Skylights and Roof Windows

Skylights and roof windows bring daylight and, where they open, ventilation into rooms under a roof, but they are not simply vertical windows tilted over. A roof window is fitted in the plane of a pitched roof, following its slope, while a skylight, in common use, may sit on a flat roof on an upstand, or may be a fixed dome or lantern, and each type has different flashing, insulation, drainage and structural requirements. The roof is a weather-critical element, so a roof window that leaks, condenses or overheats a room will cause more trouble than a vertical window with the same faults, because water, heat and light all behave differently on a slope or a flat roof.

Daylight benefit is real: a roof window can light a loft room, a landing or a deep-plan room that a vertical window cannot reach, and an opening roof window can provide stack ventilation, letting warm air escape high in a home. Overheating risk is equally real: a south-facing roof window in a loft room can admit intense summer sun, turning a bright room into an uncomfortably hot one, so glazing, shading and ventilation must be designed together, not added as afterthoughts.

Flashing is the critical detail, and it is where most roof window failures begin. Flashing kits, designed for the specific roof covering, tile profile and pitch, must be fitted exactly as the maker specifies, with underfelt, battens and tiles or membrane integrated so water runs over, not under, the flashing. A generic flashing, a flashing fitted for a different tile, or a flashing that is sealed with mastic instead of properly lapped, will eventually leak, often after a driving rain that a vertical window would have shrugged off.

Insulation and thermal bridging around the reveal are equally important. A roof window in a well-insulated roof must be surrounded by insulation that runs up to the frame without gaps, and the internal reveal should be designed to avoid a cold bridge where condensation forms, especially in a bathroom or kitchen loft where humidity is high. Safety glass, opening restrictors, blinds and, where relevant, escape requirements, all add further detailing that a vertical window does not face in the same way.

Check the roof pitch, covering type and age, the room below and its use, and whether the window must open for ventilation, for escape or for cleaning access, since those functions change the type and position required. Note the sun path for that roof slope, including summer overheating risk, and whether external shading, a blind or a solar-control glazing is proposed, and how it will be operated if the window is high or hard to reach.

Ask for the flashing kit specification for the exact covering and pitch, the whole-window U-value, the glazing type including safety and solar control, and the insulation and reveal detailing in writing. A quote that names a window size but not a flashing kit for this roof has not yet specified the job.

In the EU, roof windows follow product standards and national building regulations, with member states differing on U-value requirements, on safety glass rules, on escape and ventilation provisions for loft rooms, and on planning rules for roof alterations, especially on street-facing slopes. A roof window that is permitted development in one country may need permission in another, particularly in conservation areas.

In the UK, building regulations cover structure, weather resistance, insulation, fire safety and escape for roof windows, with permitted development rules governing size, position and projection, and devolved standards adding local differences. Loft conversions, in particular, have specific escape, headroom and fire requirements that a roof window alone does not satisfy, so the whole conversion, not just the window, should be checked.

A frequent mistake is fitting a roof window with the wrong flashing kit for the tile profile, so it leaks in driving rain and the leak is blamed on the window rather than the flashing. Another is fitting a large south-facing roof window without shading or ventilation, so a loft room that was dark but cool becomes bright but unbearably hot in summer, and occupants keep blinds shut all day, losing the daylight benefit entirely.

A third mistake is ignoring condensation on the reveal: a roof window in a humid loft bathroom, without insulated reveals and adequate ventilation, will show condensation and mould around the frame, even if the window itself is high quality. Reveal insulation and ventilation are part of the window job, not separate trades.

Keep the window specification, the flashing kit details for the exact covering and pitch, the insulation and reveal drawings, the glazing and safety details, and photographs of flashing before tiles are refitted. Those papers prove the roof window was detailed for this roof, not just fitted to a hole in it.

Ask a qualified installer: what flashing kit suits this covering and pitch, how are insulation and reveals detailed, what glazing and shading are proposed for overheating, and how will the window be cleaned and operated if it is high. Roof-specific answers show a roof window done properly.