Glulam — glued laminated timber — represents the convergence of engineering precision and architectural expression. Where solid timber is constrained by the dimensions of individual trees, glulam transcends these limits through the lamination of multiple boards into members of virtually any size, shape, and curvature. The result is a structural material that can span concert halls, curve through airport terminals, and create the kind of spatial drama that defines memorable architecture.
This guide explores glulam's design possibilities from both structural and aesthetic perspectives, providing architects with the technical knowledge to specify glulam confidently for exposed applications where the structure is the architecture.
Understanding Glulam
Manufacturing Process
Glulam is manufactured by bonding multiple timber laminations (typically 33–45 mm thick) with structural adhesive under controlled conditions:
- Lamination preparation: kiln-dried boards (12% MC) finger-jointed to required length
- Adhesive application: melamine-urea-formaldehyde (MUF) or polyurethane (PUR) applied to faces
- Assembly: laminations stacked in press jigs (straight or curved)
- Pressing: hydraulic pressure applied (0.6–1.0 N/mm²) during cure
- Curing: 6–24 hours depending on adhesive and temperature
- Finishing: planing to final dimensions, CNC machining of connections
- Quality control: delamination testing, visual inspection, dimensional check
Structural Grades
| Grade | fm,g,k (N/mm²) | E0,g,mean (N/mm²) | Lamination Grade | Application |
|---|---|---|---|---|
| GL20h | 20 | 8,400 | C20 | Light structural |
| GL22h | 22 | 10,500 | C22 | General |
| GL24h | 24 | 11,500 | C24 | Standard structural |
| GL26h | 26 | 12,100 | C27 | Enhanced |
| GL28h | 28 | 12,600 | C30 | High-performance |
| GL30h | 30 | 13,600 | C35 | Premium |
| GL32h | 32 | 14,200 | C40 | Specialist |
For most architectural applications, GL24h provides the optimal balance of performance, availability, and visual-grade material supply. Our BSH glued laminated beams are available in GL24h and GL28h grades.
Size Capabilities
| Parameter | Typical Range | Maximum (specialist) |
|---|---|---|
| Width | 90–240 mm | 280 mm |
| Depth | 200–2,000 mm | 2,500 mm+ |
| Length (straight) | 6–18 m | 40 m+ |
| Length (curved) | 6–30 m | 50 m+ |
| Minimum curve radius | 3–8 m | Depends on lamination thickness |
| Maximum transport length | 12 m (standard road) | 30 m+ (special transport) |
Architectural Profiles
Straight Beams
The simplest and most economical glulam form. Straight beams are used for:
- Floor beams (residential and commercial)
- Roof purlins and ridge beams
- Lintels and transfer beams
- Colonnade beams and canopy structures
Design considerations for exposed straight beams:
- Depth-to-width ratio: 3:1 to 6:1 for visual proportion
- Minimum width 115 mm for single-sided fire exposure
- Chamfered or rounded arrises (edges) for visual softness
- Consider pre-camber for spans > 8 m (counteracts dead-load deflection)
Tapered Beams
Beams with varying depth along their length, typically deeper at mid-span (where bending moment is maximum) and shallower at supports:
| Profile | Application | Advantage |
|---|---|---|
| Single taper | Mono-pitch roofs | Follows roof slope |
| Double taper (apex) | Pitched roofs | Maximum depth at ridge |
| Haunch (deepened at ends) | Portal frames | Depth where moment is highest |
| Fish-belly (deeper at centre) | Long-span floors | Efficient material use |
Tapered beams reduce material consumption by 15–25% compared to constant-depth beams for the same span and load, while creating a more dynamic visual profile.
