Lead Welding vs Bossing: What Is the Difference?

Bossing is the traditional skill of persuading flat lead sheet into a three-dimensional shape without cutting or joining it. The roofer works the lead cold, tapping and stretching it over a corner with a bossing mallet, a dresser and a bossing stick, which are wooden tools softer than the metal. The sheet thins slightly where it is pushed and thickens where it is gathered, and a skilled hand turns a flat piece into a neat internal or external corner, a dormer cheek detail or a rounded roll.

The big advantage is that there is no seam. A bossed corner is one continuous piece of lead, so water has no joint to find. It also moves freely with the sun and frost, because nothing is fixed rigidly across the corner. The trade-off is time: bossing needs patience, a good eye and enough sheet, and a roofer who is rushing will tear or thin the lead.

Corners on aprons, flashings and dormers are commonly formed by bossing, which is why it sits at the centre of leadwork. Our lead apron flashing answer shows a common example.

Lead welding, which the trade also calls lead burning, joins two edges by melting them together. The roofer holds a small gas flame close to the joint, and feeds in a thin lead rod as filler, so the two sheets fuse into one with a raised seam along the line. Done properly, the weld is as watertight as the sheet either side of it.

It is used where bossing would stretch the lead too far or where a join cannot be avoided. Examples include a tight corner on a lead-lined gutter, a junction on a flat roof or an awkward outlet where two sheets have to meet. It takes training to do well. Too little heat and the joint does not fuse; too much and the lead sags or burns through.

Welding also means naked flame on a roof, close to timber, felt and insulation. A careful roofer treats that as hot work, keeps water and an extinguisher to hand, and checks the area afterwards for smouldering. If a roofer proposes welding, ask how they will protect the roof structure and what they do after the last joint is made.

A good rule is to boss where you can and weld where you must. A bossed corner has no joint to fail, so it is preferred on dormer cheeks, bay window roofs, chimney corners and anywhere the lead has to wrap round a shape. A small dormer, with a cheek on each side and a front apron, can hold many corners between its roof, its walls and its window, and each corner forces the roofer to choose a method.

Welding earns its place where sheets must be joined into larger lead-lined areas, such as a flat roof or a deep gutter. A welded seam can be formed without the overlap that a rolled joint needs, which helps where the space is tight. For the longer, simpler runs, a roofer would normally use rolls, drips and lapped joints, which allow the lead to move. Lead drips and rolls explains that approach.

Most domestic leadwork needs neither at scale. Step flashing, cover flashing and soakers are cut and dressed, with a little bossing at the corners. If a quote mentions welding for a simple chimney or wall flashing, it is fair to ask why.

You do not need to judge technique, but a few questions tell you a lot. Ask which details will be bossed and which welded, and why. Ask which lead code is going on each part, because thin sheet bossed hard into a corner can crack, and our guide to lead sheet codes shows why the code matters. Ask whether there are any joints in the corners and where they sit.

It also helps to ask what is done to protect the roof from heat, if welding is planned, and how the finished work will be photographed so you can see the corners that you will never reach from the ground. Corners are where leadwork usually shows its quality, so photos of them are worth having, taken close up and in daylight, before the next layer covers them.

On our own surveys, we look at the detail first, then say plainly what each part needs. You see photos of the existing lead, and you get one written total price, with the forming method for each corner named. For the wider picture on this trade, see our lead work page.