What is a chimney back gutter?

1Where it sits and what it does

A stack that rises through the middle of a roof is a barrier in the path of rainwater. Water running down the slope above meets the back of the brickwork and has nowhere to go. On a narrow stack it simply splits and runs past on either side. On a wide stack, or on a shallow roof, there is too much water and too little room for it to find its own way round.

The back gutter solves this. It is a shallow trough of lead, laid behind the stack, with the base turned up the brick and tucked into a joint. Often there is also a small lead sloping piece, sometimes called a saddle, where it meets the ridge. The trough is angled so that water travels to each side and then drops out onto the slope beside the stack, where it is picked up by the flashings or the tile or slate courses.

It is a detail that exists only on chimneys on slopes, and on a flat-sided ridge stack you may not have one.

2Why back gutters leak

Lead is never still. It swells a little with summer heat and shrinks again in winter, and over decades long runs can creep, tear or split at the folds. Because the back gutter holds water, rather than just shedding it, any weakness is tested every time it rains.

The common failures are simple. Moss, leaves and debris build up in the trough and the water overflows behind the stack. The lead lifts away from the brick when the wedges or pointing that hold it in the joint loosen. Folds crack where the lead was bent too tightly. Old repairs of sealant or bitumen peel and let water under. And a trough that was laid too flat does not drain.

When the water breaks through, it appears indoors not under the gutter but wherever it finds its way: the chimney breast, the ceiling beside it or the timber in the loft. When two houses share the stack, the trough often runs across both, so a failure can show up next door too. Our chimney repairs page lists the neighbouring jobs on a stack.

3Lead codes and how the work is done

Lead sheet is sold by code, which is a number that tells you its thickness. Code 3 is described as suitable for soakers only, the small pieces of lead that slide under tiles. Leadworx describes code 4 as the usual pick for flashing in general, while code 5, at 2.24 mm thick, suits general, vertical and pitched work. A back gutter takes a run of water, so a sound choice of code, laid with correct laps and folds, matters more than the name of the product.

A proper repair strips the old lead and clears the gutter bed. New lead is dressed over the roof boards, up behind the stack and into a chased joint in the brickwork, with the top edge wedged and repointed. Joints are lapped, not smeared with sealant, and the sides are taken out over the tiles or slates so the water shoots clear.

The lead is cut, folded and dressed on the roof. That takes skill and time, which is why a good back gutter repair outlasts a sealant fix by decades.

4Repair, renew or rebuild

A small split in an otherwise sound gutter can sometimes be patched. A patch by lead welding, or a new section let in, is a repair, not a cover-up. If the lead is old, thin or cracked in several places, or the flashings either side are failing too, replacing the whole back gutter is better value.

MyJobQuote's guide, updated 22 September 2026, gives typical UK figures of £150 to £250 to patch the leadwork round a chimney, and £350 to £460 for full lead replacement without scaffolding, rising to £700 to £920 with it. It also says lead flashing can last over 80 years, and over 100 for higher codes.

Where the stack is sound but the brickwork behind the gutter is crumbling, the joint will not hold new lead, and the pointing comes first. See chimney frost damage for that. Photographs of the gutter and the joint are taken at the survey, and one written total follows.

5Looking after a back gutter

Clear the trough from time to time, ideally from a safe position and not a ladder against the stack. Moss and leaf litter hold water against the lead and speed up wear. Look for lifted edges, white salts or dark streaks on the brick, and a gutter bed that holds puddles after rain.

Avoid the temptation to smear sealant or bitumen along the joint. It hides the problem from the next inspection and traps water behind. If you plan to lower or take out the stack, the back gutter will need rebuilding or removing too, so reducing chimney height is worth reading first. And if you are thinking of a do-it-yourself repair of the brickwork above it, read DIY chimney repointing before you climb up.