Curved Beams
Curving is glulam's signature capability — enabling forms impossible in solid timber:
Single-radius curves:
- Arches (parabolic, circular, pointed)
- Portal frame knees
- Curved roof beams
- Minimum radius: 150–200× lamination thickness
Variable-radius curves:
- Free-form profiles
- Organic shapes
- Complex architectural geometries
- Require CNC-controlled press jigs
Design rules for curved glulam:
| Lamination Thickness | Minimum Radius | Typical Application |
|---|---|---|
| 40 mm (standard) | 6–8 m | Large arches, gentle curves |
| 33 mm | 5–6.5 m | Medium curves |
| 27 mm | 4–5.5 m | Tighter curves |
| 20 mm | 3–4 m | Tight curves, small radii |
| 13 mm | 2–2.5 m | Very tight curves (specialist) |
Stress considerations: bending laminations to a curve induces residual stresses that reduce the beam's load-carrying capacity. EN 14080 requires a reduction factor for curved members:
kcurve = 1 − (2000 × t/r)²
Where t = lamination thickness and r = radius of curvature. For standard 40 mm laminations at 8 m radius: kcurve = 0.99 (negligible reduction). At 4 m radius with 20 mm laminations: kcurve = 0.98.
Portal Frames
Glulam portal frames combine columns and rafters into continuous curved or haunched frames:
| Frame Type | Span Range | Typical Section | Application |
|---|---|---|---|
| Three-pin (curved knee) | 12–25 m | 140 × 500–900 mm | Sports halls, retail |
| Two-pin (moment knee) | 15–30 m | 180 × 700–1200 mm | Industrial, agricultural |
| Tied portal | 10–20 m | 140 × 400–700 mm | Low-eaves buildings |
| Tudor arch | 15–35 m | 200 × 800–1400 mm | Churches, halls |
Portal frames create dramatic clear-span interiors with the structural logic expressed in the curved timber form — architecture and structure unified.
Visual Quality Specification
Appearance Classes
Most glulam manufacturers offer three visual quality levels:
Industrial (non-visible):
- Finger joints visible on all faces
- Knots, colour variation, and minor defects permitted
- Suitable for concealed structural applications
- Lowest cost
Standard visual (Sichtqualität / visible quality):
- Finger joints permitted but well-executed
- Small sound knots permitted (≤ 30 mm)
- Moderate colour variation accepted
- Suitable for most exposed applications
- Standard cost
Premium visual (Auslesequalität / select quality):
- No finger joints on specified visible faces
- Minimal knots (≤ 15 mm, sound only)
- Uniform colour, matched laminations
- For high-end architectural exposure
- 20–40% cost premium
Surface Finish Options
| Finish | Process | Texture | Application |
|---|---|---|---|
| Planed (standard) | Machine planing | Smooth, 1–2 mm tolerance | Most exposed applications |
| Fine-sanded | 120–180 grit sanding | Very smooth | Premium interiors |
| Brushed | Wire brushing | Textured (raised grain) | Feature elements |
| Sand-blasted | Abrasive blasting | Deep texture | Industrial aesthetic |
| Charred (Shou Sugi Ban) | Flame treatment | Carbonised surface | External, dramatic |
| Oiled | Penetrating oil | Enhanced grain | Warm interior finish |
Colour and Species Selection
| Species | Colour (fresh) | Colour (aged) | Grain Character |
|---|---|---|---|
| Norway spruce | Pale cream-white | Warm honey | Fine, even |
| Scots pine | Cream with pink tones | Golden amber | Visible resin canals |
| European larch | Warm reddish-brown | Silver-grey (external) | Bold, distinctive |
| Douglas fir | Salmon-pink to red-brown | Warm brown | Strong grain contrast |
| European oak | Pale golden | Deep golden-brown | Bold, cathedral grain |
For projects requiring consistent colour across multiple beams, specify single-species laminations from the same source region and request colour-matching during production.
