short bays, with the welds in the quiet corners and not across the length of a run, the design is sound. Our answer on why lead cracks covers what happens when it is not. Cracks in welded lead nearly always trace back to a design choice, not to the metal itself.

You do not have to know the craft to judge a lead job from the ground or from a photograph. Look for flat, even surfaces with no creases, corners that sit tight without gaps, and welds that are smooth, even beads in the same colour as the sheet. Lead dulls to a matt grey within weeks, and that is normal. Dark pitting or bubbling at a weld is not.

Check that the lead is fixed at the top and left free to move below, and that the thickness suits the job. Lead is sold in codes, and our lead codes guide explains them. Before any work begins you receive survey photographs and a quote in writing with one overall figure, and the method for each joint is described in it, so there is nothing to guess at later. If a roofer cannot say how a joint will be made, treat that as a warning. The main lead roofing and leadwork page lists the other leadwork jobs we take on.