The joint between a chimney and a roof is the hardest one on the house to keep dry. Two different materials meet there, one rigid and one overlapping, at an awkward angle, and the water that runs down the slope has to be steered around the brickwork instead of into it. The job is done by flashing, strips of lead shaped over the joint so rain runs across the surface and never finds the gap behind it. Our chimney repairs hub puts it alongside the other stack work.
When that lead fails, the signs show indoors, not on the roof. A brown mark on the breast, a wet patch beside the stack in the loft or a damp corner below can all trace back to a lifted or split piece of lead. Because the leak is rarely where the stain is, a flashing fault is easily mistaken for something else, and patching the wrong place only delays the cure.
This guide names each part of a chimney flashing, sets out the lead codes and what they mean, explains how flashings leak, and walks through a proper replacement. It also gives the typical UK costs published by third-party guides, so that a written quote has something to be read against.
The parts of a chimney flashing
A chimney that rises through a sloping roof has four sides, and each side meets the roof in a different way. The flashing is built in sections to suit.
On the downhill side, the face of the chimney that looks down the slope, the lead is an apron. It lies flat on the tiles or slates below the stack, and its upper edge turns up the brickwork. Along the two sides the lead is cut in steps, one for each course of brick it climbs, which is why it is called step flashing. On the uphill side, where water runs down against the stack from behind, the roof needs a back gutter. This is a lead-lined channel that collects the water and carries it round both sides. The lead on that side is made with a shaped trough and a fold, and our page on the chimney back gutter shows how it works.
Soakers and the cover flashing
On a tiled roof, soakers are slipped under each course of tiles at the side of the stack and turned up against the brick. A soaker is a folded lead strip that lies under the tile and rises against the wall. Over the top of the soakers and the turned-up lead goes a second layer, the cover flashing. Its top edge sits in a slot cut into the mortar joint, called a chase, and is wedged tight with lead wedges and then pointed over. Its lower edge hangs down loosely over the soaker, so water cannot be driven behind it.
That arrangement is how a flashing copes with the fact that a roof and a chimney move at different rates. Because the lead hangs free below and is wedged only above, it can shift a little without opening a gap. One stuck down hard with sealant loses that freedom and tends to split.
Some older houses have a cement fillet, a sloping bead of mortar, in place of lead. It is cheaper to make and it cracks, so where the stack allows we replace failed fillets with lead.
Lead codes and what they mean
Rolled lead for roofing is made in six standard thicknesses, known as codes. A higher code is thicker and heavier. The codes follow BS EN 12588, and Leadworx, a lead supplier, sets out their usual uses.
According to Leadworx, code 4 suits most flashing work, and code 3 is reserved for soakers and light duty.
The table gives the figures as published, with the colour marking that each code carries on the roll.
| Code | Thickness | Weight (kg/m²) | Typical use |
|---|---|---|---|
| 3 | 1.32 mm | 14.97 | Soakers and light duty |
| 4 | 1.80 mm | 20.41 | General flashing |
| 5 | 2.24 mm | 25.40 | General, vertical and pitched work |
| 6 | 2.65 mm | 30.10 | Large roofs |
Codes 7 and 8 (3.15 mm and 3.55 mm) are heavy-duty and rarely needed on a house chimney.
Choosing a code for your stack
The choice is about length and exposure. A short soaker stays flat and sees little movement, so code 3 is enough. The cover flashings and aprons are longer and take more of the weather, so we normally use code 4 for ordinary stacks and code 5 where the lead runs a greater distance or the stack is exposed. Thicker sheet resists the slow stretching of long lead runs.
Thicker lead tends to last longer. MyJobQuote (last updated 22/09/26) puts lead flashing at over 80 years, and over 100 years for the higher codes. That is a typical range, not a promise, and it depends on workmanship and on how the lead is dressed and fixed. A sound installation is what gets a flashing to a long life. We show you the code and the lengths on the written quote, along with photographs from your roof.
Why chimney flashings leak
Lead itself rarely wears out in a few years. What fails is the way it is held or what surrounds it, and the causes fall into a short list.
The first is the mortar. The top edge of the cover flashing is held in a chase, and the pointing over it weathers away. Once the pointing has gone, the wedges loosen, the lead lifts, and driving rain gets behind it. We look for this early on a stack, and it often goes with worn brick joints, as our guide to frost and weather damage to chimneys explains.
The second is movement and fatigue. Lead swells in warm weather and shrinks in cold, and over many cycles it can split along a fold or creep down a long run. Thin lead in a long run suffers most.
The third is a blocked back gutter. Leaves, moss and silt gather in the channel behind the stack and the water banks up, then slips under the tiles or over the lead. It can pass for a flashing failure when the lead is sound.
