The roof structure is where timber construction demonstrates its most compelling advantages: high strength-to-weight ratio enabling long spans with minimal material, ease of prefabrication for rapid site assembly, and the ability to create complex geometries that would be prohibitively expensive in steel or concrete. From simple domestic trussed rafters to dramatic curved glulam portal frames, timber roof systems offer solutions across the full spectrum of building types and spans.
This guide covers the design principles, member sizing, connection details, and modern fabrication methods for timber roof structures — providing the technical foundation for specifying efficient, buildable roof systems.
Roof Structure Types
Classification by Structural Action
Timber roof structures fall into three fundamental categories based on how they resist loads:
1. Coupled Roofs (Rafter-based)
- Rafters act as inclined beams, transferring load to supports at eaves and ridge
- Require horizontal restraint at eaves (ceiling tie or ring beam)
- Suitable for spans up to 5–6 m without intermediate support
2. Purlin Roofs (Beam-based)
- Purlins span between supports (walls, trusses, or frames)
- Rafters span between purlins as secondary members
- Allow longer overall spans with intermediate support
3. Triangulated Structures (Truss-based)
- Members arranged in triangulated patterns carrying axial forces
- Span the full building width without intermediate support
- Most material-efficient for spans above 5 m
Common Truss Configurations
| Truss Type | Typical Span | Pitch Range | Application |
|---|---|---|---|
| Fink (W) | 5–12 m | 22.5°–40° | Standard domestic |
| Fan | 5–11 m | 22.5°–35° | Domestic, limited storage |
| Howe | 8–15 m | 15°–30° | Commercial, industrial |
| Pratt | 8–18 m | 5°–20° | Low-pitch commercial |
| Scissor | 5–10 m | 30°–45° | Vaulted ceilings |
| Attic (room-in-roof) | 6–9 m | 35°–50° | Habitable loft space |
| Parallel chord | 8–20 m | 0°–5° | Flat/low-pitch commercial |
| Bowstring | 12–30 m | Curved | Large-span halls |
| Raised tie | 5–9 m | 30°–45° | Exposed ceiling feature |
Rafter Design
Loading on Rafters
Rafters carry load along their slope, with the following components:
| Load Type | Typical Value (kN/m²) | Applied To |
|---|---|---|
| Concrete interlocking tiles | 0.55–0.75 | Plan area |
| Clay plain tiles | 0.65–0.85 | Plan area |
| Natural slate | 0.35–0.55 | Plan area |
| Battens and felt | 0.05–0.08 | Slope area |
| Rafter self-weight | 0.05–0.15 | Slope area |
| Insulation (between rafters) | 0.02–0.05 | Slope area |
| Snow (UK zone 1, <100 m) | 0.30–0.60 | Plan area |
| Snow (Scandinavia, lowland) | 1.0–2.5 | Plan area |
| Maintenance access | 0.60 minimum | Plan area |
| Ceiling (plasterboard + skim) | 0.20–0.25 | Plan area |
Rafter Span Tables (C24, 400 mm centres)
For standard UK domestic loading (concrete tiles + 0.6 kN/m² snow):
| Rafter Size (mm) | Maximum Span — Slope (m) | Dead Load ≤ 0.75 kN/m² | Dead Load ≤ 1.0 kN/m² |
|---|---|---|---|
| 97 × 47 | 2.47 | 2.31 | 2.18 |
| 122 × 47 | 3.38 | 3.17 | 2.99 |
| 147 × 47 | 4.27 | 4.01 | 3.78 |
| 170 × 47 | 5.04 | 4.73 | 4.47 |
| 195 × 47 | 5.80 | 5.44 | 5.14 |
| 220 × 47 | 6.54 | 6.14 | 5.80 |
For 600 mm centres, reduce spans by approximately 15%. For C24 structural construction timber, these values assume service class 2 conditions with adequate ventilation.
Rafter Design Checks
Every rafter must satisfy:
- Bending — M_Ed ≤ M_Rd (applied moment ≤ resistance)
- Shear — V_Ed ≤ V_Rd (applied shear ≤ resistance)
- Deflection — δ_final ≤ L/150 (rafters) or L/250 (with brittle ceiling)
- Bearing — σ_c,90 ≤ f_c,90,d at supports
- Lateral stability — h/b ratio ≤ 5 for unrestrained rafters (or provide lateral restraint)
Deflection typically governs for longer spans, while bending governs for shorter spans with heavy coverings.
