Roofs and flat roofs
What is a butterfly or secret gutter between two roof slopes, and why is it not an eaves gutter?
A butterfly, or secret, gutter is the channel where two slopes fall towards each other, often hidden from the street. It is not the eaves gutter on the outer edge, and if it blocks or splits, water stands between the pitches. A visual survey may never see 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?
People use “secret gutter” loosely. Here it means the hidden meeting of two inward slopes, not a gutter fixed to a fascia and not the parapet gutter that runs along the top of a wall. Those are different details and they fail in different places. A butterfly gutter is easy to miss because the roofs hide it. Leaves, moss and a tired lining then wet the ceilings under the valley while the outer tiles still look even.
A home survey records what was visible from the ground, from a window, or from a loft where access was safe. It does not strip a secret gutter and it does not pour water along it. If the report says the central valley was not seen, that is a limit. Ask a roofer with safe access to describe the lining and the outlet. Do not price an eaves-gutter replacement and assume the hidden channel is included.
What should you ask your surveyor?
- Did you see the channel between the slopes, or only the outer eaves?
- Is any stain under that meeting point, or only on an outside wall?
- Are you separating this from a parapet gutter along a wall?
What are the typical next steps?
- Point a roofer at the inward valley, not only at the gutters you can see from the pavement.
- Do not treat a clear eaves gutter as proof the secret channel is clear.
- If the loft shows a stain on that line, say so when you ask for a quote.
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 a secret gutter always behind a parapet?
No. Two pitches can fall towards each other with no parapet at all. A parapet can hide one, but the parapet is not the gutter.
Will a drone photograph prove the lining is sound?
A photograph can show debris or an obvious split. It does not prove the sole of the gutter, and it is not a water test.