A roof window is one of the few improvements that changes how a space feels immediately, and on a loft or a dark landing it usually does more than any amount of decorating. It is also a hole cut in a weatherproof surface, so it lives or dies on the flashing kit and the way the opening is formed. This guide covers choosing, positioning and fitting, and the two problems that show up afterwards when either is done carelessly.
Choosing the window
Start with how the window will be used rather than with the catalogue. A window you need to open regularly for ventilation wants a handle you can reach, which usually means a top hung or centre pivot at a height you can operate from standing. A window purely for light in a high gable or over a stair can be fixed and higher up.
Size is worth being generous about. The difference in cost between a small and a medium window is modest, and the difference in the light delivered is substantial, particularly on a north facing slope. Two smaller windows spaced apart usually light a room more evenly than one large one.
Glazing specification matters more on a roof than in a wall because the window faces the sky. Consider solar control glazing on a south or west facing slope, and laminated inner panes where the window is above a stairwell or a bed.
Position: structure, sightlines and the covering
Where the window can go is decided by three things. The rafter spacing, because the opening needs to sit between trimmed rafters with properly formed headers above and below. The sightline you want, because a window set low in the slope gives a view out and one set high gives sky and light only. And the covering, because the window needs whole tile or slate courses around it and a clear run of covering below it for the flashing apron to discharge onto.
On a room in the roof, the head height at the window also decides whether the space feels usable, and that is worth checking with a tape before anything is ordered.
- Opening trimmed between rafters with new headers, not simply cut through
- Whole courses of covering above and below, not awkward slivers
- Clear discharge below the window to the eaves or a valley
- Enough separation from a chimney, hip or valley for the flashing to work
The flashing kit is the whole job
Roof windows are supplied with flashing kits matched to the type of covering: one for flat interlocking or plain tiles, another for profiled tiles such as pantiles, another for slate. Using the wrong kit, or improvising with mortar and sealant, is the reason for almost every leaking roof window we are called to.
A correctly installed window sits with the apron dressed down over the covering below, side flashings interleaved with the courses either side, and the head flashing under the covering above so water passes over the top of the frame and away. There should be no visible sealant doing structural work anywhere.
The underlay also has to be dressed into the opening and connected to the window's own drainage, so any water that gets past the covering is carried out onto the slope rather than into the ceiling.
Conservation rooflights and older buildings
On listed buildings and in conservation areas, a standard roof window that stands proud of the slope will often not be acceptable. Conservation pattern rooflights sit flush with the covering, are usually smaller, and have a central vertical glazing bar and a dark metal frame that mimics the traditional cast iron rooflight.
On a listed building, adding any rooflight normally requires listed building consent, and the position, size and pattern will all be considered, typically with a strong preference for the least visible elevation. It is a conversation to have with the conservation officer before the design is fixed.
Condensation around a new roof window
The most frequent complaint after fitting is not a leak, it is water on the inside of the frame in winter. Roof windows are colder than the surrounding roof, and moist air in the room condenses on the coldest surface it finds.
The fix is usually in the lining rather than the window. The internal reveals should be formed with the sill returned horizontally and the head returned vertically, which lets air circulate over the glass rather than stagnate against it. Insulating the reveals fully and keeping the reveal free of curtains and blinds pressed against the frame both help.
In a bathroom or kitchen, the window will need help from ventilation. A roof window in a shower room without an extractor fan will show condensation, and that is a moisture problem rather than a window fault.
Common questions
Do I need planning permission for a roof window?
On many houses a rooflight that does not project significantly and does not face a highway falls within permitted development, but that varies with the property, the elevation, conservation area status and any Article 4 direction. On a listed building consent is normally required. Check with your local planning authority.
Can a roof window be fitted into a pantile roof?
Yes, using a flashing kit designed for profiled coverings. The profile of a pantile means the apron has to be dressed into the tile shape properly, so the correct kit and careful dressing matter more than they would on a flat tile roof.
Will fitting one make my loft a habitable room?
No. Adding light does not make a loft a bedroom. Habitable loft conversions need building regulations approval covering structure, fire escape, insulation and stair access. A rooflight is often part of that work, but it is not a substitute for it.
How long does fitting take?
A single window into an accessible slope is usually a short job, dependent on scaffold or access equipment and on the weather, since the roof is opened during the work. Multiple windows or a structural opening in an old roof take longer.