Windows, doors and glazing
Why does a window cill that slopes back towards the glass matter?
A cill should throw rain away from the window and the wall. If it slopes back, or the drip groove is blocked, water sits against the frame and the reveal. That is a route for penetrating damp. It is not, by itself, a failed damp-proof course, and a moisture meter on the plaster below does not prove which path the water took.
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?
Surveyors see the consequence inside: staining on the reveal, soft plaster, or a tide that starts under the window rather than at the floor. Outside they may see a flat cill, open joints, or mastic that has shrunk. They do not water-test the window. A dry visit does not prove the cill works in driving rain.
The repair is often the cill, the mastic and the decoration, not an injected damp-proof course. The Property Care Association’s advice is to find the building defect before a treatment is sold. Ask a joiner or a mason about that opening. Costs depend on the material of the cill. No national figure is cited here.
What should you ask your surveyor?
- Does the cill fall outwards, and is the drip clear?
- Is the staining under the window or at skirting level?
- Did you see this on a dry day only?
What are the typical next steps?
- Photograph the cill and the reveal before anyone seals over them.
- Do not commission a damp-proof course for a window that tips water inward.
- If the rating is about one opening, keep the quote to that opening.
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 cracked cill the same as a sloping one?
No. A crack lets water into the cill itself. A backward slope sends water to the frame. You can have either or both.
Will new mastic cure a cill that slopes the wrong way?
Mastic fills a gap. It does not change the fall of the cill. Ask the person quoting which of the two they are pricing.