Purlin Design
Purlin Types and Applications
| Purlin Type | Material | Typical Span | Section Depth |
|---|---|---|---|
| Solid timber | C24 softwood | 2.0–4.0 m | 150–250 mm |
| Glulam | GL24h–GL28h | 4.0–8.0 m | 200–450 mm |
| LVL | Kerto-S | 3.0–7.0 m | 200–400 mm |
| Steel (for comparison) | S275 | 4.0–8.0 m | 150–250 mm |
Purlin Span Tables (GL24h Glulam)
For rafter spacing 4.0 m, standard domestic loading:
| Section (b × h mm) | Maximum Span (m) | Governing Criterion |
|---|---|---|
| 90 × 200 | 3.2 | Deflection |
| 90 × 250 | 4.1 | Deflection |
| 90 × 300 | 5.0 | Deflection |
| 115 × 300 | 5.5 | Deflection |
| 115 × 350 | 6.3 | Deflection |
| 140 × 400 | 7.8 | Bending |
| 140 × 450 | 8.5 | Bending |
For BSH glued laminated beams, these spans assume simply-supported conditions with full lateral restraint from rafters.
Ridge Beam Design
Where a ridge beam replaces the traditional ridge board (supporting rafters rather than merely connecting them), it must be designed as a beam carrying half the roof load:
- Loading: total roof load × rafter span / 2 × ridge beam span
- Typically requires glulam or LVL for spans above 3 m
- Deflection limit: L/300 (to prevent visible sag at ridge line)
- Must be supported on adequate posts or load-bearing walls at each end
Trussed Rafter Systems
Manufacturing Process
Modern trussed rafters are manufactured using:
- Design software — 3D modelling of roof geometry, automatic member sizing per EC5
- Timber selection — C24 graded softwood, typically 35 mm or 47 mm thick
- Cutting — CNC saws cut members to exact length and angle
- Assembly — members positioned in jigs on assembly tables
- Pressing — galvanised steel nail plates pressed into both faces at each joint
- Quality control — visual inspection, plate embedment check, geometry verification
- Bundling — trusses bundled for transport, maximum 12 m length for standard delivery
Nail Plate Connections
Nail plate (punched metal plate fastener) capacity depends on:
- Plate size and tooth pattern
- Timber density and moisture content
- Grain angle relative to force direction
- Number of teeth in the effective area
Typical nail plate capacities per tooth:
- Parallel to grain: 0.35–0.50 kN
- Perpendicular to grain: 0.25–0.35 kN
Plates are sized to transfer the calculated forces at each node with appropriate safety factors per EN 14250.
Truss Spacing and Bracing
| Truss Spacing | Application | Decking/Sarking |
|---|---|---|
| 400 mm | Standard domestic (UK) | Tile battens direct |
| 450 mm | Standard domestic (Ireland) | Tile battens direct |
| 600 mm | Domestic with board sarking | 18 mm OSB/plywood |
| 900 mm | Commercial with purlins | Purlins + cladding rails |
| 1200 mm | Industrial with purlins | Heavy purlins + sheeting |
Bracing requirements (per BS 5268-3 / manufacturer guidance):
- Longitudinal brace: 35 × 97 mm C16 minimum, fixed to underside of top chord at 45° from eaves to ridge
- Diagonal web brace: 35 × 97 mm, in plane of webs, every 4–5 trusses
- Lateral restraint: to all compression members at ≤ 2400 mm centres
- Gable ladder: cantilevered from first truss, maximum 600 mm overhang without additional support
Temporary Stability During Erection
Truss erection is the most dangerous phase of timber roof construction. Requirements:
- First truss must be braced back to a stable structure (scaffold, temporary props)
- Each subsequent truss must be braced to the previous one before releasing the crane
- Minimum three trusses must be erected and braced before any worker stands on them
- Temporary bracing must remain until permanent bracing and decking are complete
- Wind speed limit for erection: typically 15–20 m/s depending on truss size
Engineered Timber Roof Solutions
Glulam Portal Frames
For spans exceeding 15 m, glulam portal frames offer an elegant structural solution:
| Span Range | Frame Type | Typical Section | Knee Connection |
|---|---|---|---|
| 12–18 m | Three-pin portal | 140 × 500–700 mm | Bolted steel plates |
| 18–25 m | Two-pin portal | 180 × 700–900 mm | Moment-resisting |
| 25–35 m | Three-pin with haunch | 200 × 900–1200 mm | Haunched knee |
| 35–50 m | Arch (curved glulam) | 200 × 800–1400 mm | Pinned base |
Glulam portal frames are widely used for:
- Sports halls and swimming pools
- Agricultural buildings
- Retail and commercial spaces
- Churches and community buildings
- Exhibition halls
The curved profiles achievable with glulam allow architecturally expressive roof forms that would be impractical in solid timber or steel.
