What Lead Flashing Does On A House

Key takeaways

• Lead is used because it can be dressed into an irregular shape and absorbs movement without cracking.
• Its thickness is specified by a code number, and using too thin a code causes early splitting.
• It must be fitted in short lengths with laps, or thermal movement will tear it.
• The top edge belongs in a raked out mortar joint, wedged and pointed, not stuck to the wall face.
• The lead itself rarely fails. The fixing, the pointing and the workmanship do.

Why lead and not something else

A junction between a sloping roof and a vertical wall is an awkward shape that no sheet material can simply be laid across. It has to be formed, and lead is uniquely suited to being formed: it is soft enough to be dressed by hand into the exact profile of a brick course or a tile, it does not spring back, and it holds the shape it is given. It is also extremely durable outdoors, resisting corrosion for many decades, and it accommodates the substantial expansion and contraction that any exposed metal undergoes without fatiguing, provided it is installed in a way that allows that movement.

Where it appears on a house

Seven places, mostly. An apron across the downhill face of a chimney. Step flashings following the brick courses up the sides of a stack, usually with soakers interleaved between the tiles beneath them. Behind the stack, a channel collecting whatever runs down from the roof above it. A cover flashing where a lower roof abuts a wall. A valley lining where two slopes meet. Around a soil vent pipe or a roof window. And on parapet gutters and small flat areas such as bay roofs. If a house has a leak at a junction, one of those is almost always the answer.

Codes, which is how thickness is specified

Lead sheet is supplied in codes, a numbering system where a higher number means a thicker sheet, and each code is traditionally identified by a colour. Domestic flashing work generally uses code four or code five, with heavier applications using thicker material. The code matters because thin lead fatigues and splits far sooner, particularly along fold lines and around fixings, so using a lighter code because it is cheaper and easier to work produces a flashing that fails early. It is one of the few roofing specifications reducible to a single number, which makes it a useful thing to ask about.

Movement, and why pieces are kept short

The property that makes lead so useful, its softness, also means it responds strongly to temperature. A sheet fixed rigidly at both ends and heated by the sun has to go somewhere, and if it cannot slide it deforms instead. The design response is to work in modest lengths with a lap between each, so that every piece can travel a little and the laps absorb the difference. Published guidance sets maximum lengths for each application for exactly this reason. It is one of the few aspects of leadwork reducible to a rule, and it is the rule most often broken by people fitting lead who do not usually work with it.

How it is attached to masonry

Lead is not fastened to a wall in the way most materials are. A slot is cut into a horizontal mortar joint, the sheet is folded and turned into it, and small folded pieces of lead are driven in alongside to grip it mechanically before the joint is closed with mortar. The result is that the wall itself is holding the metal, which is what allows the flashing to survive decades of movement without fixings tearing through it. It is worth understanding because it explains why lead cannot simply be glued, screwed or nailed to a wall face: the attachment method is part of what makes the material work.

Why it is worked by hand rather than manufactured to fit

Almost every piece of lead on a house is cut and formed on site, because no two junctions are identical. A chimney sits at a particular angle to a particular pitch, in a particular bond of brickwork, with courses at whatever height they happen to be. The metal is therefore marked out, cut, and dressed with wooden tools to take the exact profile of the tiles and the steps of the brickwork. That is why leadwork is a distinct skill rather than an assembly task, why two roofers can produce visibly different results from identical material, and why the neatness of the folds is a reasonable proxy for the care taken over everything hidden beneath them.

Cement fillets, which are a substitute rather than a detail

On many older properties the leadwork at a chimney has been replaced at some point with a wedge of sand and cement, called a fillet. It is quick, cheap and requires no leadworking skill, and it fails for a reason that never goes away: cement is rigid and everything around it moves, so it cracks. A cracked fillet performs worse than an open joint would, since the fissure gathers water and feeds it inward by capillary action instead of letting it run away. A cracked cement fillet at a stack is among the most reliable predictors of interior damp in older housing.

The white deposit, which is normal

New lead develops a pale grey or white powdery surface as it oxidises, and this is a protective layer rather than a fault. The streaking it can leave on masonry underneath is untidy rather than harmful, and applying patination oil to new work largely prevents it. What matters is not the surface appearance but whether the top edge is still captured in the wall, whether there are splits or tears particularly along creases, and whether any section has lifted enough to leave a visible gap.

