What size roof window should I get?
Start with the room, then let the roof correct you. A glazing area of around fifteen to twenty per cent of the floor below is the sensible opening position, and then the rafter spacing, the pitch, the ceiling line and the orientation of the slope all pull that number about until you arrive at a size the roof will actually accept.
The order matters. People usually choose a size from a brochure and then ask whether it fits. Doing it the other way round, roof first, is faster and produces a better answer, because the slope you already have rules out more options than any catalogue will. What follows is the sequence used on survey, and it feeds straight into VELUX window installation.
The glazing ratio that gives you a first answer
Take the floor area of the room under the slope and multiply by fifteen to twenty per cent. A three by four metre loft bedroom is twelve square metres, so somewhere between one point eight and two point four square metres of glass.
That is a target for total glazed area, not for one unit. Split across two windows it is comfortable. In one it may be more than the rafters will give you. Treat the figure as a budget of daylight to be spent, and the rest of this page is about how the roof decides the denomination it gets spent in.
The width the rafters hand you before anything is cut
Rafters in English housing sit at four hundred or six hundred millimetre centres, occasionally four hundred and fifty on older work. Between two rafters at six hundred centres you have around five hundred and fifty millimetres of clear space, which is why so many roof window widths cluster just under that figure.
Cutting one rafter and doubling the two either side typically buys you around eleven hundred millimetres. Cutting two gets you wider again, with heavier trimming and, past a point, a structural calculation. So the width question is really a question about how many rafters you are willing to disturb, and the survey answers it with a tape in the loft rather than a guess from the ground.
Height on the slope, and the two numbers that fix it
Height is usually freer than width, because you are working along the rafters rather than across them. It is bounded at the top by the purlin, the ridge or the ceiling line, and at the bottom by the eaves detail and the head of any wall below.
The practical rule is that the head of the window wants to sit high enough that a standing adult sees sky rather than the top of the reveal, and the sill wants to sit low enough that a seated adult sees out. On a forty five degree slope with a two metre ceiling, that usually places the sill somewhere between nine hundred and eleven hundred millimetres above the finished floor.
Going taller rather than wider
Where the rafters cap the width, height is the variable that still moves. A tall narrow window in a slope delivers more useful daylight than its area suggests, because the top of the pane sees a larger part of the sky and throws light further into the room.
It also solves the seeing-out problem in one move. On a steep Victorian rear slope in Chelmsford or Colchester, a tall unit will give both a standing view and a seated view from the same opening. The limit is the purlin, and on those roofs there is usually one running through at around two thirds height that the survey needs to find first.
Splitting the area into two openings
Two smaller windows in adjacent rafter bays frequently beat one large one. The light arrives from two directions, the shadows in the room soften, and neither opening needs more than one rafter cut.
The detail that matters is the gap between them. Flashing kits for side by side installations come with a defined combination width, commonly around one hundred millimetres of rafter showing between the frames, and the two units are then weathered as a pair. Setting them out at a spacing the kit does not support is a problem discovered on the roof rather than on paper, so the setting out is done before the order goes in.
Orientation, and the size a south facing slope will take
The compass bearing of the plane changes the answer more than most people expect. A north facing slope gets steady, cool, shadowless light and almost no solar gain, and it will take a generous area with neutral glass.
A south or south west facing slope collects high summer sun straight onto the pane. At the same glazing ratio, that room overheats by mid afternoon in July. Two responses work: hold the area towards the lower end of the range, and specify solar control glass. Both are decided at order. The thinking behind the glass itself is set out under energy efficient rooflights.
Purlins, ceiling lines and what the loft allows
The survey goes into the loft before it commits to a size, and it looks for four things: the rafter depth, any purlin crossing the slope, where the finished ceiling will meet the rafters, and anything already running through the void.
Old cut roofs are full of surprises. A bathroom extract discharging into the void, a tank stand on the ceiling joists, a chimney breast rising through the space where the window wanted to go. On the trussed roofs at Great Notley, South Woodham Ferrers and Beaulieu Park the constraint is simpler and harder: the webs carry load, so the window is sized to sit between trusses unless an engineer designs otherwise.
Where a rule sets the minimum rather than daylight
Sometimes the size is not a design choice at all. Where the window serves as a means of escape from a loft conversion or an inner room, Building Regulations set a minimum clear opening, and that floor overrides any daylight calculation you have done.
The same happens where a window is the only ventilation to a room and Part F sets a minimum openable area. Both are settled at design stage rather than discovered later. The escape requirement in particular has its own detail, which is covered on whether a roof window can be an escape window.
Measuring the slope before you order
Three measurements get you a long way from the ground and a loft hatch. The clear span between two rafters, taken from face to face rather than centre to centre. The distance from the top of any wall plate to the underside of the purlin or ridge. And the pitch, taken with a spirit level and a tape rather than estimated by eye.
With those three, plus the covering type and the compass bearing, a sensible size range comes back by email. Manufacturer size codes and the standard grid behind them are a subject of their own, covered under whether roof windows come in standard sizes, and the survey confirms the final figures before anything is ordered.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements