What size rooflight fits between rafters?

The gap is the number, not the spacing

Rafter spacing is quoted centre to centre, so a roof at 600mm centres does not give you 600mm of space. Subtract the width of one rafter and you have the clear gap. A 47mm modern rafter leaves 553mm. A 50mm one leaves 550mm. An old 75mm timber at nominally 600mm centres leaves 525mm.

Measure it at the point on the slope where the unit will sit, with a tape held square across the two rafter faces, and take the smallest of three readings up the bay rather than the first one you get.

What rafter spacings you find around Oxford

Post-war and modern housing is generally at 600mm centres with sawn softwood rafters of 47 or 50mm, and trussed rafter roofs from the 1970s onward are commonly at 600mm centres too. Interwar semis and 1930s bay-fronted houses often run at 450mm. Victorian and earlier roofs are set out by eye and by what the timber allowed, so anything from 350mm to 500mm turns up, sometimes within the same slope.

Stone-built cottages are a category of their own. Heavy coverings need close spacing, so the bays are frequently tight and the timber is often deeper and wider than anything modern.

Standard widths, and the gap each one needs

The common frame widths across the main ranges we install are these. The opening column allows a working tolerance; the exact figure is published per model and is what we order to.

Read the middle column carefully. It is the reason the honest answer to this question is narrower than people expect.

The tolerance that catches people out

A 550mm frame does not go into a 553mm gap. The opening has to be wider than the frame so the unit can be set square, packed, plumbed and fixed, and so the insulation collar has somewhere to sit. Depending on the model that allowance is roughly 20 to 45mm in total, split between the two sides.

Forcing a frame into a gap without it produces a unit that is square to the rafters instead of square to the world, sashes that catch as they close, and no room for the sub-frame packing. It also leaves nowhere for the underlay collar to be dressed, which is a weathering problem rather than a fitting one.

Old roofs measure differently in every bay

On a house built before the war, the bay you measure is the bay you get, and the one next to it may be 40mm different. Rafters are often hand-cut, sometimes waney on one edge, occasionally twisted, and not always parallel from eaves to ridge.

So the measurement is taken at the head and the foot of the proposed opening as well as the middle. A bay that is 480mm at the bottom and 440mm at the top will not take a unit sized off the generous reading. This is also why an inspection from inside the loft is part of the survey rather than an optional extra.

Height is set by the slope, not by the rafters

Rafters constrain width. They do nothing to height, so the vertical dimension is where you buy back daylight when the width is capped.

Standard heights run from roughly 700mm up to 1600mm. What limits them is the slope: how far the head can go before it runs out of covering below the ridge, how low the foot can go before it meets the eaves or a purlin, and the pitch, since a shallower roof needs more height to give the same view and the same light. Going taller within a single rafter bay is the cheapest extra daylight available, because nothing structural changes.

The daylight you give up to avoid a cut

It is worth knowing the size of the compromise. A 550mm by 980mm unit gives roughly half a square metre of glass. A 780mm by 1400mm unit gives around one square metre. That is double the light for one cut rafter and two trimmers.

In a loft bedroom or a converted attic, that difference is the difference between a room with a window and a room that feels lit. In a landing, a stairwell or a small bathroom, the narrow unit is often exactly right and there is no reason to cut anything.

When cutting a rafter is the better answer

Forming a wider opening is ordinary work on a rafter-and-purlin roof: the rafter is cut, trimmers span between the neighbours at head and foot, the cut ends bear on the trimmers, and the load takes a short detour. Building Control is notified and the detail is inspected.

Two situations change that. A trussed rafter roof cannot have members cut without a designed alteration, and on a heavy stone or thick clay covering the loading warrants more care about how the trimmers are sized. Our page on trimming rafters covers the detail, and whether fitting a rooflight weakens the roof answers the worry behind the question.

How the size gets settled on site

At survey we measure the clear gap in the specific bay, the rafter depth, the pitch on that slope, the distance to the ridge and to the eaves, and the head height inside the room. Those five numbers give a shortlist of units that genuinely fit, usually two or three, and then the choice is about light, reach and cost rather than about what is possible.

What we will not do is order a unit off a measurement taken through the ceiling. Fifteen years installing and replacing rooflights around Oxford has produced a firm rule about that: measure the timber you can see, in the bay the unit is going into. The roof is closed the same day it is opened, and the workmanship carries a ten-year guarantee. If you have the loft measurements already, email them to info@heritageskylightsoxford.co.uk and we will tell you what fits.