Lead Bossing Techniques: Tools, Wood-Cored Roll Method, Internal and External Angles

Summary

Bossing is one of the oldest craft skills in the building trades, and it remains the standard for high-quality leadwork. The principle is straightforward: instead of cutting lead sheet at an angle, folding it flat and patching the resulting joint with a separate piece, the leadworker uses a set of hand tools to manipulate the metal progressively into the required shape. The sheet stays in one piece; the corner, angle, or roll is formed by redistributing the metal, not by cutting it.

The result is a lead detail with no joints. Every joint is a potential leak path — no matter how well a patch or soldered joint is executed, it introduces a discontinuity in the sheet that will eventually fail under the repeated thermal expansion and contraction of the lead. A correctly bossed angle has no joints; it is the same continuous piece of lead throughout, with no welds, patches, or solder. On a roof expected to last 50–100 years, this makes a significant difference.

Bossing looks straightforward but it requires practice and a feel for the material that cannot be acquired from a written description alone. Lead work-hardens rapidly — overworking a single area without relieving the stress will crack it. The metal must be kept moving; the skilled leadworker works progressively across the surface rather than concentrating effort in one place. Knowing when and how to anneal (soften) work-hardened lead is as important as knowing how to boss in the first place. This article covers the tools, the principal techniques for internal and external angles and wood-cored rolls, the annealing process, and common errors.

Key Facts

• Bossing — forming lead without cutting; redistributes metal using hand tools rather than adding or removing material
• No-joint advantage — a bossed angle has zero joints; every patch or soldered joint is a potential leak point after 10–20 years of thermal movement
• Minimum code for bossing — Code 4 (1.80mm, 20.41 kg/m²) or Code 5 (2.24mm, 25.40 kg/m²); Code 3 (1.32mm) is too thin and will tear during working
• Heritage requirement — for Grade I, II*, and many Grade II listed buildings, bossing is expected; conservation officers and the SPAB specify bossed joints over soldered patches
• Work hardening — lead hardens rapidly as it is deformed; hardened lead is brittle and will crack if overworked; annealing restores softness
• Annealing — gentle heating with a propane torch to 150–200°C (lead melts at 327°C; do not exceed 250°C) relieves work hardening; lead is ready when a finger-tip drag leaves a bright mark
• Bossing mallet — the primary forming tool; flat-faced hardwood mallet, approximately 450g; used to spread metal outward from the corner
• Bossing stick — a smooth hardwood tool (boxwood or beech) used for finer work and to consolidate metal at the corner edge
• Setting-in stick — flat-ended hardwood tool for defining the corner line without tearing; used before final dressing
• Chase wedge — thin, hardened hardwood wedge for pushing metal into tight internal angles and behind flashings
• Lead knife — for precise trimming of the finished form if minor adjustments needed
• Wood-cored roll — a roll formed around a timber fillet, the ends closed (capped) with bossed lead; copper rivets or lead burning used to close the seam
• Roll diameter — typically 50mm, 63mm, or 75mm timber fillet (ramin or pine); match original on heritage work
• Copper rivet — 4mm diameter copper rivet used to close the seam of a wood-cored roll without the need for burning

Quick Reference Table

Detailed Guidance

Tools and Their Uses

Bossing mallet: The workhorse of bossing. A flat-faced hardwood mallet with a head approximately 60–70mm diameter and a weight of around 450g. The flat face spreads the impact energy over a larger area than a round-faced mallet, moving metal without thinning it excessively. The mallet is used with a light, fast action — many small blows rather than a few heavy ones. A heavy blow concentrates stress in one spot and risks tearing; a rapid succession of light blows moves the metal progressively and evenly.

The quality of the mallet matters. Cheap mallets with softwood heads deform after a few jobs and develop dents that mark the lead. A good bossing mallet is made from close-grained hardwood (typically lignum vitae, boxwood, or hornbeam) and will last a working lifetime. Keep the face smooth and clean; any debris bonded to the face will mark the lead and can be ground in, creating a stress point.

Bossing stick: A hand-sized smooth hardwood tool used for more detailed work close to the corner or fold line. The bossing stick is used to dress the metal into tighter angles that the mallet face cannot reach, and to consolidate the metal at the final profile. Some leadworkers use the bossing stick for the majority of bossing work on light Code 4 sheet, using the mallet only to start the movement on heavier Code 5 or 6 work.

