Can two roof windows be fitted side by side?

The gap between the two windows is not left to chance. Manufacturers supply a flashing kit made for the exact configuration you are fitting, and for two windows next to each other that means a side-by-side kit with a purpose-made section for the join. The most common spacing sits at around 100mm of roof between the two frames, which is enough to run a batten and keep the covering below the glass properly supported.

Two arrangements are worth knowing. The first keeps a visible band of tile or slate between the windows, so you see two distinct openings with roofing between them. The second uses a slim aluminium bar and a shared flashing to bring the frames closer, giving something nearer a continuous run of glass with only a thin divide. Which one you can have depends on the roof covering, the window make and how the sizes work out against the rafter spacing, so it is decided on the roof rather than off a brochure.

Whichever you pick, the flashing does the same job it does on a single window. It lifts water coming down the roof and carries it around the frames and back onto the covering below, with a gutter section above the top window and an apron below the bottom edge. The vulnerable line is the vertical channel between the two units, and that is exactly the piece the combined kit is built to seal, so fitting two ordinary single flashings next to each other and hoping is the wrong move.

A single roof window narrow enough to sit inside one rafter bay often needs no structural change at all. Put two windows side by side and you are usually spanning more than one bay, which means at least one rafter falls inside the combined opening. That rafter has to be cut, and the loads it was carrying have to go somewhere.

The standard answer is a trimmed opening. The cut rafter is stopped short and tied into horizontal timbers, called trimmers, top and bottom. Those trimmers pass the load into the rafters left and right of the whole opening, which are often doubled up to take it. On a modern trussed roof the sums are stricter, because a truss is engineered as one triangle and you cannot simply saw through a member, so cutting a truss to make room for glazing needs a structural engineer to design the support before anyone touches it.

None of this is exotic, but it is the part that decides whether two side-by-side windows are straightforward or a bigger job. The width you want, the rafter spacing you already have, and whether the roof is cut timber or trussed all feed into it. It is the first thing worth checking, because it can steer you towards two narrower windows that each fit a single bay, which keeps the structural work light.

People often reach for a pair when what they really want is more glass and more light over one room. That is a real choice, and it is worth making deliberately rather than by default.

• Two windows keep the structure simpler. Two units that each fit a rafter bay, with a bay between them, may need no rafter cut at all. That can be cheaper and quicker than one wide opening that forces a trimmed structure.
• A single large unit gives an uninterrupted view. If the aim is one clean sheet of sky with no bar across it, one large rooflight or a made-to-measure unit does what two windows cannot, at the cost of more structural work and a heavier lift onto the roof.
• A pair lets you split the function. You can make one of the two open for ventilation and leave the other fixed, or put them on separate controls, which is harder to arrange in a single pane.
• Symmetry reads well from below. Two matched windows centred over a room look considered from inside, and matched from the garden, in a way a single off-centre unit sometimes does not.

If the room is a loft conversion or a pitched rear extension, this is the same decision our pitched roof skylight installation work turns on, and it is worth settling before the glass is ordered rather than after.

Here is the part that catches people out, and it is the reason we specify light before we specify units. Fitting two roof windows instead of one does not just double the daylight. On a south-facing roof it roughly doubles the solar heat coming into the room, and that is where a pair can turn a pleasant space into one that is unusable on a hot afternoon.

Southend-on-Sea faces south across the Thames Estuary, with an open horizon and light bouncing off the water, so a south-facing rear roof here collects sun from mid-morning until it swings west with nothing in the way. Two windows in that roof is a lot of glazing pointed straight at the summer sun. The fix is not fewer windows, it is the right glass. Ask for a low G-value, solar-control specification of around 0.3 or below on a south-facing pair, so the daylight comes in and most of the heat does not. On a north-facing roof you can leave the G-value higher, nearer 0.5 to 0.6, and take the free winter warmth, because the overheating risk is not there.

The rule of thumb is simple: the more glass you put in a roof, the more the orientation and the glass specification decide how the room feels. Two windows is more glass, so it raises the stakes on both. Our solar-control glazing pages set out the numbers, and the same logic applies whether the windows are new or you are replacing a pair that already runs hot.

Start with the roof, not the windows. Note which way the roof faces, whether it is cut timber or trussed, and roughly how wide a run of glass you are picturing. Those three things tell you whether two side-by-side windows are a light job or one that needs a trimmed opening, and whether the glass wants a solar-control specification.

New or enlarged rooflights are notifiable under Building Regulations, covering thermal performance under Part L and safety glazing under Part K, and on most houses a pair of windows staying within the roof plane falls under permitted development, though conservation areas and flats are the usual exceptions. It is worth checking with the local authority for your address rather than assuming.

If you would rather we worked it out, tell us the orientation, the rough opening width and whether the roof is trussed, and we will come back with a sensible pairing, a glass specification and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the two holes in it.