A felt-covered flat roof incorporating a rectangular glass rooflight. The photograph shows the roof surface, rooflight frame and flashing where the roof meets the existing house.
About this type of work
The notes below explain this kind of work in general terms. Every home is different, so the details of a particular job depend on the property, the ground and what the owner wants.
What a flat roof and rooflight involve
A flat roof is not completely flat. It is built with a slight fall so rainwater runs off to a gutter, and it is only as good as its layers and its details. A typical build-up is a structural deck, a vapour control layer, insulation, and a waterproof covering such as the felt shown here. The weak points are the edges and anywhere the roof meets something else.
- Flashing. Where the roof meets the wall of the house, the waterproof layer is turned up the wall and covered with flashing, so water cannot get behind it. The photograph shows this detail.
- The rooflight kerb. A rooflight sits on a raised frame, or kerb, that is waterproofed and insulated like the rest of the roof. The glass is usually double or triple glazed to limit heat loss.
- Falls and drainage. Water has to have somewhere to go, and standing water shortens the life of a roof.
Rules to know about
- Building Regulations normally apply to a new flat roof on an extension, because of structure, insulation and fire safety.
- Planning. A rooflight can be permitted development on a house if it does not stand more than 150 millimetres above the roof surface, so a raised or kerbed rooflight needs checking. A new flat roof as part of an extension is covered by the extension rules.
- The roof of an attached building should be checked for how it joins the existing house, since a poor join is a common cause of leaks.
See our guide to Building Regulations for a house extension or read about our extension service.