Setting-in stick: A flat-ended tool (typically 20–30mm wide) used to define the corner line clearly before final dressing. The setting-in stick is placed along the intended fold line and struck lightly to create a clean, defined crease. This technique prevents the metal rolling indefinitely under bossing and ensures the finished angle is crisp and true.

Chase wedge: A thin tapered hardwood (or sometimes nylon) wedge used to push lead into tight corners — behind existing flashings, into mortar joints, or into the acute angle of a secret gutter. The chase wedge can also be used to open up gaps in mortar where a stepped flashing needs to be tucked in. It generates significant concentrated force and should be used carefully to avoid tearing thin lead at the fold.

Propane torch and appropriate regulator: Required for annealing. A standard roofing propane torch (pencil or medium tip) is adequate; the lead does not need to be brought to a high temperature — only to approximately 150–200°C. Never use an oxy-acetylene torch for annealing domestic-scale leadwork; the temperature is much harder to control and the risk of local melting is significant.

Soldering iron and lead burning equipment: For lead-burned seams at wood-cored roll ends, a gas-powered lead burning torch or a specialist copper-faced soldering iron is required. Lead burning (autogenous welding of lead using a lead rod filler) produces a stronger, more durable joint than traditional soft solder. It requires specific training and is not covered in detail here.

Bossing an Internal Angle

An internal angle is encountered where two lead surfaces meet at a concave junction — the bottom of a valley, the internal corner of a box gutter, or the junction between a flat roof and an upstand. The challenge is that the lead in the corner has more area than can lie flat against the two surfaces; the excess material must be redistributed without cutting.

Step 1 — Mark out: Before starting, mark the intended corner line on the underside of the lead with a scratch tool or chinagraph pencil. Include the depth of the upstand or overlap required on each face.

Step 2 — Pre-fold: At the corner, pre-fold the lead approximately 90 degrees along the marked corner line by hand. Do not crease sharply — a gentle 90-degree bend is all that is needed at this stage.

Step 3 — Start bossing: The excess metal at the internal angle will appear as a triangular surplus fold or pucker. Using the bossing mallet, begin working this surplus metal upward and outward along the fold line, away from the corner. Work on both faces alternately, keeping the movement balanced. Think of the metal as a liquid: you are guiding it to where it needs to go rather than forcing it.

Step 4 — Work progressively: As the metal moves, the pucker at the corner reduces and the lead takes on a smooth, curved form. Continue working outward and upward along both arms of the angle, keeping movement even. At no point should one area receive more than 3–4 mallet blows without moving on.

Step 5 — Check for hardening: If the lead begins to feel springy or the mallet bounces rather than sinking in, the metal is work-hardening. Anneal immediately (see annealing guidance below). Attempting to continue bossing work-hardened lead will crack it.

Step 6 — Define the corner: When the surplus metal has been redistributed and the angle is approximately in shape, use the setting-in stick to define the corner line cleanly. Work along the inside of the angle, consolidating the metal and ensuring the crease is sharp and true.

Step 7 — Final dressing: Using the bossing stick, dress the two faces of the angle flat against the substrate. Check for any high spots or ripples and work them out with light mallet or stick work. The finished internal angle should be smooth, crease-free (except at the defined corner line), and tight to the substrate.

Bossing an External Angle

An external angle is a convex junction — the corner of a dormer cheek, the hip of a lead-covered pyramid roof, or the external corner of a parapet capping. Here, the challenge is the reverse of the internal angle: the metal needs to stretch slightly at the corner rather than be redistributed. External angles are generally easier than internal angles for this reason, but careless heavy bossing can thin the metal at the apex and create a weak point.

Step 1 — Mark out and pre-fold: Mark the corner line on the face of the lead. Pre-fold to approximately 90 degrees along the line.

Step 2 — Work from the high point down: Start bossing at the top of the angle (the apex of the corner) and work progressively downward along each face. This encourages the metal to flow downward and outward rather than puckering at the corner.

Step 3 — Light, rapid action: Use lighter mallet blows than for an internal angle. The metal needs to stretch slightly, not be driven away; too heavy an action will thin the sheet at the apex.

Step 4 — Form the corner profile: As the angle develops, use the bossing stick to refine the corner profile. The standard external angle for leadwork has a slightly rounded (rather than knife-sharp) apex — this helps the metal lie evenly and is actually more weatherproof than a sharp crease.

Step 5 — Dress the faces: Dress both faces of the angle smooth with the bossing stick. Check with a straight edge that the faces are flat and that the corner line is true.

