What size rooflight fits between rafters?
Take the clear distance between two rafters, subtract about 20 to 40mm for packing and tolerance, and that is the widest frame that will drop into the bay without touching timber. On the 600mm centres of a modern trussed estate roof at Quedgeley or Bishops Cleeve, that leaves roughly 550 to 570mm clear, which is exactly why the manufacturers make a 550mm wide unit. On the 400 or 450mm centres of a Victorian terrace it leaves too little for any standard product, and the choice becomes cutting one rafter or fitting nothing. The arithmetic is short, the measuring is where people go wrong, and the length of the unit is the dimension that actually gives you the daylight. Whether an opening should be kept between rafters at all is covered at can a rooflight be fitted between rafters; this page is about the numbers. The full installation sequence is on the roof window installation page.
The width calculation, in the order it is done
Start from the rafter centres, which is the distance from the middle of one rafter to the middle of the next. Subtract the thickness of one rafter and you have the clear bay. A 47mm rafter at 600 centres gives 553mm clear. A 50mm rafter at 600 gives 550mm.
From that, take off the fitting tolerance. A frame is set with a small gap all round so it can be packed square in a roof that is rarely square, and the flashing kit and brackets need that gap to work with. Between 10 and 20mm each side is normal. So a 553mm bay comfortably takes a 550mm frame only if the timber is straight, and that is the check that gets made with a straight edge before an order goes in.
Rafter centres actually found on Gloucestershire roofs
The figures below are what turns up in this county, and they are worth knowing because the assumption that everything is at 600 causes more wrong orders than anything else.
| Roof type and where it is found | Typical centres | Clear bay | Widest standard unit |
|---|---|---|---|
| Trussed rafter estates, Quedgeley, Abbeydale, Ashchurch, Yate | 600mm | 550 to 570mm | 550mm wide |
| Interwar and post-war semis, Longlevens, Hucclecote, Warden Hill | 400 to 450mm | 350 to 400mm | none without trimming |
| Victorian terraces, Gloucester, Tewkesbury, the vale villages | 350 to 450mm, irregular | 300 to 400mm | none without trimming |
| Cotswold stone slate roofs, Painswick, Northleach, Bibury | 400 to 600mm, heavy sections | varies along the slope | measured bay by bay |
| Forest of Dean slate over sarking, Coleford, Cinderford | 400 to 500mm | 340 to 450mm | none without trimming |
| Attic trusses on newer estates | 600mm, clear rectangle by design | 550mm plus designed zone | 550mm, sometimes wider in the designed opening |
Older roofs were built to a carpenter’s judgement rather than a module, so centres wander along a slope. Measure at least four bays and take the smallest.
The narrow widths the manufacturers actually make
Roof window ranges are built on a set of fixed widths, and only the narrowest of them fits a bay without touching a rafter. In round figures the widths run 550mm, 660mm, 780mm, 940mm, 1140mm and 1340mm, each with a letter code, and the length codes run alongside them.
The 550mm width is the one designed for a 600 centre bay, and it is made in several lengths so you can gain area up the slope rather than across it. The 660mm width already needs a rafter cut on a 600 centre roof. Everything wider than that is a trimmed opening by definition. VELUX, Fakro, Keylite and Roto all publish the same broad ladder of sizes with their own code systems, and the code stamped on the sash of an existing unit is how a survey turns into an order. See VELUX window installation for how the codes read.
Length runs up the slope, and that is where the daylight comes from
Width is constrained by the timber. Length is constrained only by what is above and below, so on a between-rafters unit it is the dimension worth pushing.
Common lengths for a 550mm wide unit are around 780mm, 980mm and 1180mm. Moving from 780 to 1180 in the same bay adds well over half as much glass again for no structural change at all, no engineer and no extra rafter cut. It also raises the head of the window, which throws light further into the room, and it lowers the cill, which is what lets someone standing on the floor see out rather than only see sky.
What stops the length before the rafters do
Three things usually catch a long unit, and all three are found in the loft rather than on the roof.
A purlin crossing the slope at about mid height is the most common, and on an interwar semi it divides the slope into two zones with a usable run in each. A ceiling binder or a strut foot can do the same lower down. Above, the ridge and any ridge board set the top limit, and below, the eaves, the wall plate and the gutter line set the bottom one. There are also lap requirements: the flashing kit needs a proper run of covering above the head and below the cill to lap into, which on a double-lap plain tile roof is more than people allow for. Ridge clearance is covered at how close to the ridge a rooflight can go.
Trussed roofs, where the bay is the only option there is
On a trussed rafter roof the arithmetic stops being advice and becomes a rule. The webs and chords are part of a triangulated frame held with punched nail plates and designed as one object, so they stay whole. An opening either fits inside the 550mm clear bay with head and cill trimmers between the two trusses, or it needs a designed member carrying the affected truss off the ones either side.
That second route is achievable and it is often right for a loft conversion, but it is an engineer’s calculation, a deeper timber or a steel, and a Building Control approval before the covering comes off. The detail is on rooflights in a trussed roof.
Two narrow units against one wide one
Where the room wants more glass than one bay gives, the useful comparison is two 550mm units in adjacent bays against one wider trimmed unit.
Two narrow units need no structural design, deliver more glass in total than a single 940mm unit, and light a room more evenly because the light arrives from two positions rather than one. They also mean a shorter day and no engineer’s fee. The argument for the single wide unit is the uninterrupted view and the cleaner elevation, which on a visible slope in a conservation area can matter a great deal. Joining two units into one composition is discussed at can two rooflights be joined together.
Measuring your own centres before you enquire
You can do this in five minutes with a tape and a torch, and it makes the first reply much more useful.
- Go into the loft and find the rafters, which run up the slope, rather than the ceiling joists, which run across.
- Measure from the left edge of one rafter to the left edge of the next. That is the centre spacing.
- Repeat across four or five bays and note the smallest and the largest.
- Measure the thickness of a rafter and its depth.
- Note anything crossing the slope: a purlin, a strut, a binder, a collar, a water tank, cabling or a soil vent pipe.
- Photograph the bay you are interested in with the tape in the shot.
Send those with a photograph of the roof from outside to info@severnskylightsgloucester.co.uk. Whether cutting a rafter is the right call is covered at does fitting a rooflight weaken the roof, and what the sizing does to the figure at how much a pitched roof rooflight costs.
Next on this subject.
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