Connection Design for Exposed Glulam
Concealed Connections
The preferred approach for architectural glulam — all metalwork hidden within the timber:
Slotted-in steel plates:
- Steel plate (8–20 mm thick) inserted into CNC-machined slot
- Dowels or bolts pass through plate and timber
- Timber plugs cover dowel holes on visible faces
- Achieves clean timber-to-timber appearance
Glued-in rods:
- Threaded steel rods bonded into pre-drilled holes with structural epoxy
- Completely invisible from outside
- Very high capacity in compact connection
- Requires factory installation (controlled conditions for gluing)
Concealed hangers:
- Proprietary concealed beam hangers (e.g., Simpson, Rothoblaas)
- Installed in CNC-machined recesses
- Covered by timber infill pieces
- Suitable for beam-to-beam and beam-to-column connections
Expressed Connections
Where metalwork is celebrated as an architectural feature:
Steel brackets and plates:
- Visible steel plates, often with expressed bolt heads
- Industrial/contemporary aesthetic
- Easier to inspect and maintain
- Can be detailed as sculptural elements
Cast steel nodes:
- Custom-cast steel connectors for complex geometries
- Used in tree-column structures and branching forms
- High cost but unique architectural expression
- Requires specialist foundry work
Connection Design Considerations
| Factor | Concealed | Expressed | Traditional Joinery |
|---|---|---|---|
| Aesthetic | Clean timber | Industrial/modern | Craft/heritage |
| Fire performance | Good (timber protects steel) | Requires protection | Excellent (no steel) |
| Cost | High (CNC machining) | Medium | Very high (hand-crafted) |
| Inspection | Difficult | Easy | Moderate |
| Capacity | High | High | Limited |
| Fabrication | Factory (CNC) | Factory or site | Specialist workshop |
For detailed connection engineering, see timber connections design.
Span Tables and Sizing
Preliminary Sizing Guide (GL24h, uniformly loaded)
| Span (m) | Width (mm) | Depth (mm) | UDL Capacity (kN/m) | Governing Criterion |
|---|---|---|---|---|
| 6 | 115 | 300 | 8.5 | Bending |
| 8 | 140 | 400 | 9.2 | Deflection |
| 10 | 140 | 500 | 8.8 | Deflection |
| 12 | 160 | 600 | 9.5 | Deflection |
| 15 | 180 | 750 | 9.0 | Deflection |
| 18 | 200 | 900 | 8.5 | Deflection |
| 20 | 220 | 1000 | 8.2 | Deflection |
| 25 | 240 | 1200 | 7.8 | Deflection |
Note: these are indicative values for preliminary design. Final sizing requires project-specific calculation considering actual loads, support conditions, fire requirements, and lateral stability. For detailed span tables, see glulam beam span tables.
Pre-Camber
For spans exceeding 8 m, pre-camber (upward curvature built into the beam during manufacture) is recommended to counteract dead-load deflection:
| Span | Recommended Pre-camber | Purpose |
|---|---|---|
| 8–12 m | L/500 to L/400 | Counteract dead load deflection |
| 12–18 m | L/400 to L/300 | Visible sag prevention |
| 18–25 m | L/300 to L/250 | Essential for visual quality |
| > 25 m | Project-specific calculation | Critical for appearance |
Pre-camber is achieved by manufacturing the beam with a slight upward curve — the dead load then deflects it to approximately level. This is standard practice for exposed beams where any visible sag would be architecturally unacceptable.
