Pick the widest unit that drops between your existing rafters, then choose the height from where you want the top and bottom of the glass to sit relative to the floor. That single rule settles most domestic jobs. Width is constrained by structure, height is constrained by the roof plane and by how you use the room, and the two are decided in that order rather than together.
Rafter spacing sets the width before anything else
Roof windows are made in width bands that correspond to common rafter centres. Measure the clear gap between two adjacent rafters, not the centre to centre distance, and take the frame width from that. On a roof at 400mm centres the clear gap is typically around 350mm and the narrow width bands fit. At 600mm centres you get roughly 550mm clear and the next band up drops in.
Going wider than one bay means cutting a rafter, adding trimmers above and below, and doubling the rafters either side. That is normal carpentry and we do it regularly, but it is a decision with a cost attached, so it is worth knowing you are making it. A pair of narrow units in adjacent bays sometimes gives the same glass area with none of the structural work.
Glass area against floor area
A sensible working target for a habitable room is glazed area of somewhere between a tenth and a fifth of the floor area, weighted toward the upper end for a loft with no other window and toward the lower end for a room that already has a gable window or a dormer.
Roof glass is more efficient than that ratio suggests, because it faces open sky rather than a fence or an opposite wall. A north facing roof window still gathers steady light all day. A south facing one at the top of the range will need a solar control coating or an external awning blind, or the room will be unusable in July.
Height above the floor decides what you see
This is where most people get it wrong, because they choose from a catalogue rather than from the room. The bottom of the glass wants to be around 900mm to 1100mm above the finished floor. Below that and you are looking at your own floor from a standing position. The top of the glass wants to be at or above 2000mm so that somebody standing near the wall sees sky rather than the head of the reveal.
On a shallow pitch the sash covers a lot of horizontal distance for very little vertical rise, so a tall unit is needed to span from 900mm to 2000mm. On a steep pitch a short unit gets you there easily. Work the geometry out on the actual roof before you settle on a height code.
| Roof pitch | Approximate sash length to span 900mm to 2000mm above floor | Practical note |
|---|---|---|
| 20 degrees | Very long, often beyond stock sizes | Consider two units stacked, or a different product |
| 30 degrees | Long unit, upper end of the range | Common on 1930s roofs, plan for the reveal depth |
| 45 degrees | Mid range unit | The easiest case, most stock heights work |
| 60 degrees | Short unit | Top-hung is usually more comfortable at this pitch |
One large unit or two smaller ones
Two units spread the light across the room and remove the harsh contrast between a bright zone and a dim one. One large unit gives a better view and a stronger sense of the sky. Structurally, two narrow units in separate bays are simpler than one wide unit spanning three rafters.
Where two are fitted side by side, they need a combination flashing kit with a fixed spacing between them, and that spacing is not adjustable on site. Decide it at the order. The visual result of a pair depends heavily on getting them level with each other and symmetrical about something in the room or on the elevation.
The use of the room narrows the answer
A loft bedroom wants glass you can see out of while sitting or lying down, which pushes the cill lower and the unit longer. A bathroom wants ventilation and reach more than view, so a smaller unit positioned where you can operate it is fine. A landing or stairwell wants light only and can take a modest unit high in the slope. A home office wants daylight without glare on a screen, which usually means a north facing slope or solar control glass rather than sheer area.
Kitchens are the outlier. If you want a lot of glass over a kitchen, you are usually looking at a different product on a flat or shallow roof rather than a larger roof window.
Where an escape opening sets a minimum
If the window has to serve as the escape opening for a loft conversion, the size stops being a preference. The clear openable area must be at least 0.33 square metres with both clear height and clear width at least 450mm, and the bottom of that opening must sit no more than 1100mm above the floor. Those numbers rule out the smallest units immediately and they rule out anything mounted high in the slope.
Not every loft window needs to meet this. Only the one designated as the escape route does. The detail is on whether a roof window can be an escape window.
Reaching the handle on a tall unit
A long sash puts the top of the frame a long way up the slope. Centre-pivot units usually carry a control bar across the top of the sash, which is easy to reach on a tall window and awkward on a low one. Top-hung units put the handle at the bottom, which is easy on a low window and can be out of reach on a tall one mounted high.
If the unit is going in above a fixed obstruction, a bath, a stair or a run of units, add an electric operator at the order stage rather than fitting a pole you will lose. Retro-fitting a motor later is possible but it is a second visit and a second bill.
What the survey checks before anything is ordered
We measure the clear rafter gap, confirm the pitch with a digital level, take the height from finished floor to the proposed cill and head, check the covering and its gauge so the right flashing kit is specified, and look at what sits under the window inside. Then the size code is fixed and the order goes in. If a room needs a different size from the one you had in mind, we say so before ordering rather than after, because glass in these units is not returnable once cut.
For the broader question of proportion against the roof itself, see sizing a rooflight. We install and replace roof windows across Oxford and up to twenty five miles by road, with fifteen years in the trade, a ten-year workmanship guarantee, and VELUX, Fakro, Keylite and Roto units fitted as standard.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
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