Rooflights in a trussed roof and what cannot be cut

Telling a trussed roof from a cut one

Stand in the loft and look for four things.

Trussed roofs have thin timbers, commonly 35mm or 47mm thick and often only 72mm to 97mm deep. Cut roofs have chunkier rafters, usually 50 by 100mm or heavier. Trussed roofs have diagonal braces forming a W or a fan inside the triangle. Cut roofs have an open void with a purlin running horizontally along each slope.

Trussed roofs have flat toothed steel plates pressed into the timber at every joint. Cut roofs have joints made by nails, bolts or carpentry. And trussed roofs have no ridge board worth the name, because the trusses meet at an apex plate instead.

If you see toothed plates, you have a trussed roof. In Oxford that means the house was almost certainly built after the mid 1960s, and on most estates from the 1970s onwards it is a certainty.

Why a truss behaves as one object

A cut roof is a collection of members that each carry a share of the load in bending. A truss is different. Its timbers are sized to work in tension and compression along their length, in a triangulated arrangement where each force is balanced by the ones meeting it at a node.

Bending is expensive in timber and axial force is cheap, which is why a truss can span a house on sections half the depth of a traditional rafter. It is also why the arrangement is not negotiable. Take out one diagonal and its neighbours are suddenly being asked to resist bending in a section that was never sized for it.

The failure is not dramatic. It is a gradual sag, a ceiling line that dips, cracking along a plasterboard joint, and a roof plane that develops a hollow visible from the pavement.

The members, and the job each one does

The top chords are the sloping members that carry the battens and covering. They are in compression along their length and they also bend between the points where the webs support them.

The bottom chord runs horizontally and carries the ceiling below. It is in tension, and it is tying the two feet of the truss together against spreading. That tension is why a bottom chord is the worst possible thing to cut, and it is also the one people are most tempted to cut when they want floor space.

The webs are the internal diagonals and verticals. Each is in tension or compression depending on its position, and each is doing two jobs: transferring load between the chords, and shortening the effective span of the chord it braces.

The nodes, where members meet, are where the metal plates sit and where the forces resolve.

What cannot be cut, ever

Any chord, top or bottom, at any point, for any length.

Any web member, including the ones that appear to be doing nothing where you happen to be standing.

Any part of a nail plate, including trimming a protruding corner that snags your head.

The truss must also not be notched, drilled through, or have a service chased into it. Not a small hole for a cable. Not a shallow notch to let a pipe past. The design assumes the full net section of every timber and the full engagement of every plate tooth.

None of that means a rooflight is impossible. It means the rooflight goes between trusses, or the trusses get altered by design, and there is no third option.

Nail plates and why they must be left alone

The toothed plate at each joint is not a bracket. It is the joint itself, made by pressing hardened teeth into the timber under considerable factory pressure so that force transfers through hundreds of small bearing points at once.

Its capacity depends on the whole plate being embedded to full depth across its whole area. Lift one corner and you have not lost a corner’s worth of strength; you have introduced a lever that progressively unzips the rest. Plates are damaged more often by carelessness than by cutting: a foot planted on a plate while crawling across the loft, a board dropped edge-on, a prop wedged against one.

We work off crawling boards spread across the bottom chords for exactly this reason, and we route materials up the scaffold rather than through the loft where the geometry allows.

Truss centres, and the gap you actually get

Trussed rafters are almost always set at 600mm centres. With a 35mm or 47mm truss thickness, the clear gap between them is around 553mm to 565mm.

That is not a coincidence in the rooflight world. Manufacturers produce structural widths designed to drop into that gap, with 550mm being the standard figure across VELUX, Fakro, Keylite and Roto. A 550mm wide unit, in whatever length suits the slope, will fit a standard trussed roof with no truss touched.

The length is where the flexibility is. A 550 by 980mm unit and a 550 by 1,180mm unit occupy the same bay and let in noticeably different amounts of light, and going longer is free structurally in a way that going wider never is.

Check the actual centres before ordering. Trusses drift, and a bay that measures 540mm somewhere along its length will not take a 550mm structural width.

Short trimmers between trusses are still needed

Fitting between trusses does not mean fitting with no carpentry.

The unit still needs a level bearing at head and sill, so short trimmers go in between the two adjacent top chords. Those trimmers carry only the window and the short lengths of batten stopping at them, which is a trivial load, and they are fixed with hangers or with a properly housed connection into the side of the truss.

The important word is into the side. A trimmer is fixed to the face of the top chord, never notched into it, never let in flush. Fixings are screws or nails into the timber face, well away from the nail plates, and the specification for how close a fixing may come to a plate is worth respecting.

