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Structural Timber · For Architects
Trussed rafter specification combines timber grading, connector plate design, and a complete bracing strategy. Most truss failures come from inadequate site bracing rather than truss design itself. This template covers both manufactured trusses and the bracing required to make them work.
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NBS-format clause, common errors checklist, and standards reference. Editable PDF format ready for your tender documents.
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Specification Checklist
Fink, Howe, Attic, Mono, Scissor. Affects span capability and useable space.
C24 standard. C16 acceptable for short spans. Specify per EN 14081-1.
Galvanised steel nail plates per EN 14545. Specify finish (Z275 typical, A2 stainless for coastal).
TRADA Q-Mark, BSI Kitemark, or equivalent third-party certified manufacturer.
Diagonal bracing, longitudinal bracing, web bracing — all per BS 5268-3 / EN 1995-1-1.
Galvanised joist hangers sized per truss reaction loads.
600mm centres typical. Closer for heavy roof finishes (slate, concrete tiles).
No site cutting. Service holes at design stage only.
Example Clause (NBS Format)
TRUSSED RAFTERS Truss design: - Type: Fink truss for spans up to 11m, Attic truss where habitable space required - Timber: C24 to EN 14081-1, machine stress graded - Connector plates: Galvanised steel nail plates per EN 14545, Z275 coating - Spacing: 600mm centres - Manufacturer: TRADA Q-Mark certified or equivalent Loading: - Roof finish: Concrete interlocking tiles, 50 kg/m² nominal - Snow load: Per EN 1991-1-3 for site location - Wind load: Per EN 1991-1-4 for site location and exposure Bracing (CRITICAL): - Diagonal bracing: 22×97mm, fixed at 45° to lower chords across full roof - Longitudinal bracing: Continuous along ridge, web ties at 4m centres - Wall plate strapping: Galvanised straps at 1200mm centres, fixed to truss and masonry Hangers and connections: - Truss to wall plate: Galvanised truss clips, two per truss - Truss to truss: Designer's binders only — no site connections Penetrations: No site cutting permitted. All service openings, hatches, and chimney stacks specified at truss design stage. Certification: Per consignment, with truss design certificate from manufacturer's structural engineer.
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
No diagonal bracing
Correction: Diagonal bracing is mandatory per BS 5268-3. Without it, truss roofs fail in wind events.
Site cutting trusses for services
Correction: Cutting any truss member voids the design. Plan all penetrations at design stage.
Generic "trussed rafter" specification
Correction: Specify truss type (Fink, Attic, etc.) and complete loading. Manufacturer needs full data.
No certification requirement
Correction: Always require TRADA Q-Mark or equivalent. Uncertified truss manufacturers vary in quality.
Reference Standards
Frequently Asked
Fink trusses are economical to ~11m span. For longer spans (12–18m), consider Howe trusses, attic trusses with steel reinforcement, or glulam beams.
Standard Fink trusses cannot — webs cross through the storage zone. For useable roof space, specify Attic trusses or Room-in-the-Roof trusses.
Yes. Z275 galvanised is standard for internal use. For coastal locations or marine atmospheres, specify A2 stainless steel plates (~30% premium).
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