Spacing is counted in rafters, not centimetres
The roof decides the first answer: each window needs its own bay, or its own trimmed opening, with sound timber left between them.
A pitched roof is a row of rafters, and every roof window sits either between two of them or in an opening where one has been cut and trimmed. Older Cambridge houses, from the gault brick terraces of Romsey and Mill Road to 1930s semis in Cherry Hinton, commonly have rafters somewhere between about 350 and 450mm apart, and rarely perfectly even. Newer homes at places like Trumpington Meadows or Great Kneighton are usually built with trussed rafters at around 600mm centres. So the first way to describe the gap between two roof windows is simply how many rafters stand between them.
The safest layout keeps at least one full, uncut rafter between two openings. That rafter carries the roof load down to the wall plate, and the trimmers at the head and foot of each window hand their share of the cut rafters onto it and its neighbours. Where two openings are trimmed side by side with nothing whole between them, the load has further to travel and the timbers either side usually need doubling or a structural check. Our guide to trimming rafters for a roof window explains how that load is carried. On a trussed roof no member is cut without a design from the truss maker or an engineer, which often means choosing a width that slots between the trusses, as covered in fitting a roof window between rafters without cutting.
How far apart for light that looks even
Once the structure is settled, the spacing is chosen by where the light from each window lands, and the aim is for the pools of light to meet rather than leave a dim strip between them.
A roof window throws a band of daylight down and forward into the room, and on a sunny day a sharper sun patch that travels across the floor and walls as the hours pass. Two windows placed close together make one strong bright zone. Two windows spread along a slope make two zones, and whether the room reads as evenly lit depends on the gap. As a working guide, when the space between two frames is no wider than one window width, the light from each overlaps across the floor and the wall opposite. Once the gap stretches to two or three window widths, the area between them starts to feel like a separate, shadowed part of the room, which is fine over a wardrobe or a stair and poor over a desk or a sofa.
Orientation changes how much this matters. On a north slope the light is soft and diffuse, so windows can sit a little further apart without an obvious dark band. On a south, east or west slope the direct sun makes each pool brighter and its edge harder, so the gap shows more. The light planning guide sets out how each slope behaves across the day and the year, and angled linings, described in splayed reveals for roof windows, widen each pool so a pair can sit further apart.
| Layout | Typical gap | What the room gets |
|---|---|---|
| Combination pair | Set by the kit | One strong bright zone |
| One rafter between | A single bay | Near one zone, simple structure |
| About one window width | Two or so bays | Even light, pools overlap |
| Wide spread | Several bays | Two zones, dimmer middle |
Keeping clear of the ridge, eaves, hips and walls
Spacing is also about the gap between each window and the edges of the slope, because the flashing, the roof covering and the fire rules all need room.
Every flashing kit needs a run of tiles or slates above and below it so rainwater is gathered and shed properly, so a roof window is not pushed hard up under the ridge or right down onto the gutter line. The makers publish minimum distances for their kits, and the exact figure varies with the product and the roof covering. On a hipped roof the hip rafters and the shorter jack rafters leave less room to play with, which is covered in installing a roof window in a hipped roof. Chimney stacks and valleys need a clear margin too, so their own flashings and the window's do not clash.
On terraced and semi-detached houses there is a fire rule as well. Approved Document B guidance generally keeps rooflights out of a strip of roof about 1500mm either side of a party wall, so fire cannot pass across the roof to next door. On a narrow Petersfield or Romsey terrace, that strip on both sides can take a real share of the slope, and it often decides whether two windows fit at all and how far apart they can be.
When close together is the right call
Grouping windows is sometimes better than spreading them, and then the gap is fixed by the combination flashing rather than chosen freely.
A pair side by side over a bed or a desk gives a wide view and a concentrated pool of light where it is needed most. A pair stacked one above the other on a long slope gives the view at the bottom and the reach at the top. Combination kits are made for a small range of standard gaps between frames, so the rafters, the kit and the lining inside all have to agree before anything is ordered. Roof window combinations goes into the layouts in detail.
Spreading windows along the room suits a long loft space, a bedroom with a desk at one end, or a vaulted room where the light needs to reach both ends. If you are still working out how many windows a room needs, how many roof windows for a loft conversion covers the number, and positioning a roof window in a room covers sill and head height. At a survey for roof window installation in Cambridge we measure the real rafters and mark the spacing on the written light plan, so you can see where each skylight will throw its light before the roof is opened. You can book a light survey or message us on WhatsApp.
Questions people also ask
Can two roof windows share one trimmed opening?
Two windows can sit in one wide trimmed opening under a combination flashing, with a short vertical or horizontal trimmer between the frames. It works, but more rafters are cut, so the trimming has to be sized properly and, on a trussed roof, designed by the truss maker or an engineer.
How far apart should pitched roof skylights be on opposite slopes?
Windows on the front and rear slopes are independent structurally, so the question is purely about light. Facing pairs on a loft room light it from both sides and soften the glare, and lining them up across the room lets the light meet in the middle.
Does spacing affect Building Regulations approval?
Spacing itself is not a separate test, but the structure, the party wall fire strip and any escape window position are all checked. New roof windows are generally notifiable, and we handle that notification where it applies.
Is it cheaper to put windows close together?
A matching pair under one combination kit can share some of the labour, while spread windows each need their own flashing and lining. The overall cost depends on the roof, the access and the units, which our costs page sets out in general terms.