Battens either side stop on the trimmers and the covering is cut to suit, which is the same detail as any other pitched roof installation.

When a truss genuinely has to be altered

Sometimes the position matters more than the width. A rooflight over a stairwell, a unit that has to align with something below, or a room in roof scheme wanting a larger opening.

Altering a truss is possible. It is done by the truss manufacturer’s own designer or by a structural engineer, and it produces a drawing showing the exact members to be removed, the replacement framing, the connection details and the fixing schedule. Usually that means new posts and beams installed to take over the load before anything comes out, sometimes down to a supporting wall that has to be checked in its own right.

It is not expensive as engineering goes. It is simply not something that can be improvised on the day by an installer with a reciprocating saw and confidence.

Attic trusses are a different animal

Attic trusses, sometimes called room in roof trusses, are manufactured with a clear space inside the triangle so the loft is usable from the outset. They have heavy top and bottom chords, vertical posts forming the room walls, and no diagonal webs crossing the middle.

Houses built with them, and a fair number of Oxfordshire estate homes since the 1990s were, are the easiest trussed roofs to put a rooflight into, because the sloping members are already sized for a habitable room and the space between them is open.

The rule is unchanged: the chords and posts are still designed members and still cannot be cut. But the useful bays are wider and the geometry cooperates. On some attic truss designs the manufacturer’s original drawings even show rooflight positions that were allowed for.

The shapes you meet, and what they tell you

The fink truss, with its distinctive W of webs, is the standard domestic shape for spans up to around 11 metres. Its webs cross the middle of the void, which is why fink roofs feel unusable.

The fan truss looks similar with an extra vertical, and turns up on slightly longer spans. Mono pitch trusses appear over extensions and garages. Raised tie and vaulted designs occur where a partially open ceiling was wanted.

Recognising which you have does not change the cutting rule. It does change how much of the loft is reachable, where a rooflight can be reached from inside, and how difficult the internal lining will be around the finished opening.

Bracing, the part nobody looks at

A trussed roof is only stable because of the bracing that ties the trusses to each other: longitudinal binders along the underside of the top chords, diagonal braces running from eaves to ridge across several trusses, and chevron bracing on the webs of longer spans.

Without bracing, a row of trusses is a set of playing cards. With it, the roof acts as a stiff box. Bracing is also the thing that most often gets removed by somebody boarding out a loft, because it sits exactly where you would like to walk.

A rooflight installed between trusses rarely conflicts with bracing, but where a binder crosses the intended opening it has to be rerouted rather than shortened, and the reroute has to maintain continuity across the same number of trusses.

What can and cannot be touched

Signs a previous installer took a shortcut

Look up at the truss beside an existing rooflight and check whether any web stops short and starts again on the other side of the opening. A cut web is sometimes disguised with a length of timber screwed alongside, which does not restore the connection at either node.

Look for plates with a corner lifted, or a plate on one face and nothing on the other. Look for a trimmer notched into the top chord rather than fixed to its face. Look for a bottom chord with a section removed to make a hatch, which is a serious matter regardless of the rooflight.

And look at the ceiling below in raking light for a shallow dip across the width of one or two trusses. That is the cheapest diagnostic available and it costs nothing.

Loft conversions, and why trusses often come out entirely

Where the whole loft is being converted rather than lit, the usual engineering answer is not to alter trusses but to replace the structural system.

Steel beams go in at floor level and at the eaves and ridge lines, a new floor is built off the steels, new rafters or purlins pick up the roof, and the original trusses are then cut out because they are no longer carrying anything. Once that has happened, rooflight positions become almost free, constrained only by the new structure.

If a conversion is on your horizon within a couple of years, it is worth telling us. The rooflight positions that make sense inside a converted room are not always the ones that make sense in an unconverted loft, and the order of works can save you paying twice.

What we settle at survey, and what we will decline

We measure real truss centres in three places, not one. We record truss thickness, chord depth, and the position of every web crossing the area you are interested in. We photograph the plates near the intended opening. We check the bracing runs and where they cross.

Then we tell you one of three things. The unit fits between trusses as it stands, which is the common outcome. Or a slightly different size or position makes it fit, which is the next most common. Or the opening you want requires a designed truss alteration, in which case you need a drawing before we book a date.

The narrow scope helps here. We fit and replace rooflights, so we have no incentive to talk you into structural work and no reason to pretend a constraint is not there. The four details that decide whether an installation lasts are all invisible once the plasterboard is on, and in a trussed roof the structure joins that list. That is what the ten year workmanship guarantee is standing behind.