The short answer
Correctly installed lead flashing is reported to last well over 80 years, and over 100 years in the heavier codes. Mortar flashings, a cement fillet at the stack, are a weaker detail and tend to crack long before lead does, because the mortar cannot flex. Poor fixing, the wrong thickness or lengths that are too long cut a lead flashing short.
Lead flashing: a long life if it is laid properly
A flashing seals the junction where a chimney stack rises through the roof. In lead, it is made up of several pieces: an apron at the front, step pieces up the sides, and at the high side a back gutter that carries water round the stack. Between the roof covering and the flashing sit soakers, small folded sheets of lead that interleave with each slate or tile.
The MyJobQuote leadwork guide, updated in September 2026, says chimney lead lasts over 80 years when it is correctly installed, and over 100 years for the higher lead codes. Lead sheet is graded by code, which describes thickness. Royston Lead lists code 4 for soakers, flashings, pitched valley gutters and dormers, and code 5 for flashings, valleys, and parapet or box gutters. Code 4 is 1.8 mm thick and code 5 is 2.24 mm.
Those are figures for good work in a normal setting. They are typical lives, depending on exposure and workmanship, not a promise for any particular stack.
Mortar flashings and why they do not last
Older houses and quick repairs often have a different detail: a sloped fillet of sand and cement running along the junction of stack and roof. It is cheap and it looks tidy for a while, but it asks rigid mortar to do the job of a flexible metal. The stack and the roof move at slightly different rates through the year as they heat, cool, soak and dry. Mortar cannot follow that movement, so it opens at the joint in time, and rainwater runs behind it.
We found no dependable published lifespan for a mortar fillet, and we would rather leave the figure blank than guess. What you can say is that the detail is a repair rather than a long-term solution, and that it is the usual starting point when a roofer is called to a stained ceiling beside a chimney breast.
The other problem is hidden. Water that gets behind a cracked fillet soaks the timber and the brick at the base of the stack, which can lead to a larger repair than a new flashing would have been. Where an older flashing is in good condition and only the mortar around it has failed, repointing may be all that is needed.
What shortens the life of lead
Lead is long-lived, but it can be spoiled by the way it is installed. Pieces cut too long are one cause. Lead expands as it warms and contracts as it cools, and across a long run it creeps down the slope and fatigues until it splits. The MyJobQuote guide mentions lengths of 1.5 metres for this reason, and Royston Lead gives the same maximum piece length for flashings in codes 4 and 5.
Fixing matters equally. Lead folded into a joint is held with wedges, and the Lead Sheet Association says a cover flashing should be wedged at no more than 500 mm apart. Too few wedges and the flashing lifts in the wind. Using a sealant as the only fixing, without wedging and pointing, gives a short life.
The wrong code causes trouble as well. Code 3 is meant for soakers only, and flashings are normally cut from code 4 or heavier, so a thin sheet used where a thicker one belongs can split sooner. Rot in the timber beneath, contact with fresh cement or damp mortar, and a stack that has moved all take their toll. On a stack that has moved, see our note on a cracked chimney stack.
Signs a flashing is nearing the end
The earliest sign is often indoors. A damp patch or brown stain on a ceiling next to the chimney breast, or a bloom of mould on the breast itself, can mean water is entering at the base of the stack. Outside, the clues are physical.
- Lifted or curled edges. Lead standing proud of the stack, or a gap along its upper edge, lets wind-driven rain behind it.
- Splits or cracks. Cracks usually run across the lead at a fold or corner, where the metal has fatigued.
- Missing or loose wedges. Lead hanging free of the joint shows its fixing has gone.
- Gaps in the pointing. Where mortar has fallen from the joint above the flashing, the lead is loose behind it.
- A mortar fillet with a crack along it. That is the end of the detail, not a fault to seal over.
One cracked corner is a repair. A flashing that has split in several places, or that has lifted along most of its length, is better replaced. Where the brick behind it has started to decay, see brick chimney repair, and for the whole service see chimney repairs.
Repair or replace, and what it costs
If the lead is sound and only one joint has failed, a patch or fresh wedging will do. Where the lead is thin, split or fixed badly, a full replacement is the lasting answer, and it is best done with the pointing around it, since the two are linked. Birds can disturb flashings too, as a cowl or guard that is badly fitted pulls at the lead. See our note on bird guards.
MyBuilder's 2026 chimney repair guide reports £250 to £1,200 for flashing repair. MyJobQuote, in September 2026, gives £150 to £250 for a patch and about £570 on average for full chimney lead replacement, or roughly £350 to £460 without scaffold and £700 to £920 with it. These are typical UK ranges from national cost guides, not our prices.
We survey free and photograph the lead on your own stack, so you see the fault for yourself. The price is then fixed in writing.
