What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingtrussed rafter
also roof truss, fink truss, cut roof, traditional roof, gang-nail
A CUT ROOF is the site-assembled alternative and is a different thing entirely — different members, different span rules, and alterable where a trussed rafter is not.
Australia
Close, not identicalroof truss
also trussed rafter, nail plate, conventional roof, pitched roof framing
CONVENTIONAL ROOF is the site-cut alternative, and wind classification drives the tie-down detail far more explicitly than in British practice.
United Statesyours
Same word, different thingtruss
also roof truss, stick-built, rafter framing, gusset plate, truss uplift
STICK-BUILT is the site-cut alternative. TRUSS UPLIFT names a specific seasonal movement of the bottom chord that cracks ceilings and is not a defect.
Canada
Close, not identicaltruss
also roof truss, stick framing, truss uplift, heel height
As the United States, with HEEL HEIGHT — the depth at the wall plate — a routine specification because it decides whether full insulation depth fits over the wall.
The governing standard, by market
United Kingdom
BS EN 14250 / BS 5268-3 legacy
Prefabricated structural members assembled with punched metal plate fasteners, and the historic code of practice for trussed rafter roofs.
United States
ANSI/TPI 1 / BCSI
National design standard for metal plate connected wood truss construction, and the guide to good practice for handling, installing, restraining and bracing them.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Why can a truss never be cut?
- Because every member in it is carrying a calculated force, and there is no redundancy to absorb the loss of one. A truss is not a beam with decoration: it is a triangulated frame in which the top chords are in compression, the bottom chord is in tension along its whole length, and each web transfers force between them at a designed angle. Cut a web to get a water tank through, notch a bottom chord for a pipe, or trim a top chord for a rooflight, and the forces that member was carrying have nowhere to go — the frame deflects, the plates at the joints start to withdraw, and the failure is of the whole truss rather than of the piece that was cut. The bottom chord is the worst case, because a member in tension has no reserve at all and is often mistaken for a ceiling joist that happens to be there. Any alteration needs a design, which in practice means new trusses or a steel to carry the ones removed.
- What is truss uplift, and is the ceiling cracking a defect?
- It is seasonal movement, and the crack is real while the truss is fine. In a cold climate the bottom chord of a truss sits buried in loft insulation and stays at room-ish humidity, while the top chords are in the cold, dry loft air above it. Over winter the top chords lose moisture and shrink slightly, which bows the whole truss upward in the middle and lifts the bottom chord — and the ceiling fixed to it — away from the internal partition walls, which do not move. A gap opens along the top of the partition, typically a few millimetres, and closes again in summer. Nothing is overloaded and nothing is failing. The cure is detailing rather than structure: fix the ceiling boards to clips that allow the movement rather than to the partition head, and cover the junction with a coving fixed to the CEILING only, so the gap opens behind it where it cannot be seen.
- When is a cut roof still the right answer?
- When the shape is complicated, when the loft is to be used, or when the roof cannot be craned. Trussed rafters are cheaper and faster over a simple rectangular plan because the design and the cutting happen in a factory, but they fill the roof space with webs and they arrive as large, fragile assemblies that need access and lifting. A cut roof is assembled from members a person can carry up a ladder, so it suits a restricted site, a small extension or a repair; it can follow a complicated plan with hips, valleys and dormers without a bespoke truss for every position; and with a purlin and struts carried down to loadbearing walls it leaves the middle of the roof space open, which is what makes a room in the roof possible. Attic trusses exist to bridge that last gap, and they cost and weigh substantially more than a standard one.