Skylight and roof window installation: what to expect
People search for skylights, rooflights and roof windows interchangeably. On a pitched roof they usually mean the same thing: a factory-made window set into the slope. Flat-roof rooflights and lanterns are a different product and are covered in a separate guide.
Installing a skylight or roof window means cutting an opening between the rafters, trimming the structure around it, fitting the window with the manufacturer's own flashing kit so it sheds water into the tiles or slates, then finishing the reveal inside. The flashing kit, not the glass, decides whether it stays dry.
Choosing where the window goes
Position is set by the rafters, the room below and the view you want. A window placed between existing rafters needs the least structural change. Wider units, or pairs side by side, mean cutting a rafter and adding trimmers to carry the load around the opening. Height matters too: in a loft room the bottom of the window should be low enough to see out when standing, and on an escape route the opening size and height have to suit that purpose. A survey checks all of this from inside the loft before anything is ordered.
Centre-pivot, top-hung and fixed units
Centre-pivot windows rotate in the middle and are the usual choice where furniture sits under the window. Top-hung windows open outward from the top, give a clearer view and are often chosen where the window must double as an escape opening. Fixed units let in light only and suit stairwells or high positions that nobody can reach. Blinds, remote operation and glazing options are chosen at the same time, because retrofitting some of them later is awkward.
Cutting the opening and trimming the structure
Tiles or slates are stripped from a working area around the new position and stacked for reuse. The membrane is cut back, the battens are removed, and if a rafter has to be cut it is supported first, then trimmed with new timbers top and bottom so the loads pass around the opening. The frame is fixed to the structure with the brackets supplied, checked for square and level, and set at the depth the flashing kit expects.
Why the flashing kit matters most
Each manufacturer makes flashing kits for different roof coverings: profiled tiles need a deeper kit, flat tiles and slate a lower one. The kit's collars, gutters and aprons interlock with the frame and lap under and over the surrounding covering in a set order. The site fits skylights with the manufacturer's own flashing kit rather than a generic lead apron, because improvised flashings are the leading cause of skylight leaks. The membrane is dressed up to the frame so any water that gets past the tiles is carried away.
Finishing inside
Inside, the reveal around the window is lined, insulated and plastered or boarded. Splaying the top and bottom of the reveal lets more light into the room and helps air move across the glass, which reduces condensation. The surrounding ceiling normally stays intact, but the new reveal itself needs finishing and decorating, so plan for that when arranging the work.
Planning and building control
Most roof windows on a house fall within permitted development, provided they do not project far above the roof plane and are not on a principal elevation in a restricted area. Conservation areas and listed buildings are different: approval may be needed and a flush, conservation-style unit is often expected. Cutting rafters is structural work, so building regulations apply to the trimming, and to escape and insulation where the window serves a habitable room. Ask your roofer who is handling any notification.
When an existing skylight leaks
Water appearing around an existing roof window is usually a flashing problem, a blocked gutter at the head of the kit, or condensation running off cold glass. Staining that appears only in cold weather, without rain, points to condensation. Staining after heavy rain points to the flashing or the seals. Old units with failed seals or rotten frames are often better replaced than repaired, and a replacement can usually reuse the existing opening.
Questions to ask before you book
Ask which make and model is proposed, which flashing kit suits your tiles or slates, whether any rafters will be cut and how they will be trimmed, what internal finishing is included and who arranges any approvals. A clear written scope makes quotes comparable, and a free survey is the right place to settle these points.
FAQs
On a pitched roof the terms are used for the same product: a factory-made window set into the slope with its own flashing kit.
Usually not on a house, but conservation areas and listed buildings can need approval. Structural trimming falls under building regulations.
The surrounding ceiling normally stays intact. The new reveal around the window needs lining, plastering and decorating.
If it happens without rain, condensation on cold glass is the likely cause. Drips after heavy rain point to the flashing or seals.
Related reading
More on process & paperwork and the jobs that usually come with it.
- Planning permission for a loft conversion in south-west LondonProcess & paperwork
- Dormer loft conversion: what to expect from start to finishProcess & paperwork
- Party walls, shared roofs and semi-detached housesProcess & paperwork
- Loft insulation and your roof: getting both rightMaterials & systems
Looking for something specific? See our roofing services, cost guides, common roof problems or the areas we cover.
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