Roofs and flat roofs
What is a roof light set into a flat roof, and how is it different from a roof window on a tiled slope?
A flat-roof light is a kerb, dome or glass panel in the flat covering, with the waterproofing dressed up the upstand, whereas a roof window on a slope sits among tiles or slates with its own flashing kit. The weak points differ: a low upstand, a split dressing or a cracked dome, rather than a tile apron. A survey comments on the light if it could be seen and does not walk the flat roof as standard.
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?
Buyers mix the two up because both let light in. The repairer is not the same detail. On a flat roof the covering must turn up a kerb high enough to stay above standing water. If the upstand is low, or the felt has parted from it, rain enters at the dome while the middle of the flat roof looks fine. A pitched roof window fails at soakers, aprons and kerbs cut into the slope. The report should say which opening this is.
From inside, both can stain a ceiling. The survey should follow the stain to the opening it can see, and say if the top was not visible. Cracked glass or a misted panel is the glazing. A split dressing is the roof. Do not replace the flat covering because the dome is crazed, and do not replace the dome because the outlet is blocked. There is no standard price. Ask for the junction to be quoted separately from the rest of the flat roof.
What should you ask your surveyor?
- Is this light in a flat roof or in a pitched slope?
- Did you see the upstand and the dressing, or only the glass from below?
- Is any stain at the light, or elsewhere on the flat roof?
What are the typical next steps?
- Ask a roofer to look at the kerb, not only a window fitter to look at the glass.
- Do not walk on the flat roof to photograph it.
- Keep this opening separate from any roof window in the tiled slope.
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 lantern over a stair the same thing?
It is still a glazed opening in a roof. The junctions and the bars may differ. The survey should describe the one it saw, not a generic roof window.
Does a new dome mean the kerb is new?
No. Domes are swapped without the covering being redressed. Ask about the upstand.