Roofs and flat roofs
A secret gutter over a 1930s bay is blocked with moss and the tiles in front of it look even. Why is the gutter the first place to look?
The secret gutter is the first place to look because it is blocked with moss, while the tiles in front of it look even. On a 1930s bay the gutter is a hidden channel behind the tiles, and a blockage there will overflow into the bay long before a slipped tile shows on the face. Even tiles do not clear moss out of that channel. The survey has recorded the blockage and the even covering in front. It has not cleared the gutter, and it has not seen the lining under the moss. The risk starts in the hidden gutter, not on the tidy slope.
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?
A secret gutter on a bay sits behind the tiles, where you barely see it from the ground. Moss fills it, holds water, and sends the overflow into the bay or down the wall. The tiles in front can stay in neat courses the whole time, because they are not the blocked piece. The lining only shows once the moss is out. The survey saw the moss but did not clear the channel to the sole, and it did not prove that a ceiling is already stained.
Ask a roofer to clear the secret gutter and then report on the lining and the outlets, with the seller's agreement. Tell the solicitor the tiles in front look even, so the first place to look is the blocked channel, not a new covering on the bay face. The survey has not specified new tiles. Overflow from a secret gutter often wets the bay or the room below that line. A neat slope should not send you to the tiles first.
What should you ask your surveyor?
- Is the secret gutter over the bay blocked with moss?
- Do the tiles in front still look even?
- Was the lining under the moss left unseen?
What are the typical next steps?
- Look at the blocked secret gutter before the even tiles.
- Ask a roofer to clear it and report on the lining.
- Do not specify a new tile covering from this note.
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
If the bay ceiling is dry, can the moss stay?
A dry ceiling does not empty the gutter. The secret gutter is blocked with moss, which is why it is the first place to look, and the tiles in front looking even do not change that. The lining was not seen. Dry plaster today can still be over a channel that overflows in rain. The survey has not cleared the moss.
Are the tiles even because the gutter is doing its job?
No. Even tiles are the face of the bay. The gutter behind them is blocked with moss, so it is not doing its job. A neat course can sit in front of a full channel. The survey has not seen the lining under the moss, and it has not said the tiles were newly laid.