What is a chimney back gutter?

A chimney that rises through a sloping roof is an obstacle in the path of rainwater. On the upslope side of the stack, water flows straight at the brickwork. A back gutter, sometimes called a back valley or a cricket, is the lead-lined channel sitting behind the chimney that splits the flow so it runs round both sides and down the slope.

You cannot see it from the ground. It is tucked between the back face of the stack and the roof covering, usually with a lead apron turned up the brickwork and flashings running into the courses. On Victorian and Edwardian terraces with central stacks, and on semi-detached pairs where a stack straddles the party wall, it works hard, because the stack is wide and the slope above it long.

Because the gutter sits hidden, it is often the last place anyone looks. Many chimney leaks are blamed on the pointing or the pot when the water is really getting in behind the stack. A back gutter is also high on our checklist when a stain appears high on a chimney breast after a long wet spell, because it takes the water from the whole slope above.

Lead is long-lasting when properly laid. MyJobQuote reckons well-fitted lead flashing can outlast 80 years, with higher codes going beyond 100. A back gutter can still fail, and the causes are fairly consistent.

Leaves, moss and debris collect in the channel, so water ponds and spills over the sides and under the tiles. The lead splits along sharp folds, or where the fall is too flat to drain. Laps between sheets lift or open as the metal expands and contracts. The upstand against the chimney comes away where the pointing has perished, and water runs in behind it. Sometimes the wrong thickness was used, because lighter lead such as Code 3 or Code 4 splits more readily in a gutter than a heavier code.

On a poorly built gutter, a mortar fillet or a smear of mastic is a sign of an earlier patch. Those often last a few winters and then give up.

The usual clue is a damp mark on the ceiling or chimney breast, and it tends to show after heavy or prolonged rain rather than a brief shower. Stains on the breast are brown or yellow and may have a tide line. In the loft, damp timber around the stack, or a drip following the brickwork down, points the same way.

Back gutter faults can resemble other chimney problems, such as failed flaunching, cracked brickwork or a lifted flashing. Our pages on why a chimney leaks and waterproofing a chimney help separate them. A survey is the only reliable way to tell which one you have, and we photograph the gutter on the roof so you can look at it yourself. Typical pictures show the channel from above, the lead turned up the brickwork, the laps and the tiles on either side. Where a leak is long-running, expect dark staining on nearby rafters, and that evidence helps decide how far the repair must go. It also explains why we ask you to note when the stain first appeared and whether it grows after rain.

A small split or a lifted lap can often be repaired by cleaning the channel and dressing or patching the lead. If the gutter is simply blocked, clearing it may be all that is needed. If it has several cracks, drains too slowly, or uses lead that is too thin, patching moves the leak along, and renewing the whole gutter is the better answer.

Costs depend heavily on access. MyJobQuote's September 2026 chimney leadwork guide gives £150 to £250 for a patch, £350 to £460 for full replacement without scaffold and £700 to £920 with scaffold. MyBuilder's August 2026 guide puts a flashing repair at £250 to £1,200 and high-level chimney scaffold at £400 to £900. Those are national figures, not a price for your roof. The price for your stack comes in writing after we have surveyed it.

A good back gutter is cut to a pattern, dressed into the corners without sharp creases, laid with enough fall to drain, and lapped so water cannot run backwards. Code 5 lead suits many gutters, and the upstand into the brickwork needs a proper chase, not a smear of sealant. Tiles on either side are lifted and relaid over the lead, and the stack is repointed where the flashing meets it.

Doing the gutter while a stack is being repointed or rebuilt saves scaffold, so it makes sense to check it whenever other chimney work is planned. See our pages on chimney repointing, reducing a chimney's height and repointing it yourself, and our chimney repairs team will look at the whole stack.

Do not climb up there yourself, however simple it looks from the garden. Ring 01793 626096, drop us a line on WhatsApp or through the contact form, and tell us where the stain appeared and how long it has been there. Photographs of the ceiling, the chimney breast and the stack from the ground give us a good first picture before we visit.