Rafter spacing decides how wide your rooflight can be, and it decides it before style, budget or which room the window is going into. Most UK pitched roofs have rafters at 400mm or 600mm centres. Subtract the rafter thickness, usually 47 or 50mm, and a 600mm centre leaves roughly 550mm of clear space and a 400mm centre leaves roughly 350mm. That is why standard rooflight widths start at 550mm. Anything wider means cutting a rafter and supporting it with trimmers. Anything much beyond 1140mm means cutting two, which needs a structural design. And on a trussed roof, nothing gets cut at all without an engineer.
This page is about choosing the size. The site work that follows once the size is fixed, propping, cutting, trimming and load transfer, is covered separately on forming the structural opening, and how much glass the room actually needs is on daylight levels in a loft room. Those three questions get tangled together constantly, and the order matters. Work out what the room needs, then check what the roof will give you, then price the difference.
The arithmetic, done properly
Rafter centres are measured centre to centre, not between the faces. A roof described as 600mm centres has 600mm from the middle of one rafter to the middle of the next, so the clear gap is 600mm minus one rafter thickness. On modern timber that is 47mm planed, which leaves 553mm. On older roofs cut from sawn timber it might be 50mm, or 44mm, or genuinely variable across the same slope.
Then there is a second deduction. A rooflight frame needs tolerance either side for packing and adjustment, and the manufacturer publishes a required structural opening a little larger than the frame itself. So a nominal 550mm unit does not slide into a 553mm gap without thought. It usually needs a rafter face planed back or the opening formed slightly asymmetric, worked out at survey.
| Rafter centres | Rafter thickness | Clear gap | Widest unit with nothing cut | Typical roof |
|---|---|---|---|---|
| 400mm | 47mm | 353mm | None in the standard range | Long spans, older cut roofs, some conversions |
| 450mm | 47mm | 403mm | None in the standard range | Occasional Victorian and Edwardian slate roofs |
| 600mm | 47mm | 553mm | 550mm nominal, with care | Most post-war housing, most trussed roofs |
| 600mm | 38mm | 562mm | 550mm nominal, comfortably | Trussed rafter roofs, 1970s onward |
| Irregular, 380 to 520mm | Variable sawn | Variable | Measured individually | Historic and timber-framed Suffolk stock |
Clear gap is before any allowance for frame tolerance. The survey measures the actual roof rather than assuming the nominal figure.
The last row matters more here than it would in most of the country. Inland Suffolk carries a lot of timber-framed and historic roof structure where rafters were cut on site from whatever the carpenter had, spacing drifts across the slope, and members have been spliced or replaced over centuries. On those roofs there is no nominal centre. There is only what a tape measure says, taken inside the loft.
The standard width ladder, and what each rung costs you structurally
Rooflight manufacturers did not pick their widths at random. The common ladder of 550, 660, 780, 940, 1140 and 1340mm exists because those figures sit sensibly against 400 and 600mm centres with a predictable amount of trimming.
| Nominal width | Into 600mm centres | Into 400mm centres | Structural implication |
|---|---|---|---|
| 550mm | Fits between, nothing cut | One rafter cut | Trimmers only where a rafter is cut. Cheapest installation available |
| 660mm | One rafter cut | One rafter cut | Standard trimming detail, no engineer in a normal cut roof |
| 780mm | One rafter cut | One or two cut | Standard trimming, the most common size we fit |
| 940mm | One rafter cut | Two cut | One cut is routine. Two triggers a structural check |
| 1140mm | One or two cut, depending where it lands | Two or three cut | Position on the slope decides. Survey before ordering |
| 1340mm and above | Two cut, almost always | Three cut | Engineer’s calculation. Budget a few hundred pounds for it |
Read the 1140mm row again, because it is the one that catches people. Whether a 1140mm unit cuts one rafter or two is not a property of the window. It is a property of where the window lands. Slide the same unit 200mm along the slope and it can go from one cut to two, and from a routine trimming detail to a job that needs a calculation and a Building Control conversation. At survey we measure the actual rafter positions and then move the opening, sometimes by as little as 150mm, to land it on one rafter instead of two. That single decision is often worth several hundred pounds, and it almost never harms the room.
