Can you fit a roof window between rafters?

A pitched roof is held up by a row of rafters running from ridge to eaves at a set spacing. The space between any two of them is a bay. Fitting a window into a single bay means the rafters either side stay whole and keep doing their job, so nothing structural changes. The window frame fixes to the rafters and to a pair of small timbers at the head and sill, and that is the whole of the carpentry.

The moment the window is wider than one bay, the picture changes. Now a rafter stands in the way of the glass, and it has to be removed across the height of the opening. That rafter was carrying its share of the roof, so the load it was taking has to be handed to the rafters on either side. This is done with trimmers, which are horizontal timbers fixed across the top and bottom of the opening, and by doubling up the rafters that flank it. Both jobs put a window in a pitched roof. Only the first leaves the structure alone.

Rafters in most houses sit at 400mm or 600mm centres, meaning that spacing is measured from the middle of one rafter to the middle of the next. A rafter is around 47 to 50mm wide, so the clear gap you actually have to play with is smaller than the centres figure suggests.

This is why manufacturers publish a narrow width in their range. A window near 550mm across is designed to land in a 600mm bay with a few millimetres of tolerance each side for the frame and the fixing. Ask for the structural opening size rather than the glass size, because the opening you have to form is always larger than the pane you see, and it is the opening that has to fit the gap.

One caution on older houses. A roof built by hand a century ago will not have the even spacing a modern trussed roof does, and the timbers may have moved. Do not assume a bay is 600mm because the house looks the part. Put a tape across every bay you are considering, because a 40mm difference is the difference between a window that drops in and one that needs a rafter cut.

Anything from a wide single window upwards will cross a rafter, and then the opening has to be trimmed properly. This is ordinary, well-understood carpentry, but it is structural work and it is worth knowing what a good job looks like.

• Trimmers top and bottom. Horizontal timbers close off the head and sill of the opening and pick up the cut rafter, passing its load sideways.
• Doubled trimming rafters. The two rafters framing the opening now carry extra load, so they are usually doubled, meaning a second rafter is fixed alongside each to share it.
• Sized to the span. On a wide lantern-style pitched opening, or where more than one rafter is cut, the trimmer sizes should be worked out for the span rather than guessed. This is Part A of the Building Regulations, the structural part, and it is the same principle as forming an opening for a loft-conversion stair.

Cutting more than one rafter is where a straightforward install turns into a genuinely structural one, and it is the point at which you want the sizing checked rather than eyeballed. The rest of the pitched-roof method, from the opening through the flashing to the finish, sits on our pitched roof skylight installation page.

The width decides whether you cut a rafter. The depth of the rafter decides something else entirely: how much insulation fits around the finished opening. A shallow rafter, common in older roofs, leaves little room for insulation once the window liner and the ventilation gap are accounted for, and a cold bridge round a rooflight is where condensation shows up first. If the rafters are shallow, the reveals around the window may need insulated liner board to keep the surfaces warm.

The pitch matters too. A roof window is designed to be fitted within a range of pitches, often somewhere between about 15 and 90 degrees, and a very shallow pitch may need a specific unit or a kerb to lift it. Check the window is rated for the pitch you have before anything is cut.

Before you settle on where the window goes, look at which way that slope faces, because in Southend it changes what should go in the opening. The city faces south across the Thames Estuary with an open horizon and no inland hill or terrace to shade it, so a south-facing pitch collects sun from mid-morning until the afternoon through the whole warm season. A roof window put into that slope with plain glass will pour heat and glare into the room below, and the reflected light off the water only adds to it.

The fix is not structural, it is in the glass. On a south or west-facing pitch, specify solar-control glazing with a low G-value, so the unit lets the daylight in while turning much of the solar heat away. On a north-facing slope you can leave that off and take the free winter warmth instead. The rafter bay tells you whether the window fits. The compass tells you what glass belongs in it, and that is the part worth getting right, covered on our energy-efficient skylight installation page.

Start with a tape measure. Find the rafters, measure the clear gap between them and note the depth of the timber, then measure the width of window you want. If your window is narrower than the bay, you have a single-bay job and the structure is untouched. If it is wider, plan for a rafter to be cut and the opening trimmed, and have the sizing checked on anything beyond one rafter. Either way, a new or enlarged rooflight is notifiable under the Building Regulations, so that has to be handled rather than skipped.

If you would rather not work it out from the loft, tell us the rough opening you have in mind and which way the roof faces, and we will confirm whether it sits between the rafters or needs trimming, and what glass suits the aspect. Ring 01702 898232 or request a quote, and we will look at the whole slope rather than just the gap.