CLT Roof Panels
Cross-laminated timber (CLT) panels can function as both structure and ceiling finish:
| CLT Thickness | Maximum Span (simply supported) | Fire Resistance |
|---|---|---|
| 100 mm (3-layer) | 3.5 m | REI 30 |
| 140 mm (5-layer) | 4.5 m | REI 45 |
| 180 mm (5-layer) | 5.5 m | REI 60 |
| 220 mm (7-layer) | 6.5 m | REI 60 |
| 280 mm (7-layer) | 8.0 m | REI 90 |
CLT roof panels are discussed in detail in our CLT modular homes and timber frame vs CLT guides.
I-Joists and Metal Web Joists for Roofs
Engineered I-joists and metal web joists (Posi-joists) offer advantages for roof applications:
- Lightweight: 40–60% lighter than solid timber for equivalent span
- Long spans: up to 7–8 m without intermediate support
- Service integration: open webs allow services to pass through
- Dimensional stability: manufactured from kiln-dried components
- Consistent properties: factory quality control eliminates natural defects
Typical I-joist roof applications:
- Flat roof structures (warm deck construction)
- Low-pitch roofs where truss depth is limited
- Roofs requiring extensive service runs (mechanical ventilation ducting)
Thermal Performance and Insulation
Insulation Strategies for Timber Roofs
| Strategy | U-Value Achievable | Advantages | Limitations |
|---|---|---|---|
| Between rafters only | 0.20–0.25 W/m²K | Simple, no depth increase | Limited by rafter depth |
| Between + below rafters | 0.13–0.18 W/m²K | Good performance | Reduces headroom |
| Between + above rafters | 0.10–0.15 W/m²K | No thermal bridging | Increases roof build-up |
| Full-fill between deep I-joists | 0.10–0.13 W/m²K | Excellent performance | Requires deep section |
For passive house timber construction, roof U-values of 0.10–0.12 W/m²K are required, typically achieved with 300–400 mm total insulation thickness.
Ventilation Requirements
Cold roof construction (insulation at ceiling level):
- Minimum 50 mm clear ventilation space above insulation
- Eaves ventilation: 10 mm continuous slot or equivalent
- Ridge ventilation: 5 mm continuous slot or equivalent
- Cross-ventilation for roofs > 10 m span
Warm roof construction (insulation at rafter level):
- Vapour control layer on warm side (Sd ≥ 100 m)
- Breathable membrane on cold side (Sd ≤ 0.3 m)
- No ventilation void required if vapour control is adequate
- Counter-battens create drainage plane above membrane
Proper ventilation design prevents interstitial condensation — the primary cause of timber decay in roof structures. See timber moisture content for detailed guidance on moisture management.
Connection Details
Rafter-to-Wall Plate
| Connection Type | Uplift Capacity | Application |
|---|---|---|
| Skew nailing (3 × 3.35 × 75 mm) | 0.5–0.8 kN | Sheltered locations, light roofing |
| Framing anchor (galvanised) | 2.0–5.0 kN | Standard domestic |
| Truss clip (proprietary) | 3.0–8.0 kN | Exposed locations, heavy roofing |
| Hold-down strap (30 × 5 mm) | 8.0–12.0 kN | High wind zones, tall buildings |
Ridge Connections
- Ridge board: 25 mm board provides alignment only, no structural function
- Ridge beam: supported beam carrying rafter loads, requires bearing calculation
- Apex plate: steel connector joining opposing rafters at ridge (trussed rafters)
- Mechanical ridge: bolted connection for traditional king-post or queen-post trusses
Purlin-to-Rafter/Truss
- Joist hangers: galvanised steel, capacity 5–25 kN depending on size
- Bearing on top chord: direct bearing with anti-rotation cleats
- Bolted connection: through-bolts with timber or steel cleats
- Proprietary brackets: concealed connections for exposed timber
Roof Geometry and Drainage
Minimum Pitch Requirements
| Covering Material | Minimum Pitch | Recommended Pitch |
|---|---|---|
| Concrete interlocking tiles | 15° | 22.5°–30° |
| Clay plain tiles | 35° | 40°–50° |
| Natural slate (large format) | 20° | 25°–35° |
| Natural slate (small format) | 25° | 30°–40° |
| Cedar shingles | 14° | 20°–30° |
| Standing seam metal | 3° | 5°–15° |
| Single-ply membrane | 1.5° (flat) | 3°–5° |
| Green roof (extensive) | 2° | 5°–15° |
Valley and Hip Design
Valleys and hips create concentrated loads and complex geometry:
- Valley rafters: carry load from both roof slopes, typically 2× the section of common rafters
- Hip rafters: similar loading to valleys, plus additional length (diagonal span)
- Jack rafters: shortened rafters meeting valley or hip, require compound angle cuts
- Layboard: board fixed to side of valley rafter to support tile battens at correct angle
For complex roof geometries, 3D structural modelling is essential to capture the load paths accurately. Traditional rules of thumb (doubling up valley rafters) may be unconservative for modern heavy tile coverings.
