Guide · Roof Window Installation
Rooflights in a trussed roof and what cannot be cut
A trussed rafter roof will take a rooflight. What it will not take is a rooflight sized without reference to the trusses, because the timbers in a truss are not rafters and they behave nothing like them. The short version is that units are designed to fit between trusses rather than through them, which on standard 600mm spacing caps the practical width at around 550mm unless an engineer designs something else.
That single constraint governs a large share of the roof window work across Essex, because the county is full of trussed roofs. Every planned estate built from the nineteen sixties onwards uses them. This page explains what a truss is doing, which members carry what, why a saw through a web is a structural event, and what the realistic options are when the room below wants more glass than 550mm.
Telling a truss from a rafter in ten seconds
Put your head through the loft hatch and look at the shape. A traditional cut roof reads as a simple triangle: rafters running from wall plate to ridge, a ridge board at the top, ceiling joists across the bottom, and usually a purlin halfway up with struts under it. The middle of the loft is open space.
A trussed roof reads as a series of Ws. Between the sloping top members and the horizontal bottom member sits a lattice of diagonal timbers, and the same pattern repeats every 600mm down the length of the roof. The timbers are thinner than you expect, commonly 35mm or 47mm thick, and at every junction there is a galvanised toothed plate pressed into both faces. If you can see those plates, it is a truss.
What a truss is as a structure
A trussed rafter is a single engineered component that arrives on site complete. It is designed as a whole, by software, for a specific span, pitch, covering weight and wind zone, and every timber in it is sized for the force it personally carries. Nothing in it is generous. That is the entire point: a truss uses less timber than a cut roof because the geometry does the work instead.
The consequence is that a truss has no spare capacity to lend you. A cut roof is built from members that are individually oversized and it tolerates interference. A truss is a closed calculation, and removing any part of it invalidates the calculation rather than reducing it slightly.
The members, and the job each one does
Four elements make up a standard fink truss, and each behaves differently under load.
- Rafters, or top chords. The two sloping members. They carry the covering and are in compression, with bending between the points where the webs support them.
- Ceiling tie, or bottom chord. The horizontal member across the base. It carries the ceiling and the loft contents, and critically it is in tension: it is holding the two feet of the truss together against the roof’s tendency to spread.
- Webs. The diagonals in the middle. They shorten the effective span of the top chord and transfer load down into the bottom chord. Some are in compression, some in tension, depending on their angle.
- Plates. The toothed connectors at every node, pressed in under factory conditions.
Why cutting a web is not like cutting a rafter
Cut a rafter in a traditional roof and you have removed one member from a set of similar members, each carrying a similar share. Trim it properly and the neighbours take the load. The system is redundant.
Cut a web in a truss and you have not removed a share of the load, you have removed a support point from the top chord above it. That top chord was sized on the assumption of being propped at that node. Without it the chord is spanning a much greater distance than it was ever designed for, and it deflects. The failure is not dramatic and it rarely happens on the day. It shows up as a dipped roof line along one bay, cracked ceilings below, and a sash that stops closing.
The ceiling tie, and the reason it is the worst one to touch
Of all the members, the bottom chord is the one to leave entirely alone. It is in tension, which means it is actively pulling the two halves of the roof together. Every roof wants to flatten out and push its walls outwards, and the ceiling tie is what stops it.
Cut through one and the load path does not simply weaken, it disappears. The truss spreads at the feet, the pitch reduces slightly, and the load arrives at the top of the external walls as an outward push rather than a downward one. Masonry is poor at resisting that. This is the mechanism behind bulging gable walls and cracking under the wall plate, and it is why an opening in a trussed roof is planned between trusses rather than across them.
The toothed plates, and why they cannot be made again on site
The connectors are pressed into the timber by a hydraulic press applying several tonnes across the whole plate at once, with the moisture content of the timber controlled. That is not reproducible with a hammer in a loft.
This matters because it removes the obvious workaround. You cannot simply cut a web, add timber, and plate it back together. Any modification to a truss is made with new members that bypass the original geometry entirely, bolted or nailed through with a designed connection, rather than by attempting to remake the joint that was there.
Truss spacing, and the width it leaves
Trusses are almost always set at 600mm centres. Timber thickness is commonly 35mm or 47mm. That leaves a clear internal gap between adjacent trusses of roughly 553mm to 565mm, and the structural opening for a rooflight has to sit inside it.
| Truss spacing | Truss timber | Clear gap | Largest sensible unit width |
|---|---|---|---|
| 600mm | 35mm | Around 565mm | Around 550mm nominal |
| 600mm | 47mm | Around 553mm | Around 550mm nominal, tight |
| 450mm | 35mm | Around 415mm | Narrowest unit only, or a designed opening |
This is exactly why manufacturers offer a width around 550mm and why it sells in the volume it does. That size exists because of trusses.
What fits between trusses with nothing cut
Where the unit fits the gap, the work is unusually clean. No structural member is touched. The frame is supported off timber battens fixed between the two flanking trusses at the head and the sill, which carry nothing but the window itself and the small area of roof it displaces.
Length is unconstrained by the trusses, so height is where the daylight is recovered. A tall narrow unit in a trussed slope gives a surprising amount of light because the opening reaches further up and down the roof pitch, catching more of the sky at the top and delivering light deeper into the room from the bottom.
How a 550mm unit reads in a room
Narrower than people expect on paper, better than they expect in practice. The reason is that daylight through a roof is not judged by the width of the aperture but by the area of sky visible from where you stand, and a roof opening faces the whole dome rather than half of it.
