Roofs and flat roofs
What should you ask about the internal gutter on an M-shaped roof?
Ask where the water between the two ridges goes. An M-shaped roof has a central trough as well as outer eaves, a long channel that is easy to miss from the street, and if it blocks or splits, water sits above the middle while the outside gutters look clear. A visual survey may see it only from a loft, or not at all.
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?
M-roofs appear on some terraces, double-pile houses and older pairs, with two pitches back to back and a valley between the ridges. The central gutter is not an eaves gutter, and it is not a short secret gutter behind one chimney. It has length, outlets and a lining that can be lead, felt or an old timber sole. A blockage at the outlet floods the middle. A home survey should say whether that gutter was seen.
Quotes go wrong when a contractor prices the front and rear eaves and never opens the middle. Ask them to include the internal gutter and to say how they will reach it. There is no RICS price. If the loft shows staining along the centre and the surveyor could not see the lining, treat that as unseen, not as satisfactory. Do not water-test it yourself by running a hose from a bedroom window.
What should you ask your surveyor?
- Did you see the central gutter, or only the outer eaves?
- Where does it discharge?
- Is any ceiling stain under the middle of the roof?
What are the typical next steps?
- Put the internal gutter on the roofer’s brief in writing.
- Do not accept an eaves-only quote as a whole-roof rainwater repair.
- If access to the loft was blocked, say that the central lining was not proved.
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
Is an M-roof the same as a butterfly roof?
They are related ideas: water falls inward. An M-roof is specifically two ridges with a trough between them. Use the shape this house has.
Can the neighbour share that gutter?
On a terrace they might. The survey of this house does not decide the legal share. Ask your solicitor if the trough crosses a boundary.