Chimney flashing: lead, soakers and why stacks leak

Picture the stack as an island in a stream. Rain running down the slope meets it from above and from the sides, and the lowest face sheds its share onto the tiles beneath. A good junction deals with all four faces, each with its own piece of lead.

Roofers use set names for them:

• Front apron. The strip across the lowest face of the stack. It is tucked into the brickwork and laid over the tiles below so water runs off the front.
• Step flashings. A run of lead cut in stages that follows the sloping side of the stack, rising with the roof. Each step is turned into a joint in the brickwork above the tiles. Our stepped flashing answer explains the layout.
• Soakers. Small folded sheets of lead, each interleaved with a tile or slate course along the side of the stack. The step flashing is laid over them, so water has two layers to get through.
• Back gutter. A channel of lead set on the uphill side of the stack. It gathers the water running down the slope and carries it round to either side.

The top edge of each piece does not rely on glue. It is turned into a slot cut along a mortar joint, called a raggle, and held there with lead wedges before being pointed. The lower edge simply lies over the roof covering, and that overlap is what carries water away.

A cheaper habit on some old repairs is a band of mortar or sealant filling the angle between stack and tile. It looks tidy for a season, then cracks, and the leak returns. Lead dressed properly is the lasting answer and the one the rest of this guide assumes.

Sheet lead carries a code number that stands for its thickness and weight. Royston Lead's cast-lead brochure lists codes 3 to 8, and it shows how the lighter codes suit the smaller, more flexible pieces while the heavier ones go where lead has to span or stand up on its own.

Two things stand out. Code 3 is recommended for soakers only, so it is the wrong choice for a flashing that has to be dressed into brickwork and bent over the tile profile. And code 4 is the lightest code the brochure lists for flashings, which means a roofer who proposes anything thinner for the apron or the step flashings is going below that guide.

Why heavier is not always better

A thicker sheet lasts longer and resists damage, but it is harder to dress tightly around awkward corners, and on a small flashing the extra weight buys nothing. Choosing the code is part of the job, and it depends on the length of the run, the exposure of the stack and how the lead has to be formed.

If you want to see the same comparison for a single piece of flashing, our lead code for chimney flashing answer goes straight to the choice. For lead across the wider roof, the lead roofing and leadwork page explains what we do with it.

Lead is a long-lived material. MyJobQuote, updated in September 2026, says chimney flashing can last for over 80 years. That is a typical figure rather than a promise, and a leaking junction is often not the lead wearing out at all but the lead being let down by something around it.

Pointing that has given up

The raggle that holds the top edge is filled with mortar. When that mortar crumbles, the edge of the lead is no longer gripped, and wind can lift it or rain can run behind it. Lead that looks intact from the ground may be hanging loose at the top.

Movement

Lead expands and contracts with temperature. A long, rigid piece with no allowance for movement can crack along a fold or pull away from the wall. A stack that has itself shifted, leaning slightly after years of weather, will drag its flashings with it.

Sealant instead of lead

A bead of silicone or a bitumen strip is a common quick fix. It seals a gap for a while and hides the true fault. When it fails, water gets under it and the damp spreads quietly behind.

Damaged tiles and slates beside the stack

The flashing relies on the tiles to carry water to the gutter. If the courses beside the stack have slipped or cracked, water can enter beneath the lead, even when the lead itself is sound.

Signs to look for from outside include lead that has curled away from the brickwork, white or grey streaking on the stack below the flashing, and gaps between the lead and the wall. Indoors, a brown tide-mark on the wall above the fireplace, or damp timbers in the loft close to the stack, is the usual giveaway.

Not every flashing leak needs the whole set of lead renewed. The sensible first question is how much of the lead is still sound, and the honest answer comes only from looking.

Re-pointing the raggle and re-dressing. If the lead is in good condition but has lifted because the mortar failed, it can be pushed back, wedged and pointed again. This is the smallest job and often enough.

Patch repair. A local split or a short missing section can be cut out and replaced with a new piece, properly lapped under and over the lead around it. A patch is right when the surrounding lead is healthy and the fault is one spot.

Full replacement. If the lead is split in several places, thin from age, wrongly sized or covered in sealant, patching starts to cost more in repeat visits than the job would have cost done fully. Then the old flashings come off, the raggles are cleaned and pointed, and the stack is re-flashed from the front apron round to the back gutter.

At the survey we photograph the junction and give you a verdict on the flashings: mend them, patch them for now, or renew them, with the reasons. We recommend the smallest job that cures the leak for good. There is little sense in a new set of lead round a stack that is itself failing, so the verdict also looks at the pointing, the flaunching and the brickwork, and our chimney repointing guide explains when those need attention first.

Where the stack itself is in poor shape the better route may be to take it down and build again.

Costs come from national guides, and they cover a wide range because the length of lead and the access vary so much. Checkatrade's table of roof repairs, last updated in June 2026, shows lead flashing repairs at £50 to £70 per metre, with VAT left off. For chimneys specifically, MyJobQuote (last updated 22 September 2026) gives a patch repair at £150 to £250, and full replacement of the lead at £350 to £460 without scaffolding, or £700 to £920 with it.

The gap between the two replacement prices is access. Checkatrade's scaffolding table, last updated in October 2026, puts a chimney scaffold at around £660 a week, and a stack over a pavement or a high roof usually needs one. Scaffolding that stands on the pavement needs a licence under section 169 of the Highways Act 1980, and West Sussex County Council wants 10 working days' notice, so allow time if the repair can wait.

Lead also pays back over time. A repair in the right code, fixed into sound raggles, should outlast several cheaper attempts. Read these as national typical ranges, which say little about one particular roof. Once we have surveyed yours, a single total comes to you in writing, covering the job as a whole.

For a closer look at the numbers, see how much it costs to replace chimney lead flashing.

You do not need to go on the roof to learn something useful. In fact please don't. A pair of binoculars and ten minutes on a dry day will tell you a good deal.

Stand back far enough to see the whole stack against the sky and work down it:

1. Look at the lead on the front face. It should lie flat and tight against the brickwork, with a straight upper edge.
2. Follow the stepped lead up the side. Look for gaps at the top edge, lifted corners or a stripe of white mortar crumbs beneath.
3. Check the side that faces the high ground, where the back gutter sits. Debris and moss collect there and hold water against the stack.
4. Look at the tiles or slates beside the stack, for slips, cracks and gaps in the courses.
5. Look at any sealant. A visible bead of grey or black material along the joint is a flag that someone has patched it before.

Indoors, a loft inspection with a torch can show water tracks on the timbers beside the stack and darkened insulation. Only do this from a safe position on boards or the hatch ladder, and never step between the joists.

When to call a roofer

If you see any of these, get it looked at before winter. Water that gets in at a flashing seldom stays in one place; it travels along rafters and shows up somewhere unexpected. A short repair now costs far less than a stained ceiling, wet insulation and rotten timber later.

Whatever we find, a loose raggle or a failed bead of sealant, goes into photographs that arrive with the quote.