Lead flashing and leadwork
Lead is the material that makes a roof work where it stops being a simple slope. It has been used for centuries, it lasts for decades, and almost every failure we see is a detailing mistake rather than the lead wearing out.
This is a reference page explaining how leadwork is supposed to be built, so you can tell whether what is on your roof was done properly. If you have a leak at a chimney and want it fixed, go straight to chimney flashing repair.
Why lead
Roofs leak at junctions, not in the middle. Wherever a roof meets a chimney, a wall, a valley, a parapet or a dormer, something has to bridge two materials that expand, contract and move at different rates, while staying watertight for decades in full weather.
Lead does that because it is:
- Malleable — it can be dressed by hand to the exact shape of a tile profile or a brick course, without heat or fixings through the face
- Extremely durable — it forms a stable protective patina and does not corrode away
- Able to move — it accommodates thermal expansion, provided the pieces are sized correctly
- Repairable and recyclable — old lead has scrap value and new lead can be dressed into existing work
The parts, and what each one does
Soakers
Individual L-shaped pieces of lead interleaved with each course of tiles or slates where a roof meets a wall or a chimney. Each soaker sits under one slate and turns up against the masonry. They are invisible once the roof is complete — you only ever see the cover flashing over them.
Soakers are the part most often left out on a cheap job, because nobody can see whether they are there. A step flashing dressed straight over the tiles without soakers beneath will let water track behind it.
Apron flashing
A single piece running along the up-slope face of a chimney, throwing water out onto the roof covering and away from the base of the stack.
Step flashing
The stepped pieces running up each side of a chimney or along a sloping abutment, cut to follow the stepped line of the mortar courses and tucked into them.
Back gutter
The piece behind a chimney, on the down-slope side, where water coming down the roof has to be collected and led around the stack. It is the hardest part of a chimney flashing to form correctly and the most common single source of chimney leaks. A stack with no proper back gutter — just a cement fillet, or nothing — will leak.
Cover flashing
The visible outer piece, dressed into a chase cut into the brickwork, held with lead wedges and pointed in. It covers the tops of the soakers and steps.
Valleys, parapet gutters and dormers
The same principles apply wherever water is concentrated: lead lining, correct falls, laps in the direction of flow, and sections sized so thermal movement is accommodated. See flat roof repair for parapet and box gutters.
Lead codes: why thickness matters
Lead sheet is specified by code number — a scale where a higher number means thicker lead. Codes 3 to 6 cover most domestic roofing, and different applications need different codes: soakers can use thinner lead than a valley or a parapet gutter, which take far more water and far more foot traffic.
Using lead that is too thin for the job is a false economy, because it fatigues and splits sooner. If you are getting quotes for leadwork, asking which code is specified for which element is a fair question and a good indicator of whether you are dealing with someone who knows the trade.
The single most important rule in leadwork: keep the pieces short enough. Lead expands and contracts significantly with temperature. If a piece is made too long, it cannot move at the ends, so it works against itself in the middle and eventually splits along a fatigue line. Correctly sized lead, with proper laps between pieces, moves freely and lasts. Nearly every split lead flashing we replace failed because someone made it in one long run instead of several.
How leadwork actually fails
- Thermal fatigue splits from oversized pieces — a straight crack across the sheet
- Slipped cover flashing where the pointing holding it in the chase has eroded
- Missing soakers, so water runs behind the visible flashing
- No back gutter behind a chimney
- Cement fillets used instead of lead — the most common bad substitute, and it always cracks
- Lead dressed over tiles rather than interleaved
- Chase cut too shallow, so the flashing was never properly held
- Storm lift where fixings were inadequate
- Theft — lead has scrap value and is occasionally stripped from accessible roofs
The cement fillet problem
By some distance the most common thing we correct. Instead of forming lead soakers and a cover flashing, someone has run a triangular band of sand-and-cement mortar around the base of the chimney or along the abutment.
It is quicker, it is cheaper, and it fails without exception. Brick and roof covering move independently; a rigid mortar fillet spanning between them cannot accommodate that movement, so it cracks. Water then enters through the crack and sits behind the fillet, where it cannot dry.
Patching a cracked fillet with more mortar or with mastic buys a season. The durable fix is to remove it and form the junction properly in lead.
Patina, staining and what is normal
New lead is bright and develops a grey-white patina over its first months. That is normal and protective. What is not ideal is white carbonate run-off staining the brickwork or tiles below during that period — which is why new lead is usually treated with a patination oil on installation. If your new leadwork was not patinated, expect some staining; it is cosmetic rather than a failure.
Can lead be repaired, or must it be replaced?
Lead is repairable — splits can be lead-welded and sections can be replaced without renewing everything. Whether that is the right answer depends on why it failed. If a split is the result of a piece being too long, welding it simply moves the failure: the underlying dimension is still wrong, and it will crack again. In that case the section needs remaking to the correct size. Where a piece has been damaged by an isolated event, a repair is entirely reasonable.
Leak at a chimney, valley or abutment?
Send a photograph of the junction. Whether there is lead there, and what shape it is in, is usually visible.
Leadwork questions
How long does lead flashing last?
Correctly sized and installed lead commonly lasts several decades — often outlasting the roof covering around it. When flashing fails early it is almost always because pieces were made too long, the chase was too shallow, or soakers were omitted, rather than the material being at fault.
Is there a cheaper alternative to lead?
There are proprietary flashing alternatives, and some are legitimate in the right application. But the common "cheaper alternative" in practice is a cement fillet, which is not an alternative at all — it is a detail that fails. If cost is the constraint, it is better to do a smaller area properly in lead than a larger area in mortar.
Can I just seal the flashing with mastic?
As a stopgap to slow water entry before a proper repair, occasionally. As a fix, no. Mastic sits on the surface, degrades in UV, and fails invisibly — so the leak returns with no visible change outside, and the next person to diagnose it has to work around your repair.
Why is there white staining on the bricks below my new lead?
Lead carbonate washing off new lead before it has fully patinated. It is cosmetic, it weathers off in time, and it is prevented by applying patination oil at installation. It is not a sign the leadwork is failing.