Roofs and flat roofs
What should you ask if the upstand of a roof window finishes below the tile line?
Ask whether water on the slope can run behind the upstand instead of onto the flashing below it. An upstand that finishes below the tiles around the opening is a geometry problem at that window, not a comment on the glass and not the same thing as a ceiling stain. A home survey can say this only if the upstand was in view, and it does not lift the tiles to prove the lap.
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?
On a pitched roof the tiles above a roof window should lap so that water is delivered over the apron, not under it. If the kerb is proud of nothing, or the side tiles finish short, rain can track sideways under the covering while the glass looks clean. That is a flashing and kerb question. A ceiling mark under the window can have the same cause, or a different one, and one dry visit does not separate them.
RICS home surveys are visual. They describe the junction they could see from a ladder only where that was safe, or more often from the ground and from inside. They do not strip the secret gutter around a frame. If you need a price, ask a roofer to look at that kerb and to say whether the frame, the flashing kit or only the tile line is wrong. Do not replace the window because the kerb looks low until someone has seen the lap.
What should you ask your surveyor?
- Did you see the kerb and the tile line, or only a stain inside?
- Is the glass misted, or is the issue the junction with the tiles?
- Which roof windows did you actually see from outside?
What are the typical next steps?
- Photograph the kerb only if you can do it safely. Do not climb the roof on a viewing.
- Ask a roofer to separate a low kerb from a failed glass unit.
- Do not reroof the slope because one opening sits low.
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 low kerb the same as a missing tile?
No. The tiles can all be present and still finish below the kerb, or the kerb can sit in a hollow. The lap is the point.
Does a roof window with a clear pane prove the kerb?
No. The pane can be clear while water bypasses the frame. Clear glass is not a dry junction.