Sealant and bodged repairs
The fourth cause is a previous repair. Bitumen strip, flashing tape and silicone are often used to stop a drip quickly, and some hold for a season or two. Sealant over a split fold does not mend the lead, and it hides the real fault from the next person to look.
What else can look like a flashing fault
A stain beside a chimney is not always the flashing. Cracked flaunching, open brick joints and a porous stack can each admit water nearby, and condensation in a cold flue leaves marks too. The survey checks all of them before anything is priced. Our guide to leaking chimneys sets out the full list of ways in.
How a proper replacement is done
Replacement starts with taking the old lead off. The cover flashings come out of their chase, the soakers are drawn out, and the roof and brickwork underneath are checked for rot, damp timber and loose tiles. Hidden faults show up here, so each stage of a stripped joint is photographed.
The new lead is then made up on site, because every stack is slightly different. Soakers are cut and fitted course by course under the tiles, the apron is dressed into place at the front and the back gutter is formed at the rear. The cover flashing is laid over the top, with its upper edge set in a freshly cut chase and held with lead wedges. Dressing means working the lead by hand so it follows roof and brick without gaps or kinks.
Pointing the chase
The last step on the brickwork is to point the chase. The mortar that closes it should suit the stack: a lime-based mix on an older stack, a mix that matches the existing joints on a newer one. Our page on repointing a chimney stack explains why, and flashing and repointing are best done in the same visit because they share the same scaffold.
When the stack changes shape
Reducing a chimney's height or taking it down to roof level changes what the lead has to do, so new flashing is fitted as part of that work. Our page on reducing a chimney's height sets out what is involved, including the rules for a flue still in use.
The quote is written, with photographs of the stack, and the site is left clear. Fixing for the coast means the lead is wedged high, left loose low and never left to depend on sealant.
Published UK costs for chimney leadwork
Costs for chimney flashing vary more with access than with the lead, and the published figures show that. MyJobQuote (last updated 22/09/26) gives a typical UK range of £150 to £250 for a patch repair to chimney leadwork. For a full replacement of the lead, it gives £350 to £460 without scaffolding and £700 to £920 with it. Scaffold accounts for the difference between the two.
Per metre, Checkatrade (table last updated June 2026, figures exclusive of VAT) gives £50 to £70 for code 5 lead flashing and £80 to £120 for stepped flashing. MyBuilder (updated 21/08/2026) puts the scaffolding for a chimney at £400 to £900. All of these are national guide figures and not Watertight prices.
| Job | Typical UK range | Source |
|---|---|---|
| Patch repair to chimney lead | £150 to £250 | MyJobQuote, 22/09/26 |
| Full replacement, no scaffold | £350 to £460 | MyJobQuote, 22/09/26 |
| Full replacement, with scaffold | £700 to £920 | MyJobQuote, 22/09/26 |
| Stepped flashing per metre | £80 to £120 (ex VAT) | Checkatrade, June 2026 |
What drives the price on your house
Stack height and roof pitch decide the access. A tall stack on a steep roof needs scaffold, while a low one on a bungalow may need only a tower. The number of faces to flash, a back gutter to rebuild and whatever the strip-out turns up all move the figure.
Your written quote comes after the survey, at a single total that is good and fair. Whether a quote includes scaffolding and VAT is worth checking, because published ranges differ on both. For the broader picture, see our chimney repair cost guide.
Patch or replace, and what to ask for
A patch is the right answer when the lead is sound and a small area has lifted. Re-wedging a loose cover flashing, repointing a worn chase or sealing a short split in a fold with a proper lead repair can all hold for years. A patch is the wrong answer when the lead has thinned or cracked in several places, when the soakers are missing or when a back gutter is failing along its length. In those cases repeated call-outs soon cost more than a proper job, and we say which case yours is.
A written quote that names the lead code, the parts to be renewed and the position of the chase shows the work is thought through. A vague line about sealing round the chimney does not.
Questions worth putting to any roofer
- Which lead code will go on, and for which parts?
- Will the cover flashing be set into a chase, or just stuck on with sealant?
- Is the back gutter being renewed or reused?
- Does the price include scaffold, VAT and taking the old lead away?
- Will the roof under the old lead be photographed?
On our side, the answers come with the survey. We trace the entry point, photograph your own roof and set the written quote beside the pictures. If the stack needs other work, such as repointing or new flaunching, we say which parts are urgent and which can wait. A chimney breast taken out always needs Building Regulations approval, as the Planning Portal says, so plans to alter the stack belong in the same conversation.
Where water is coming in now, we cover the opening first and cure the cause after.