Wood-Cored Roll Method

A wood-cored roll is formed by wrapping lead sheet around a solid timber fillet (typically 50mm × 50mm or 63mm × 63mm rounded ramin or treated softwood) to create a seam between adjacent bays of lead sheet. The roll provides a weathertight joint that accommodates thermal movement while keeping the two bays firmly located relative to each other.

Forming the roll: The timber fillet is positioned at the bay edge and the lead sheet is wrapped around it. One edge of the lead (from the bay on one side) is cut cleanly to height and folded under the roll, forming the lower layer. The adjacent bay's edge is then wrapped over the top of the roll and down the other side, forming the cap. The seam between the two layers of lead on the underside of the roll is fixed with copper rivets at maximum 150mm spacing.

Copper riveting: Drill a 4mm hole through both layers of lead at the seam, insert a 4mm diameter copper rivet, and peen the rivet head on both sides using a ball-peen hammer and a steel backing tool. The rivet compresses both lead layers tightly against the timber core, pulling them together. Copper rivets must be used — steel or zinc rivets will corrode in contact with the lead (see lead fixings and tacks and lead and other metals compatibility).

Bossed end caps: At the end of each bay, the roll requires a capped end piece. This is typically bossed from a square of Code 4 or Code 5 lead by first pre-forming a rough cylinder around a short timber core, then bossing the surplus metal at the circular face progressively inward to close the end. The bossed end cap is then soldered or lead-burned to the roll seam to create a weathertight closure.

Alternative — lead-burned seam: On higher-specification work, the seam of the roll can be closed by lead burning (autogenous welding) rather than riveting. This produces a completely seamless roll but requires the use of a lead burning torch and appropriate training. Lead burning is the preferred method on heritage work where rivets may be considered anachronistic.

Annealing: Restoring Softness to Work-Hardened Lead

Lead work-hardens faster than most metals because it is at approximately 50% of its melting temperature at room temperature. This means that normal plastic deformation during bossing immediately begins to cause dislocation movement and strengthening of the crystal structure. After as few as 20–30 mallet blows on the same area, the lead can become noticeably stiffer and, if pushed further, brittle enough to crack.

Annealing reverses work hardening by heating the lead to approximately 150–200°C, which allows the crystal structure to relax and the dislocations to annihilate. The practical test for readiness to anneal is: if the mallet begins to bounce rather than sink smoothly into the metal, or if you can hear the lead squeaking (a sign of crystal fracture beginning), anneal immediately.

Annealing procedure:

1. Support the lead sheet on a clean, flat surface or leave it in position if annealing in situ.
2. Light a propane torch (pencil or medium tip) and adjust to a soft, bushy flame — not a needle flame.
3. Move the flame continuously over the area being annealed; never hold it in one place. A circular or figure-eight motion is best.
4. The annealing temperature (150–200°C) is below the visible red heat of most metals. Assess temperature by the tactile test: scrape the surface gently with your fingernail — annealed lead will leave a bright, shiny mark easily; work-hardened lead resists the scratch.
5. Alternatively, use a temperature indicator crayon (Tempilstik) marked at 150°C — place a small mark on the lead surface and heat until the mark just melts, then immediately remove the flame.
6. Allow the lead to cool for 60–90 seconds before resuming bossing. Do not quench with water — allow natural air cooling.

Lead melts at 327°C. It is possible to accidentally melt through thin Code 3 or Code 4 lead with a propane torch if the flame is held stationary. Never point the torch at a single spot for more than 2–3 seconds. If the lead surface begins to look liquid or slumps, remove the heat immediately — partial melting creates a weak zone.

Code Requirements and Common Errors

Code requirements: Code 3 (1.32mm, 14.97 kg/m²) is too thin for bossing — it tears during working. Code 4 (1.80mm, 20.41 kg/m²) is the minimum for bossing simple internal and external angles. Code 5 (2.24mm, 25.40 kg/m²) is more forgiving and is recommended for beginners and for complex profiles where multiple annealing cycles will be needed. Code 6 and above can be bossed but are physically more demanding; the heavier the code, the more force and skill required.

Common errors in bossing:

Over-working one spot: The most common cause of cracking. The rule is: never give more than 3–4 blows to one location without moving. If the metal is not moving after 3–4 blows, anneal — do not hit harder.

