When does chimney flashing need replacing?

Where a stack rises through a roof, there is a joint that has to move slightly with the weather and still shed water. Flashing is the sheet lead that bridges it. A chimney normally carries several pieces. Apron flashing runs across the lower face. Step flashing, cut in a stair-step pattern, follows the roof slope up each side, and each step tucks into a mortar joint. Behind the stack, a back gutter or lead tray carries water round to the sides, and our answer on the chimney back gutter deals with that part separately.

Soakers are small lead sheets that sit under each slate or tile and turn up against the stack, with the step flashing laid over them. Lead is chosen because it can be dressed tight to uneven masonry and keeps its shape through temperature swings.

From the ground, flashing is hard to judge. Binoculars show some faults. A roof survey from a ladder or platform shows the rest. Signs that matter:

• lead lifted away from the brickwork, leaving daylight between metal and mortar
• splits or hairline cracks along bends, usually where the metal has fatigued
• mortar fillets, or a bead of sealant, running along the joint in place of lead
• white powder or dark staining on the lead, which points to a poorly detailed or aged section
• pooled water or green growth along the lower face of the stack

Indoors, brown patches on the ceiling beside the breast, or damp that comes and goes with the rain, can also trace back to flashing. Water can travel some way before it appears, so the stain is not always beneath the fault.

For leak paths in general, our guide to why Stroud chimneys leak compares flashing with the other routes.

Age is not the only cause. A stack that moves slightly with the roof, a ladder or branch that has knocked the lead, or a fillet of cement that has pulled the metal away as it cracked can all open the joint long before the lead is worn out. Birds and squirrels also pick at poorly sealed edges. We note the likely cause on the survey, because replacing lead without fixing what damaged it only repeats the problem.

Not every fault means new lead. A section that has lifted but is sound can be dressed back and wedged again into a fresh, raked-out joint. A localised split can take a patch. Cement fillets and sealant are different. They crack within a season or two, because mortar cannot move the way lead does, and a roofer will normally cut them out and fit proper lead in their place.

Replacement makes sense when the lead is thin, creased by repeated movement or cracked in many places, or when earlier patches have failed beside each other. Flashing laid in too long a run is another problem, as lead expands and contracts and needs breaks. MyJobQuote's 2026 cost guide gives lead flashing over 80 years when it is fitted correctly, and beyond a century for heavier sheet, so a roof that needs it replaced early has usually suffered poor detailing or a bad patch, not simple age.

Matching the old method is part of the decision. On a stone stack the stepped lead is usually cut into joints that run at irregular heights, so each step has to be measured to the stone and not to a template. A flat price per metre would hide that difference until the scaffold is up.

Lead comes in numbered codes, 3 to 8, and the number tells you the thickness. Leadworx describes Code 4 as the usual pick for general flashing. Code 5, which is thicker at 2.24 mm, is sometimes used where more durability is wanted, and it is listed by Royston among the grades suited to flashings. Thicker sheet costs more and is heavier to dress, so the code is a decision for the survey, not a default.

Fixing matters as much as thickness. The upper edge of each step is wedged into a raked-out mortar joint and pointed, and the lower edge laps over the soaker or the slate. Where a stack is stone, the joints are deeper and the lead can sit better, but the mortar has to be right, so we raise it in the quote.

These are national figures from published guides, not our own price list, and scaffold is the cost that moves most.

After a survey, your written quote covers the lead, the scaffold or access, the repointing and the tidying up afterwards. A chimney that is also being repaired or lowered can share the access, which is worth asking about. If you are weighing who should look at the stack first, our note on a chimney sweep or roofer sets out the split. Where a breast has been taken out below, the stack may need attention before any flashing is worth fitting; see chimney breast removed.

When the lead is in, the rubbish goes with us and a 10-year workmanship guarantee is given in writing. Other stack problems, from pointing to rebuilds, are covered at chimney repairs page.