Crane lifting prefabricated timber roof trusses onto a new-build UK house

Roof trusses, explained

Fink, king post and attic trusses vs traditional cut roofs: how your roof structure works, and the one rule you must never break.

Quick answer: roof trusses are factory-engineered timber frames that form the skeleton of most UK roofs built since the late 1960s. The common domestic type is the fink truss with its W-shaped webs; attic trusses leave a clear room-shaped void for liveable lofts. Every part of a truss is structural: never cut one without an engineer's design.

Look up in a modern loft and you will see the truss system at work: identical triangulated frames at 600mm centres, joined by slim punched-metal plates at every node. Each frame is engineered as a complete unit, designed by software, pressed together in a factory and lifted on in hours. It is why a whole estate's roofs can be structurally complete in a week.

The common truss types

Type	Shape	Where you find it
Fink	W-shaped webs	The standard truss on most UK homes since the 1960s
King post	Single central post	Short spans: garages, porches, extensions
Attic (room-in-roof)	Clear central void	New builds and extensions planned with a loft room
Raised tie	Lifted bottom chord	Vaulted ceilings and extra headroom

Trussed roof vs traditional cut roof

A cut roof is built on site, stick by stick: carpenters cut rafters, ridge boards, purlins and ceiling joists from loose timber. Pre-war and Victorian homes are almost all cut roofs, and the open space between purlins is why their lofts convert so readily. Trussed roofs are cheaper, faster and use less timber, but they fill the loft with webs and rely on every member staying exactly where the engineer put it. Neither is better; they are different systems with different rules, and the rules matter most when you start changing things.

The one rule: never cut a truss

Red flag: cutting truss webs to make loft storage or a DIY conversion, without an engineered design, is one of the most common causes of structural roof damage in modern UK homes. The roof rarely fails on the day: it sags and spreads over months as loads find paths the timber was never sized for. If a previous owner has been in your loft with a saw, or your ridge line is dipping, see our sagging roof guide and get a structural assessment quickly.

Every element of a truss is load-bearing, including the slender diagonals that look expendable. Even drilling and notching has limits. If you want storage in a trussed loft, the safe route is a purpose-made raised storage floor that spans over the bottom chords without loading the webs, fitted to a design, not a Saturday with a jigsaw.

Attic trusses and loft conversions

If a new build or extension might ever need a loft room, specify attic trusses at design stage: the room-shaped void is engineered in from day one and the cost uplift is modest against doing it later. Converting an existing fink-trussed roof is possible but properly structural: webs come out progressively while new steel or timber beams take over, all designed by an engineer and signed off under building regulations. It is routinely several thousand pounds more than the equivalent conversion in a cut roof.

What trusses cost

Truss packages are priced per design by the manufacturer: span, pitch, spacing, loading (heavy concrete tiles need more than lightweight steel) and truss type all move the number, and attic trusses cost several times a plain fink. In a whole re-roof or new build, the truss package is usually a modest slice of the total next to coverings, labour and scaffold. Get the design priced properly through free quotes rather than budgeting from generic per-truss figures.

Worried about your roof structure?

Dipping ridge lines, doors sticking upstairs, cracked ceilings below the loft or visibly altered timbers are all reasons to look closer. A professional survey will confirm whether the structure has been compromised and what the fix involves; book one through our roof inspections service.

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Roof truss FAQs

Truss questions, answered

A trussed roof uses factory-made triangulated frames, engineered as complete units and craned or lifted into place, typically at 600mm spacing. A cut roof is built stick by stick on site from rafters, purlins and ceiling joists by carpenters. Most UK homes built since the late 1960s have trussed roofs; older homes usually have cut roofs.

No. Every member of a truss is structural, including the thin diagonal webs that look like they are in the way. Cutting any of them without an engineered design redistributes loads the roof was never designed for, and the result is a sagging or spreading roof, sometimes months later. Any alteration needs a structural engineer's design first.

An attic truss (or room-in-roof truss) is engineered with a clear rectangular void in the middle, so the loft can be a habitable room without webs crossing the space. Specified on a new build or extension, it makes a future loft room straightforward. Converting a standard fink-trussed roof to open space later is far more invasive and expensive.

Usually yes, but it is an engineered structural job, not a clear-out: the webs are removed progressively while new beams, purlins and floor joists take over the loads, all to a structural engineer's design with building regulations sign-off. It costs meaningfully more than converting a cut roof, which already has open space between purlins.

Trusses are priced per design by truss manufacturers: span, pitch, spacing, loading and type (fink, attic, raised tie) all change the figure, and attic trusses cost several times a standard fink. For a whole project the truss package is usually a modest slice of the roof budget next to coverings and labour, so get the design quoted rather than working from generic figures.