Roofs and flat roofs
What is a back gutter behind a chimney stack, and why might a ground-level survey miss it?
A back gutter is the short channel on the uphill side of a stack, where the slope meets the back of the chimney, not the main valley and not a parapet gutter. Debris or a split lining there wets the stack and the ceiling beside it while the tiles look even. From the pavement the stack hides the channel, and a loft may show the stain without showing the lining.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
Chimneys that sit part-way down a slope need a way for water to pass behind them. That short gutter, often lead, is one of the least visible details on a house. It blocks with moss and mortar, or splits at the corners, and the leak is then blamed on the tiles or on “the chimney” in general. A home survey should say if the back gutter was not seen. Silence is not a pass. Soakers at the sides of the stack are a different detail again.
A roofer needs to see that gutter from the roof or from a safe high point. There is no standard price for relining a back gutter. Ask for it by name so it is not left off a quote that only lists ridge and tiles. Do not lean out of a roof window to inspect it. If the survey rates the ceiling beside the stack and could not see the gutter, the next step is access, not a new damp-proof course in the bedroom.
What should you ask your surveyor?
- Could you see the back of the stack, or only the front from the street?
- Is the stain beside the chimney, under a valley, or both?
- Are you separating the back gutter from the side flashings?
What are the typical next steps?
- Name the back gutter in the roofer’s instructions.
- Do not assume a clear front apron means the back is clear.
- Use the loft stain as a clue, not as a drawing of the lining.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Do all chimneys have a back gutter?
Only those with a slope falling against the back of the stack. A ridge stack may not. The survey should describe this stack.
Is a secret gutter between two roofs the same detail?
No. That is a long channel between pitches. A back gutter is the short one hard against a chimney.