Cut roof or trussed roof: the question that changes everything
Everything above assumes a traditional cut roof, where rafters run from wall plate to ridge and are supported by purlins, struts and a ridge board. Cut one rafter, put trimmers in, and the load finds its way out through the rafters either side. That is a well understood detail.
A trussed rafter roof is a completely different structure. Prefabricated triangulated frames, each one engineered as a whole, with thin timbers, usually 35 to 47mm, joined by pressed steel gang-nail plates. Every member is in the design because it is carrying something, including the ones that look like they are just holding the others apart. Cutting any part of a truss without a structural design is not a shortcut. It is a defect.
How to tell which you have, from inside the loft, standing on the joists with a torch:
- Trussed: W-shaped or fan-shaped webs filling the roof void, flat metal plates with rows of spikes at every joint, thin uniform timbers at a consistent 600mm centre, no purlins, and a loft space you cannot easily walk through.
- Cut: Open triangular void, chunky rafters, a horizontal purlin partway up the slope carrying them, struts running down to a load-bearing wall, joints that are notched and nailed rather than plated.
- Attic truss: A trussed roof designed from new with a clear room-shaped void in the middle. Common on newer estate housing. Still a truss, still not cuttable without design, but the openings are often already allowed for.
Locally this splits along fairly clean lines. Victorian and Edwardian terraces across central Ipswich, the Norwich Road streets and the villages are cut roofs. The 1960s to early 1970s estate housing at Chantry, Whitton and Rushmere St Andrew is usually cut. Everything from roughly 1970 onward, and all of the new-build growth at Grange Farm, Sproughton, Martlesham, Ravenswood and Stanway, is trussed. We check at survey rather than guessing from the build date, because plenty of older houses have had a trussed roof put on after a fire, a re-roof or an extension.
Designing around a trussed roof
A trussed roof does not mean no rooflight. It means the size question gets answered differently, and there are four routes.
- Fit between the trusses. A 550mm nominal unit will usually drop into the clear space between trusses at 600mm centres with nothing cut and no design required. This is the fastest, cheapest and least disruptive rooflight available on a modern house. If the roof is trussed and you are chasing daylight rather than a view, take this option seriously before you spend money on a structural engineer.
- Fit two or three narrow units instead of one wide one. Three 550mm units spaced across a slope give you more glazed area than a single 1140mm unit, deliver better light distribution across the room, and still cut nothing. They cost more in labour and flashing kits than one unit, less than one unit plus a structural design and the trimming work that follows.
- Go taller instead of wider. Rooflight length runs up the slope, parallel to the rafters, so it is limited by purlins, ridge and eaves rather than by rafter centres. A 550 x 1400mm unit has more glass than a 780 x 980mm one and does not touch a truss. This is the single most useful trick in the whole subject and it is covered in more depth on the daylight page, because window height is also what drives light deep into a room.
- Get the truss designed. An engineer specifies how the cut truss is supported, usually by doubling the trusses either side and forming a trimmed opening off them. It is entirely doable. It costs a few hundred pounds for the calculation plus the additional timber and labour, and it is what a proper loft conversion does as a matter of course.
The question to ask any installer quoting on a modern house. Is my roof trussed, and does your price include a structural design for cutting it? A quote that neither mentions trusses nor includes a design, on a house built after 1970, has either not looked in the loft or is planning to cut something it should not. Ask which.
A worked example: a 1970s loft at Kesgrave
A 1970s detached house, trussed roof, 600mm centres, 38mm members. The owner wants daylight into a 4.2 by 3.6 metre loft room and has priced a 1140 x 1180mm unit at £2,600 to £3,600 fitted.
The survey finds that unit needs two trusses cut. So the real figure is that price plus an engineer’s calculation, plus doubled trusses either side, plus the extra day of work, and a Building Control application that would not otherwise have been needed.
The alternative alongside it: two 550 x 1400mm units spaced roughly 2 metres apart, nothing cut, no engineer. Slightly more glazed area than the single wide unit, light reaching both ends of the room instead of pooling in the middle, and a fitted cost close to the published range for two units on the same slope.
We show both and let the owner choose. Plenty of people still want the single wide window because it frames a view, which is a perfectly good reason to spend the money. What is not acceptable is finding out about the second option after the first has been ordered.