Quality Control and Inspection
Pre-Installation Checks
Before any timber roof structure is installed, verify:
- Timber grade: CE marking visible, grade stamp matches specification
- Moisture content: ≤ 20% for structural timber (check with pin meter)
- Dimensions: within tolerance per EN 336 (±1 mm for thickness, ±2 mm for depth)
- Straightness: bow ≤ L/500, twist ≤ 2 mm/25 mm width
- Defects: no splits, shakes, or insect damage beyond grading limits
- Treatment: preservative treatment certificate where specified
- Storage: timber stored off ground, protected from rain, ventilated
For guidance on site storage practices, see timber storage on site.
During Installation
- Verify truss spacing matches design (±10 mm tolerance)
- Check bracing installed per manufacturer's layout drawing
- Confirm all nail plates fully embedded (no plate lift-off)
- Verify bearing length at supports (minimum 75 mm for trusses)
- Check temporary bracing adequate before loading roof
Post-Installation
- Confirm permanent bracing complete before removing temporary bracing
- Verify no trusses cut or modified without designer approval
- Check all connections tight and complete
- Inspect for damage during subsequent trades (plumbing, electrical penetrations)
Cost Comparison of Roof Systems
| System | Cost (relative) | Speed | Span Capability | Loft Potential |
|---|---|---|---|---|
| Trussed rafters (fink) | 1.0× | Fast | 5–12 m | None |
| Trussed rafters (attic) | 1.4× | Fast | 6–9 m | Full room |
| Traditional cut roof | 1.6× | Slow | 4–6 m | Full conversion |
| Glulam purlins + rafters | 2.0× | Medium | 6–8 m | Full conversion |
| CLT panels | 2.5× | Fast | 4–8 m | Full (flat ceiling) |
| Glulam portal frame | 3.0× | Medium | 12–35 m | N/A (open span) |
Trussed rafters remain the most cost-effective solution for standard domestic construction, but the premium for alternative systems is often justified by the additional usable space or architectural quality they provide.
Sustainability Considerations
Timber roof structures have significantly lower embodied carbon than steel or concrete alternatives:
| Material | Embodied Carbon (kgCO₂e/m² roof) | Notes |
|---|---|---|
| Timber trussed rafters | 15–25 | Including plates and bracing |
| Timber glulam frame | 20–35 | Including steel connections |
| Steel portal frame | 45–70 | Including purlins and bracing |
| Concrete frame + precast | 80–120 | Including reinforcement |
When the biogenic carbon stored in the timber is included, timber roofs can achieve net-negative embodied carbon — actively removing CO₂ from the atmosphere. See timber carbon footprint and net zero timber buildings for detailed carbon accounting.
Timber roof structures also offer excellent end-of-life options: reuse of trusses in new buildings, recycling into particleboard, or energy recovery through biomass combustion. The timber recycling and circular economy guide covers these pathways in detail.
Related Resources
- Structural Timber Guide — comprehensive timber construction overview
- C24 Timber Grading — understanding structural softwood grades
- Glulam Beam Span Tables — engineered timber sizing
- Timber Frame Insulation — thermal performance of timber structures
- Passive House Timber — achieving passive house with timber
- Timber Connections Design — fastener and joint specification
- Timber Fire Resistance — fire engineering for timber
- C24 Structural Construction Timber — structural softwood products
- BSH Glued Laminated Beams — engineered beam products