The short answer
Repoint lead flashing when the mortar or sealant holding its top edge in the wall has cracked, shrunk or fallen out, because rain can then run behind the lead. The old filling is raked out of the groove, the lead is checked and wedged, and the groove is refilled with fresh mortar or a lead sealant. If the lead itself is split, repointing alone will not cure the leak.
When the filling around a flashing needs renewing, how the groove is cleared and refilled, and how mortar compares with sealant.
What is being repointed and why it fails
Beside a gable or a chimney stack, the upper edge of a lead flashing is turned into a slot cut along a mortar course. That slot is called a chase or reglet. The lead sits inside it, held by small lead wedges, and the slot is closed up with mortar or sealant so rain cannot run in behind the metal. The filling is the weather seal at the top of the flashing, so when it fails the lead can look fine while water gets in.
Mortar fails in a few ordinary ways. It shrinks as it cures, it cracks because the brickwork and the lead expand and contract by different amounts, and it crumbles away once water has got into it and frozen. You can often see the signs from the ground: a dark line along the top of the flashing, gaps in the joint, or loose crumbs on the roof below.
Look too for lead that has slipped out of its slot or lifted away from the brick. Our answer on lead flashing coming away from the wall goes through that fault, which needs more than a repoint.
How the job is done
The first step is to rake out the old filling to a depth that leaves room for a proper new joint. A roofer does this with a small chisel or a raking tool, taking care not to cut into the lead or crack the brick. Dust and loose bits are brushed out, so the new filling bonds to clean masonry.
Next the lead itself is inspected. If the top edge has loosened, it is re-seated and held with fresh lead wedges, spaced along its length, because the wedges, not the filling, are what hold the lead in place. Any split, hole or torn fold in the lead gets fixed or replaced at this point, since repointing over a damaged sheet does not stop water.
Last comes the new filling, pressed well into the groove and finished neatly. On a chimney, the work usually follows the step line of the roof slope, so a long run of stepped chase is worth planning around the access. Checkatrade's June 2026 table gives £50 to £70 per metre for a lead flashing repair and £80 to £120 per metre for stepped, in Code 5, as national guide figures. The price for your wall is set by the survey.
Mortar or lead sealant in the chase
Both are used, and each has its place. Mortar is the traditional choice. It looks right on older brickwork, and it is hard-wearing when the mix and the joint are good. The risk is cracking, since mortar is rigid and the wall and lead do not move together.
A lead sealant, which stays flexible, takes up some of that movement and does not crack in the same way. It suits modern buildings and places where the lead moves a lot. The drawback is looks and age: some sealants harden or lose their grip over time, and on a traditional wall they can look out of place. They also need a clean, dry surface to stick to, so they are not a job for a wet day.
How long either filling lasts depends on exposure, workmanship and the wall, so nobody can honestly promise a number. Lead itself is quoted at over 80 years by one cost guide, so the filling is nearly always the part that is renewed first. A roofer should say which they recommend and why, for your wall, and the written price should state it. Our can lead flashing be repaired answer covers the wider question.
When repointing is not enough
Repointing is the right job when the sheet is healthy and the filling alone has let go. It is the wrong fix when the lead is cracked, thin or lifting along its length, or when it was fitted without wedges and keeps working loose. Smearing sealant over a failing flashing hides the fault for a while and then fails.
It is also not enough when the flashing is the wrong kind for the detail, or the brickwork around the chase is itself decayed. Soft, flaking brick will not hold either mortar or sealant, and the wall needs attention first. On a chimney, repointing the stack and its flashings can be sensible to do in one visit if the access is shared. Checkatrade's October 2026 guide gives an average of around £750 for chimney repointing.
We start with the survey, check the filling, the wedges and the lead, and photograph what we find. You then get one written total price, and the choice between mortar and sealant is explained in the quote. See our lead work page for the full trade, or step flashing vs cover flashing for the parts of a chimney detail.
More on this in Lead Work & Lead Flashing in Crawley
Two more repointing questions
Can I repoint a flashing myself from a ladder?
It is not one for a homeowner. The work is at height, often at a chimney or gable where a ladder is not a safe base, and raking out masonry on a slope is awkward. It is also easy to cut the lead or leave wedges missing. Look from the ground, take photos if you can, and let a roofer judge it.
Does the new filling need to match the old?
Not always, but it should suit the wall. On old soft brick, a mortar that is far harder than the brick can damage the brick, so the mix matters. On newer walls a flexible sealant may be a better fit. A roofer should explain which they are using and why, and say so in the quote.