What good work looks like from the ground

You can assess a fair amount with a zoom lens. At a chimney you want to see metal following the brick courses in steps up each side, an apron across the front face dressed down over the tiles, and no visible mortar wedge doing that job instead. The top edges should disappear into mortar lines rather than sitting on the brick face. There should be no long continuous strip running the full length of a wall abutment without any visible joint. And there should be no beads of mastic, no patches, and nothing that looks like it has been added later to stop something.

How long it should last

Correctly specified and correctly installed, lead outlasts most of what surrounds it. Sheet lead on buildings has demonstrated service lives measured in centuries, and on a domestic property the flashing will generally still be sound when the covering around it has been replaced. That durability is precisely why the shorter lived elements matter so much: the mortar holding its edge, the workmanship of the folds and laps, and the code chosen. Lead almost never reaches the end of its own life on a house. It is retired early by something done to it, which is a useful thing to know when somebody proposes replacing it.

The alternatives, and where they sit

Proprietary flashing products exist, including self adhesive tapes and flexible composite materials, and they have legitimate uses, particularly where lead would be difficult to work or where theft is a genuine concern on a substantial installation. They generally have shorter expected lives than correctly installed lead and their performance depends heavily on surface preparation. On a domestic chimney or abutment, lead remains the standard answer for good reason, and a proposal to use something else is worth asking about rather than assuming it is either a saving or a compromise.

Why lead work costs what it does

Leadwork is a distinct skill within roofing, and pricing reflects that. The material has a real cost that moves with the metals market, the correct code is heavier and dearer than a thin one, and the labour is slow because each piece is cut, dressed and fitted individually to a shape that exists only on that building. Cutting a chase into masonry, wedging and pointing adds more time again. A price that treats a chimney flashing as a quick job is either using a substitute detail, using material that is too thin, or has not understood the work, and comparing two quotations for the same chimney stack usually makes that difference visible immediately, since one of them will have allowed for the work and the other will not have. That is usually the whole of the difference between them.

Looking after it

Lead requires almost nothing, which is part of its appeal, and the small amount it does need is worth knowing. The pointing above the top edge is mortar and has the same service life as any other mortar, so it will eventually need renewing even where the lead itself is perfect. Debris collecting against an apron or in a back gutter holds water against the metal and against the tiles, so anything that accumulates there should be cleared when gutters are done. And any repair carried out nearby is a moment to check that the flashing has not been disturbed, since a great deal of lead damage is caused by other trades working around it rather than by weather.

Frequently Asked Questions

What is lead flashing for?

Closing the joint wherever a roof meets something vertical: chimneys, wall abutments, dormers, roof windows and soil pipes, plus valley linings and small flat areas. Lead is used because it can be dressed by hand into an irregular shape, holds that shape, and tolerates large thermal movement without cracking.

What do lead codes mean?

They specify thickness, with a higher number meaning a thicker sheet. Domestic flashing work generally uses code four or five. It matters because thinner lead fatigues and splits earlier, particularly along fold lines and around fixings, so a lighter code chosen for cost produces a flashing that fails early.

Why does lead flashing split?

Almost always because it was fitted in one long continuous piece. Temperature makes the metal travel. Broken into modest pieces with laps between them, each can move on its own. Formed as one unbroken strip it has nowhere to go, so it deforms, hardens where it keeps folding, and eventually tears.

How should the top edge be fixed?

A slot is cut into a mortar joint, the sheet is folded into it and gripped by small folded lead pieces driven in alongside, then the joint is closed with mortar. The masonry itself is what holds the metal, which is why lead cannot simply be glued or screwed to a wall face.

What is a cement fillet and is it acceptable?

A wedge of sand and cement used at a chimney instead of lead. It is a substitute rather than a detail, and it cracks because cement is rigid while everything around it moves. Once cracked it channels water into the joint rather than shedding it, which makes it worse than no fillet at all.

Is the white powder on my lead a problem?

No. It is oxidation and it forms a protective skin on the surface. On new work an applied patination oil reduces the streaking that can run onto masonry beneath while that skin develops. It has no bearing on how long the lead will last.

Can I just seal a lifting flashing with mastic?

As a short term measure before a proper repair, possibly. Not as the repair. Adhesives struggle on porous brick, break down under sunlight, and hide the problem while water keeps travelling behind them. Where you find mastic on a flashing, you are usually looking at somewhere that has been patched more than once already.

Are there alternatives to lead?

Proprietary tapes and flexible composites exist and have legitimate uses, particularly where lead is difficult to work or theft is a real concern on a large installation. They generally have shorter expected lives and depend heavily on surface preparation. On domestic chimneys and abutments lead remains the standard for good reason.