What is a roof truss?

A roof truss (or trussed rafter) is a factory-made structural unit comprising timber rafters, a bottom chord (ceiling joist), and internal web members, all connected at their joints by galvanised pressed steel nail plates. The triangulated form distributes loads through the members in pure tension and compression, allowing smaller timber sections than an equivalent cut roof. Trusses are engineered for the specific span, loading, and pitch of each project and are delivered to site ready to lift and fix to the wall plates.

What are the different types of roof truss?

The Fink truss (W-type) is the most common for simple pitched roofs - its W-shaped web efficiently carries loads from both slopes. The Howe truss has a different web pattern suited to longer spans. The attic (room-in-roof) truss has a raised central section to create usable space within the roof void, with vertical studwork framing the room. The scissors truss creates a vaulted internal ceiling while maintaining a pitched roof externally. Mono-pitch trusses are used for lean-to roofs and extensions. Girder trusses are multiple trusses bolted together to carry the end loads of hip sets.

Can you convert a trussed rafter loft into a room?

Yes, but a trussed rafter loft cannot simply have the internal webs cut out to create space - the trusses must be completely replaced with a purpose-designed structural system. A loft conversion in a house with trussed rafters typically requires steel beams (ridge beam and lateral beams), new rafter pairs, and a new floor structure, all designed by a structural engineer. The existing trusses must remain in place and be propped while the new structure is built around and above them. Building Regulations approval is always required for a loft conversion.

How are roof trusses fixed in place?

Trusses are lifted onto the wall plates at the specified truss centres (typically 600mm) and fixed with proprietary galvanised truss clips nailed to both the truss bottom chord and the wall plate. Trusses must be braced diagonally before loading to prevent racking - longitudinal binders (typically 25x100mm timbers) are fixed diagonally across the top chords and vertically through the web members to the bottom chords. Bracing is a critical structural requirement - an unbraced trussed roof is unstable and several collapses have occurred during construction from inadequate temporary and permanent bracing.

Roofing noun

Roof Truss

/ ruːf trʌs /

Also known as: trussed rafter, fink truss, attic truss, W-truss, engineered roof truss

Definition

A roof truss is a factory-manufactured structural unit of timber members connected by galvanised nail plates, forming a triangulated frame that spans the full width of a building to carry roof and ceiling loads. The triangulated geometry converts loads into pure tension and compression in the individual members, allowing efficient use of small timber sections. Trusses are engineered for each project and delivered to site ready to install - they are the standard roof structure in modern UK housebuilding, replacing the traditional cut roof.

In practice

The most common type is the Fink (or W-type) truss for standard pitched roofs. Trusses are craned or manually lifted onto the wall plates at 600mm centres and fixed with galvanised truss clips. Diagonal and longitudinal bracing is fitted immediately to stabilise the assembly - typically 25x100mm binders diagonally across the top chords and vertically through the web members. Without adequate bracing, an unloaded trussed roof is unstable and can collapse during construction.

Attic trusses incorporate a raised central opening (framed with vertical studs) to create a habitable room within the roof - they are significantly heavier and more expensive than standard fink trusses but allow an additional storey without planning permission for an extension in many cases. The bottom chord of any truss must not be cut, notched, or drilled without manufacturer approval - the bottom chord is a critical tension member and damage will cause collapse. Any plumbing, wiring, or storage requirements must be accommodated within the truss design from the outset.

Building Regulations

Relevant PartPart A - Structure

Design standardBS EN 14250 - Timber structures, prefabricated roof trusses

BracingBS 8000-5 - Workmanship, carpentry and joinery

Loft conversionRequires full structural redesign and Building Regs approval

Roof trusses are engineered products supplied with a structural design certificate. Approved Document A requires them to be installed strictly in accordance with that design - including bracing details, truss clips, and any special fixings. Alterations to trusses (cutting webs or the bottom chord) are structural alterations requiring Building Regulations approval and a new structural design. Loft conversions of trussed rafter roofs always require Building Regulations approval and must be designed by a structural engineer.

Full Building Regulations guidance

Roof Truss

Just so you know...