Roof windows are made for pitched roofs and sit in the plane of the tiles with a flashing kit; flat rooflights are made for flat and low-pitch roofs and sit on an upstand. They are not interchangeable — the roof pitch decides which you need.
Which roofs suit which product?
Roof windows work on pitched roofs within the manufacturer's stated pitch range, commonly from around 15°. Flat rooflights are for flat and very low-pitch roofs, with a fall built into the unit or upstand.
How are they weathered?
A roof window uses a flashing kit lapped under and over the tiles or slates. A flat rooflight relies on an upstand with the roof membrane dressed up it.
Do the energy rules differ?
Yes. In England a roof window needs 1.4 W/m²K or WER Band B in an existing home; a rooflight needs 2.2 W/m²K measured horizontally.
Which costs less?
Fitting a roof window into an existing pitched roof is usually quicker and cheaper than forming a new flat-roof opening, because rafter trimming and flashing kits are routine.
This guide gives general information, not legal or structural advice. Rules change and every property is different — check with your council or building control for your address. We review our guides regularly; the date above shows the last review.
Sources
- Approved Document L, Volume 1 (2021 incorporating 2023 amendments)
- The Future Homes and Buildings Standards: Building Circular 01/2026
- GPDO 2015, Schedule 2 Part 1 Class C (England)
- Permitted development rights for householders: technical guidance
- Approved Document L Volume 1, Wales (2022 incorporating 2024 amendments)
- Town and Country Planning (GPDO) (Amendment) (Wales) Order 2013
- Building Standards Technical Handbook 2022: Domestic, section 6
- Circular 1/2024: householder permitted development rights (Scotland)
- Technical Booklet F1 (Northern Ireland), June 2022
- Planning (General Permitted Development) Order (NI) 2015, Schedule Part 1
- Building Regulations 2010, Schedule 3 (competent person self-certification)
- Approved Document F, Volume 1
- Approved Document O: overheating FAQs
- Approved Document K
- HSE: fragile roofs
- Regulatory Reform (Fire Safety) Order 2005, article 17
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