Where the opening lands on the slope
Width is decided by rafters. Position is decided by three other things, and they interact with the size you have chosen.
- Purlins. On a cut roof the purlin runs horizontally partway up the slope, and a tall unit can collide with it. Either the opening sits above or below the purlin, or the purlin needs structural attention, which is a bigger job than trimming a rafter.
- Ridge and eaves clearance. Keep roughly 600mm clear of both. Nearer than that and the trimming detail gets awkward, the flashing has less covering to discharge onto, and an engineer’s view is usually wanted.
- Valleys, hips and party walls. An opening close to any of these changes how load travels. On a terrace with a party wall close to the proposed opening, that is an early structural conversation rather than a late one.
The other constraint is what the room needs the window to do. Sill height governs whether you get a view when seated, head height governs how far light travels into the room, and a Part B escape requirement fixes a minimum clear opening that can rule out the size you first wanted. Escape sizing is covered on our loft conversion rooflight page, and it is worth checking before you fall in love with a size, because it constrains the choice harder than the rafters do.
What we do at survey
Every quote for a new opening starts inside the loft, not on the roof. We measure the actual rafter or truss centres at three points rather than one, because a single measurement will happily read 600mm on a roof that drifts to 640mm two bays along. We record the timber section, identify whether the structure is cut or trussed, find the purlins, and check clearances to ridge, eaves, valleys and party walls.
Then we come back with sizes rather than a size. Usually the one that fits with nothing cut, the one that cuts a single rafter, and where it genuinely adds something, the larger one with the structural work priced in. Each with a fixed figure against it.
Nobody needs to go up on the roof for any of this, and we would not ask you to. A photograph of the loft interior taken from the hatch, showing the rafters or trusses, tells us most of what we need before we visit.
Common questions about rafter spacing and rooflight sizes
How do I measure my rafter spacing?
From inside the loft, with a torch and a tape. Measure from the centre of one rafter to the centre of the next, and do it in at least three places across the area where you want the window. Most modern roofs read 600mm and most will be consistent. Older Suffolk roofs frequently are not, and a roof that measures 600mm at the hatch can be at 520mm eight bays along. Also note the thickness of the timber, because the clear gap is the centre dimension minus that thickness. Do not go onto the roof to measure anything.
Can a rooflight be fitted without cutting any rafters?
Yes, if you accept a 550mm nominal width and your rafters are at 600mm centres. That gives roughly 553mm of clear space, which a 550mm unit will take with the frame tolerance worked out at survey. It is the quickest and cheapest installation available and it removes the structural question entirely. At 400mm centres there is no standard unit narrow enough, so a cut is unavoidable. Height is not restricted the same way, so a narrow tall unit is usually the better answer than a narrow short one.
Why can I not just cut a truss and put a trimmer in?
Because a trussed rafter is engineered as a single triangulated unit, not as a rafter with extra bits attached. Load travels around the whole frame, and the gang-nail plates at the joints are designed for that specific arrangement of forces. Remove a member and the remaining ones are carrying loads they were never sized for, often without any visible sign for years. The correct route is a structural engineer’s design, which normally doubles the trusses either side and forms a trimmed opening off them. It is a routine piece of design work, it just has to happen before anyone picks up a saw.
Does rafter spacing limit how tall the rooflight can be?
No. Rooflight length runs up the slope, parallel to the rafters, so it is limited by purlins, ridge and eaves rather than by rafter centres. That is why a taller unit is so often the right answer on a constrained roof. On a cut roof the usual obstacle is the purlin partway up the slope. Keep roughly 600mm clear of ridge and eaves as well, and check the head height you end up with, because that is what drives how far daylight travels into the room.
Get the size right before you order
Send us the address, whether it is a new opening or a replacement, and roughly what the room is for. We will get into the loft, measure the actual rafter or truss positions rather than the nominal ones, and come back with two or three sizes and a fixed price against each. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them. More on the whole subject is on our pitched roof skylight installation page.
Request a fixed quote for a rooflight in Ipswich, Suffolk or north Essex, and we will tell you what your rafters will actually take before you commit to a size.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.