Not annealing in time: Waiting until the lead is fully work-hardened before annealing significantly increases the risk of cracking during the next bossing cycle. Anneal early and anneal often. There is no penalty for annealing too frequently; there is a severe penalty for annealing too late.

Too-thin code: Using Code 3 for bossed work to save cost. Code 3 will always tear at the point of maximum deformation in an internal angle. Use Code 4 as the absolute minimum.

Heavy, slow mallet action: Many beginners use their natural carpentry mallet technique — slow, heavy blows. Bossing requires the opposite: light, fast blows with the mallet held closer to the head for control. Heavy blows cause local thinning.

Ignoring substrate preparation: Bossing against a rough or uneven substrate prevents the metal from lying flat and creates local stress concentrations. Ensure the substrate is smooth and clean before bossing, particularly at corners where offcuts of timber or adhesive may have accumulated.

Pulling instead of pushing: When repositioning metal, the temptation is to grip an edge and pull. Pulling thins and tears the sheet; only push and work with tools. Move the metal by working from behind and adjacent, not by pulling the surface.

Frequently Asked Questions

How do I know if a corner needs bossing or if I can just cut and solder it?

If the work is on a listed or historic building, boss it — conservation officers and heritage clients expect bossed corners and a soldered patch on a heritage job is often considered unacceptable workmanship. For standard domestic work, a high-quality soldered corner patch is structurally adequate, but a bossed corner is the mark of skilled craft. If the roof has a long service life ahead of it, bossed corners are always preferable because they eliminate the joint.

My bossed corner keeps cracking. What am I doing wrong?

Almost certainly over-working one area without annealing. Check: are you giving the same spot more than 3–4 blows? Is the mallet bouncing rather than sinking? Has the metal started to squeak under the mallet? All three are signs of work hardening. Stop, anneal the entire region of the corner (not just the crack), allow to cool, and resume with a lighter, faster mallet action. Also check your code weight — Code 3 will crack under almost any bossing.

Can bossing be done in cold weather?

Yes, but with extra care. Below 5°C, lead is noticeably stiffer and more brittle, and work-hardens faster. Anneal more frequently — after every 10–15 blows rather than 20–30. Avoid bossing when the temperature is below 0°C if possible, as the risk of cracking is significantly elevated. Warmed tools (store the mallet inside, not on the van) help marginally.

What is lead burning and how does it differ from soldering?

Lead burning (or autogenous welding) involves melting the surfaces of two lead pieces together using a gas torch (typically oxy-propane) with a lead rod as filler. The joint becomes part of the parent metal — there is no separate solder alloy with different expansion properties. Lead burning produces stronger, more weathertight joints than soft soldering and is the preferred method for heritage and high-specification work. It requires training and practice; a poorly executed burn can melt through the substrate lead and create a hole. Soft soldering (using tin-lead alloy solder) is faster and easier but the joint quality and longevity are inferior.

How long does it take to learn to boss lead properly?

Most leadwork apprentices can produce a functional bossed internal angle after approximately 20–30 hours of practice. A clean, professional bossed corner that a conservation officer would accept takes considerably longer — typically 6–12 months of regular bossing work to develop the light, fast mallet action and the feel for when to anneal. If you are learning, practise on Code 4 offcuts before attempting heritage work or any visible application.

Regulations & Standards

• BS 6915:2001+A1:2014 — Design and construction of fully supported lead sheet roof and wall coverings; code weight and bay size requirements that apply to bossed work

• LCA Manual (Lead Contractors Association / Lead Sheet Association) — Primary technical reference for bossing techniques, tool specifications, and code requirements

• LCA Heritage Scheme — Requires demonstrated competency in bossing for heritage certification

• Building Regulations Approved Document H — Drainage and water management; relevant where bossed lead gutters and outlets are involved

• COSHH Regulations 2002 — Relevant to dust control during lead cutting operations associated with preparation for bossing

• Lead Sheet Association Technical Manual — Full detail on bossing techniques, tool specifications, and code weight requirements

• Historic England: Practical Building Conservation — Metals — Conservation context for bossing on historic buildings

• SPAB Technical Guidance: Leadwork — Traditional repair principles including bossing versus patching

• lead fixings and tacks — Copper rivets and clips for wood-cored rolls

• lead and other metals compatibility — Why copper rivets are required (not zinc or steel)

• lead on listed buildings — Heritage context where bossing is required over patching

• lead recycling and waste — Dust control and COSHH for lead cutting and working