Roofs and flat roofs
Moss is sitting in the valley and holding water. Is the moss itself a structural defect, or a maintenance note kept apart from the valley metal?
Not moss sitting in the valley and holding water is a structural defect. Moss is a maintenance note. It can slow the flow and keep the channel wetter, which matters to the metal or the tiles that form it, but the plant is not a failed rafter, truss or wall. Keep the moss apart from any comment on the valley metal. Clearing growth is not a structural repair, and the survey has not redesigned the valley because vegetation was sitting in it on the day.
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?
Valleys that hold debris stay damp, and a wet joint wears faster than a clear one. That is a reason to clean the channel, not a reason to call the roof frame unsound. If the surveyor also saw a split, a hole or a soft board, that will be its own sentence. Do not fold those into the moss, and do not upgrade moss into those defects because water was being held during the visit.
Have the valley cleared and then looked at again, or ask the roofer to clear it as the first line of a quotation and to say what metal remains once the moss is gone. If they price a new valley before they have seen the sole, ask them to separate the clean from any repair. The maintenance note is the growth holding water. It is not, by itself, a new roof structure.
What should you ask your surveyor?
- Is the moss the whole of the valley note, or did you also see a defect in the metal?
- Was water being held on the day, and could you see the outlet past the moss?
- Are you rating maintenance, or the structure of the roof?
What are the typical next steps?
- Clear the moss, or have it cleared, before the valley metal is condemned.
- Keep the maintenance note separate from any split, hole or soft board.
- Ask a quotation to price the cleaning on its own line.
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
Does moss in the valley mean the lead has failed?
No. Moss is growth holding water. The valley metal needs its own sentence. The plant is not a split sheet and is not a structural failure.
Is removing the moss a structural repair?
No. Clearing moss is maintenance, not a structural repair. It does not become structural work because the valley sat wetter while the growth was there.