Roof Window Sizes and Rafter Spacing

Rafter centres run from the middle of one rafter to the middle of its neighbour. The clear gap between two rafters is smaller, because each rafter has its own thickness. On a roof with 600 mm centres and rafters 50 mm thick, the clear gap is about 550 mm. On 400 mm centres the gap is about 350 mm.

Roof windows are made in widths that suit common rafter spacings. The narrowest standard width in the VELUX range is 55 cm, the C width, and it is the size aimed at rafters set at 600 mm centres. With 50 mm rafters that leaves a clear gap of about 550 mm, the same as the frame, so whether it goes in without trimming depends on the real rafter thickness and on the side clearance the maker's installation instructions ask for. Wider windows do not fit between rafters at that spacing.

Rafters are not always evenly spaced. A roof that has been extended or opened for a dormer can have a closer pair here and a wider gap there, so the survey reads the spacing at the exact position where the window is proposed and not from one sample.

These are example figures for a roof with 50 mm rafters. The real thickness is measured, since rafters in older houses can be thicker or thinner, and the maker's installation instructions give the clearance the frame needs on either side.

A window wider than the clear gap needs at least one rafter cut and the load carried around the opening. A pair of horizontal timbers called trimmers are fixed across the opening at its top and bottom, and the cut rafter ends are fixed to those. The load that rafter carried travels sideways through the trimmers to the rafters on either side.

A window 78 cm wide, such as the M width, spans more than one gap at 600 mm centres, so one rafter is trimmed. A window 94 cm wide, the P width, also needs one trimmed: with one rafter out, the clear width across two bays in the example is about 1,150 mm. The U width, at 134 cm, is wider than that, so at 600 mm centres two rafters are trimmed. The wider the window, the more of the structure it interrupts.

Trimming also affects the room below. The cut rafter no longer runs unbroken from the wall plate to the ridge, so any lining fixed to it needs support at the cut, and the window lining is built to close the gap neatly. That work is part of the quote, along with the plasterboard and finishing around the opening.

Cutting or trimming rafters to make a new opening changes the structure of the roof, so Building Regulations approval is needed before the work, and the details are in the Building Regulations guide. At the survey we note which rafters would be cut, what size timber the trimmers need and the size of the rafters either side that take the extra load.

In some lofts there is a purlin, a long horizontal beam running the length of the roof under the rafters, or a collar tie across the rafters. Those sit in the way of some window positions. The surveyor looks for them from the inside before choosing a position.

Choosing the narrow window or a wider one

A narrow window avoids trimming but gives less light, less view and, in a loft room, a smaller opening. Where the space is a bedroom or a study, a wider unit with a trimmed rafter is a common choice. Where it lights a landing or a store, the narrow window can be enough.

Every VELUX window carries a code such as MK04. Width is given by the first letter of the code and height by its last two digits, each standing for a fixed size in the maker's range. The widths run from the narrow C up to the wide U, and the common heights are 02, 04, 06, 08 and 10. The full list of letters and numbers is in the size codes guide, and four examples show the pattern.

The width and height behind the code are the outside dimensions of the window frame. The opening in the roof has to be larger by the installation gap the maker's instructions give, and the flashing covers more again on the roof surface. We take the opening from those instructions and compare it with the rafter positions on the survey sheet.

Other makers use their own systems, with sizes given in millimetres and not codes. The principle is the same, since the width has to be reconciled with the rafters.

Height has a second influence. On a centre-pivot window the sash turns about a point part way up the frame, so the upper part of the sash swings into the room as it opens. In a low room, the height of the window and where it sits on the slope matter for headroom when the sash is open.

A roof window is cut into the slope, so its height along the rafter is not its height on a wall. The same window looks and acts differently on a shallow roof and a steep one. On a steep slope the window faces the sky less directly, with more of its light coming at an angle. On a shallow slope it faces upward and collects more of the sky.

Each model and flashing kit has a rated pitch range from the maker. A window installed outside its range is not covered by the maker's instructions. We do not quote a minimum pitch for any named product here: the range for the model you choose goes on the survey sheet next to the pitch we measure with an inclinometer. For the question in general, our answer on minimum roof pitch explains the principle.

Position on the slope affects which part of the room the window lights. A window high in the slope sends light deep into the room. A window set low in the slope lights the floor near the eaves and gives a view out. The position along the slope is chosen to suit what the room needs.

A taller window on a steep slope reaches further up the rafters, and that matters when a purlin or collar tie crosses the line. A shorter window can sit beneath the obstruction where a taller one would not. Height is therefore tested against the roof timbers as well as against the room.

• Top of the window at a height that clears the ridge board and any purlin.
• Bottom edge at a height that allows a view out from standing or seated, depending on the room.
• Clear space under the sash for furniture or a bed.

In a chalet bungalow the upstairs rooms sit partly within the roof slope, and the floor space under the lowest part of the roof is limited. The window is often the only source of light and air, so its size is not a luxury. It also may be needed for escape, and a roof window of suitable size can serve as an escape window in a room in the roof of a bungalow.

The conditions for escape are in Approved Document B. Paragraph 2.10 asks for a clear opening no smaller than 0.33 m² in area and no less than 450 mm in either direction, with the bottom of that openable area no more than 1,100 mm above the floor, and it must stay open without being held. On a sloping roof those figures are checked against the window's real opening and the floor level, which is why the escape size can push the choice up a width or a height. Loft rooms on a two-storey house cannot rely on a roof window for escape, because an escape window only serves a storey within 4.5 m of the ground, and those rooms need a protected stair instead.

This is covered in detail on its own page: roof windows for chalet bungalows. The same size and rafter points still apply, and the escape size is an additional check.

Eaves height matters here more than almost anywhere. Where the roof meets the wall at knee height, the lowest part of the slope cannot carry a window at all, and the glazing starts higher up the rafter. The window position is set by the top of the usable ceiling and by what a person can reach when the sash is opened.

One large window and two small ones can give the same light and a very different feel. Two narrow windows side by side between pairs of rafters avoid trimming and give an even spread of light, and a single wide window gives a bigger view and needs a trimmed rafter.

There is no Building Regulations daylight requirement for existing houses in England, so the number and size of windows is a design choice. A useful starting point is to look at how the room is used and where people sit or stand. Our answer on how many roof windows a loft needs works through that for common loft layouts.

If you are thinking of going bigger than the window you have now, the opening has to be recut to suit. Our answer on fitting a bigger VELUX explains what that involves, and our guide to replacing an old VELUX window covers the like-for-like case.

Pairs and groups of windows are worth a mention. Two windows set side by side with a rafter left between them can be weathered with a combination flashing the maker designs for that layout, and the maker's instructions give the spacing between frames it expects. Stacked windows, one above the other, work in a long room under a steep slope. Each of these is laid out on the survey sheet against the rafter positions.

The size is chosen after the measurements, not before them. On the visit we go into the loft first and onto the roof second.

1. Rafter centres, measured in several places because spacing can change along the roof.
2. Rafter thickness and depth, to work out the clear gap and the trimmer sizes.
3. Purlins, collar ties, wiring or pipework that cross the proposed opening.
4. Roof pitch, read with an inclinometer on the slope itself.
5. Roof covering and batten spacing, which affect the flashing kit.
6. Floor to underside of rafter at the position of the window, to check headroom and sill height.

These readings are written on the survey sheet you keep. Together they give the size options that fit your roof, the ones that need a trimmed rafter, and the ones that can be ruled out. The window page, roof window installation, shows the ranges we fit, and the next step is a written quote that names the size, the flashing kit and the structural work in the same line.