What size roof window do I need?

The honest starting point is the floor plan, not the roof. Daylight from a roof window falls more evenly than daylight from a wall window, because the opening looks up at open sky rather than across at whatever is opposite, so a smaller area of glass goes further than it would in a vertical wall. That is why the 10 to 20 per cent guide sits lower than the figure you might use for a normal window.

Where you land inside that band depends on the job the room does. A bedroom used mostly at night is comfortable near the bottom of the range. A kitchen, a home office or a converted loft that doubles as a workspace wants the upper end, because those are daytime rooms and you notice every dull corner. A deep room needs its glass split rather than piled into one opening: two windows a metre or so apart light the near and far walls, where a single large unit throws a bright pool under itself and leaves the ends grey.

Height matters as much as area. So that a standing adult can see out and the sky reaches the back wall, the top of the glass ideally sits about two metres above the floor, and the bottom low enough to see the ground from a chair, usually somewhere between 900mm and 1100mm. On a shallow pitch you get that reach by choosing a taller unit; on a steep pitch a shorter one covers the same span of view.

Roof windows are made in a fixed grid of sizes rather than cut to order, so in practice you are picking the nearest standard unit and letting the size drive the opening. The common widths and heights look like this:

Widths tend to run in steps of roughly 110 to 200mm and heights in similar jumps, and every maker uses its own code letters for them. When you compare a quote against a brochure, work in the actual millimetres rather than the codes, because the letters do not match across brands. If your specification is built around a particular range, our notes on VELUX window installation set out how those size codes read.

The roof sets limits the room plan cannot argue with. Rafters usually sit at 400mm or 600mm centres, and a window wider than that gap needs one or more rafters cut and the load carried around the opening with trimmers. That is ordinary structural work, but it is the difference between a straightforward fit between two rafters and a framed opening, and it steers many people towards a tall narrow unit rather than a short wide one.

Position on the slope counts too. A window needs to sit clear of the ridge at the top and clear of the eaves at the bottom, with enough tile or slate above and below to form the flashing and shed water properly, so the usable band on a short roof plane is narrower than the plane looks. On a loft conversion there is also the matter of escape: a room designated for sleeping generally needs at least one openable window big enough to climb out of, with a clear opening around 450mm each way and roughly a third of a square metre of area, positioned so the bottom of the opening is within reach of the floor. Treat that as a floor for the size, not a target, and check it with your local authority for the specific room.

Size and orientation cannot be decided separately, and this is where a lot of otherwise sensible plans come undone. Every extra square metre of glass collects daylight in winter and solar heat in summer, and on a roof facing south across the estuary the summer side of that bargain is the one that bites. A large window that makes a north-facing room feel bright and calm can turn a south-facing loft into a room nobody uses between June and September.

The fix is not always a smaller window, because you still want the daylight. More often it is the right size paired with the right glass: as the area goes up, the G-value of the glazing wants to come down, so a big south-facing opening is specified with solar-control glass that lets the light through while turning back much of the heat. Getting that balance right for a sloping roof is the heart of pitched roof skylight installation, and the reasoning runs right through our energy-efficient skylight installation work. Decide the size and the orientation together, then choose the glass to match, rather than sizing the hole and hoping.

You can do most of this yourself in ten minutes. Measure the room, take 10 to 20 per cent of the floor area for a daytime space and a little less for a bedroom, and that is your target glass area. Note which way the slope faces and roughly how much clear run there is between ridge and eaves. Decide whether one window or a pair suits the shape of the room, and check the rafter spacing if you can see it in the loft.

That gives you a size, an orientation and a rough position, which is everything needed to price the job properly and to specify the glass. If you would rather hand that part over, tell us the room dimensions and which way the roof faces and we will come back with a size, a glass specification and a figure. Ring 01702 898232 or request a quote, and we will size it to the room and the sun rather than to the first standard unit that fits the gap.