A tall 550mm unit over a landing or a bedroom lights the space properly. Where it disappoints is in a wide room where a single narrow slot leaves the corners dark, and that is a spacing problem rather than a size problem.
In a trussed roof the honest question is not how wide one unit can be, but how many bays you are willing to use.
Using more than one bay instead of one wide opening
The standard answer to a wide room under a trussed roof is two units in adjacent or near adjacent bays. It costs more in flashing and internal lining than one wide unit would, and it delivers markedly better light distribution because the light arrives from two positions rather than one.
Getting the spacing, the alignment and the internal lining right is its own subject, covered on two rooflights side by side on one slope. The short version is that the gap between them is a design decision rather than whatever the trusses happen to leave.
When a truss can be modified, and who decides
Trusses are modified regularly. It is a designed operation rather than an improvised one, and the design comes from a structural engineer or from the truss manufacturer’s own technical department.
The usual approach is to install new members that take the load around the opening rather than through it. Typically that means a pair of timber or steel beams running horizontally across the top chords, picking up the trusses that are being interrupted and bearing onto trusses either side that remain whole, or onto the supporting walls. The interrupted truss then hangs off that beam. The engineer produces a drawing and a calculation, and the connections are specified down to the bolt.
What a designed trussed opening costs in programme
Not usually a fortune in materials. The cost is in time and coordination. There is a site visit by the engineer, a design period, a calculation to submit to Building Control, and materials with a lead time. The carpentry itself is a day or two.
What it changes is the decision, because the same money spent on a second and third narrow unit across three bays often buys better daylight than one wide opening in a modified structure. That comparison is worth doing before the engineer is instructed rather than after.
Attic trusses, and the roofs built ready
Not every truss is a fink. An attic truss, sometimes called a room-in-roof truss, has a clear rectangular void in the middle and heavier top and bottom chords to make it work. Estates built from the nineteen nineties onwards in Essex frequently used them, particularly where the developer offered a loft room as an option.
Where an attic truss is present, the situation is far better. The floor is already designed to carry a room, the void is usable and the vertical members either side of the void are effectively studs. The spacing still governs the width, but the rest of the conversion is much simpler. Rooms in the roof are covered on rooflights for a loft conversion.
Bracing, and the timbers that are easy to dismiss
A trussed roof is not just the trusses. Running at right angles to them is a set of bracing: diagonal braces from the ridge down to the wall plate at roughly 45 degrees, longitudinal binders along the ceiling tie, and chevron bracing on the webs of larger spans. These are thin timbers, usually 100mm by 25mm, and they look like offcuts.
They are structural. Without them the trusses are a row of dominoes with no resistance to being pushed sideways, and the whole roof relies on them for stability under wind load. Where a rooflight opening requires a brace to be interrupted, it is re-routed and refixed around the opening rather than simply removed. That is straightforward work as long as somebody recognises what the timber is.
Water tanks, services and what else lives in the bay
Trussed lofts are congested. The cold water storage tank sits on spreader boards across several trusses and its position is fixed by the plumbing. Bathroom extract ducting, soil vent pipes and cabling all thread through the webs.
All of it is movable, and none of it is free. Establishing what is in the intended bay before the size is fixed is part of the survey, because moving a tank changes the plumbing and moving a soil vent pipe changes the roof penetration alongside the new one.
Where the trussed roofs are in Essex
The pattern across the county is fairly clean. The Victorian terraces in Chelmsford, Colchester and Maldon are cut roofs. The interwar semis around Brentwood, Billericay and Westcliff are cut roofs with purlins. From the nineteen sixties the picture changes: the new town housing in Harlow and Basildon, then the later estates at Great Notley, South Woodham Ferrers, Springfield and Beaulieu Park, are trussed almost without exception.
Age is a reasonable first guess but not proof. Roofs re-covered after storm damage or fire have sometimes been rebuilt in trusses within an older shell, and some late fifties housing used early trussed forms. The loft hatch settles it in seconds and the answer changes what can be ordered.
Building Control, and the paperwork on a trussed job
A rooflight fitted between trusses with nothing cut is notifiable for the same reasons any rooflight is: thermal performance under Part L, and Part K or Part B where they apply. Sunspire handles that notification as part of the job.
Where a truss is modified, the structural calculation is submitted alongside and Building Control will want to inspect the connections before they are covered. That means the inspection point falls in the middle of the installation rather than at the end, and the programme is planned around it. Keeping the roof watertight through that period is a sequencing matter, and the covering goes back on the same day the opening is formed.
What the survey establishes in a trussed loft
Six measurements decide the whole job. The centre to centre spacing of the trusses. The thickness of the truss timber. The clear gap between two adjacent trusses at the position you want. The truss type, fink or attic or raised tie. The position of the nearest web node to the intended head and sill. And what services or storage occupy the bay.
With those six figures, the available unit sizes are known exactly and the price follows. Without them, any number quoted is an estimate of a roof nobody has looked inside, and trussed roofs are the ones where that guess goes wrong most often.
Keep reading
More on roof window installation
Soakers, aprons and head flashings explained
Read the guide →Rooflight reveals and why splaying them matters
Read the guide →Two rooflights side by side on one slope
Read the guide →Rooflights on a shallow pitch below 15 degrees
Read the guide →Underlay and sarking around a pitched rooflight
Read the guide →
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