Durability and Protection
Internal Applications (Service Class 1)
Glulam in heated, dry interiors requires no treatment:
- Moisture content remains stable at 8–12%
- No risk of fungal decay or insect attack
- Surface treatment is aesthetic only (oil, wax, or lacquer)
- Design life: unlimited (if conditions maintained)
Semi-Exposed Applications (Service Class 2)
Covered external structures (canopies, covered walkways, open-sided buildings):
- Moisture content may reach 20% periodically
- Low decay risk if well-ventilated
- Consider preservative treatment for critical members
- Design life: 50+ years with appropriate detailing
Fully Exposed Applications (Service Class 3)
External glulam exposed to weather:
- Requires either naturally durable species (larch, oak) or preservative treatment
- End-grain protection essential (sealed with end-grain sealer)
- Drainage details to prevent water pooling
- Regular maintenance of surface treatment
- Design life: 25–50 years depending on exposure and maintenance
Detailing principles for external glulam:
- Slope all horizontal surfaces (minimum 5°) for drainage
- Protect end grain with metal caps or generous overhangs
- Raise base of columns above ground level (minimum 300 mm)
- Provide ventilation around all timber surfaces
- Use stainless steel fixings (A4 grade minimum)
- Apply end-grain sealer to all cut surfaces
Case Studies
Swimming Pool Roof — Curved Glulam Arches
- Span: 28 m clear
- Profile: Parabolic arch, GL28h larch
- Section: 200 × 1,100 mm at crown, 200 × 1,400 mm at springing
- Finish: Premium visual, oiled
- Environment: High humidity (pool hall) — larch selected for durability
- Connections: Concealed steel pins at base, bolted crown connection
Residential Extension — Exposed Ridge Beam
- Span: 7.5 m
- Profile: Straight beam, GL24h spruce
- Section: 140 × 450 mm with 15 mm pre-camber
- Finish: Standard visual, clear oil
- Fire: REI 30 achieved with existing section (no oversizing needed)
- Connections: Concealed hangers at wall plates, expressed steel at ridge
Commercial Atrium — Tree Columns
- Height: 12 m
- Profile: Branching columns (trunk + 4 branches)
- Material: GL28h Douglas fir
- Section: 300 × 300 mm trunk, 200 × 200 mm branches
- Finish: Premium visual, wax finish
- Connections: Cast steel nodes at branch points
- Effect: Biophilic design — structural trees in interior space
Procurement and Programme
Lead Times
| Item | Typical Lead Time | Notes |
|---|---|---|
| Standard straight beams (stock sizes) | 2–4 weeks | From manufacturer stock |
| Custom straight beams | 4–8 weeks | Made to order |
| Curved beams (standard radius) | 6–12 weeks | Custom jig required |
| Complex curved/tapered beams | 10–16 weeks | Design + manufacture |
| Premium visual grade | Add 2–4 weeks | Material selection time |
| Large projects (>50 m³) | 8–16 weeks | Production scheduling |
For procurement planning, see timber lead times in Europe.
Cost Factors
| Factor | Impact on Cost | Notes |
|---|---|---|
| Strength class (GL24h → GL28h) | +10–15% | Higher-grade laminations |
| Visual grade (standard → premium) | +20–40% | Material selection, waste |
| Curved profile | +30–80% | Jig cost, thinner laminations |
| Species (spruce → larch → oak) | +20–100% | Raw material cost |
| CNC machining (connections) | +15–25% | Programming + machine time |
| Surface treatment (factory-applied) | +5–15% | Labour + materials |
| Transport (>12 m length) | +20–50% | Special vehicle required |
Specification Template
A complete glulam specification for architectural applications:
GLULAM BEAM SPECIFICATION
─────────────────────────
Strength class: GL24h per EN 14080
Species: Norway spruce (Picea abies)
Visual grade: Standard visual (no finger joints on bottom face)
Surface finish: Planed all round, arrises chamfered 5 mm
Moisture content: 12% ±2% at delivery
Dimensions: 140 × 500 × 8,400 mm (w × d × L)
Pre-camber: 12 mm upward at mid-span
Fire requirement: REI 60 (three-sided exposure)
Treatment: None (internal application, SC1)
Certification: FSC Chain of Custody required
Connections: Concealed slotted steel plates (see structural drawings)
Finish: Factory-applied clear hard wax oil (one coat)
Delivery: Protected from weather, stored flat on bearers
Related Resources
- Glulam Beam Span Tables — detailed sizing guidance
- Timber Connections Design — connection engineering
- Timber Fire Resistance — fire design for exposed timber
- Biophilic Design Timber — timber in interior design
- Timber Facade Design — external timber applications
- Structural Timber Guide — comprehensive overview
- Timber Lead Times Europe — procurement planning
- BSH Glued Laminated Beams — glulam products
- C24 Structural Construction Timber — lamination material