Fire Resistance of Structural Timber: Charring Rates & Eurocode 5

Fire safety is often cited as the primary concern when specifying structural timber for multi-storey or complex buildings. Yet timber has been used in fire-resistant construction for centuries, and modern engineering methods — codified in Eurocode 5 Part 1-2 (EN 1995-1-2) — provide rigorous, predictable design approaches that allow timber structures to achieve 60, 90, or even 120 minutes of fire resistance.

This guide explains the fundamental principles of timber fire design, provides calculation methods per Eurocode 5, compares timber's fire performance with steel and concrete, and offers practical guidance for architects and engineers specifying fire-resistant timber structures.

Fundamental Principles: Why Timber Works in Fire

The Charring Mechanism

When timber is exposed to fire, the outer layers char at a predictable, well-documented rate. This char layer — composed of carbon with very low thermal conductivity — acts as an insulating blanket that protects the uncharred timber beneath.

The key insight is: the uncharred core of a timber member retains its full ambient-temperature strength and stiffness. Unlike steel, which progressively weakens as temperature rises throughout the entire cross-section, timber maintains a clear boundary between the sacrificed char layer and the fully functional structural core.

Predictability: Timber's Key Advantage

Timber's fire behaviour is:

• Predictable: Charring rates are well-established and consistent
• Slow: 0.65 mm/min means significant time before structural capacity is compromised
• Self-insulating: The char layer slows further charring
• Visible: Char depth can be measured post-fire to assess residual capacity
• Warning: Timber structures give visible and audible warning before failure (creaking, deflection)

Compare this with unprotected steel, which:

• Conducts heat rapidly throughout the entire section
• Loses strength progressively (50% at ~550°C)
• Can fail suddenly without warning
• Requires applied fire protection (intumescent paint, board, spray) to achieve any fire rating

Eurocode 5 Part 1-2: Design Framework

EN 1995-1-2 provides two primary methods for fire design of timber structures:

Method 1: Reduced Cross-Section Method (Simplified)

The most commonly used approach for standard fire resistance design:

1. Calculate the charring depth for the required fire resistance period
2. Add the zero-strength layer (7 mm)
3. Determine the residual cross-section
4. Verify that the residual section can carry the fire-condition loads

Method 2: Reduced Properties Method

An alternative approach that uses the full cross-section but applies reduction factors to strength and stiffness properties. Less commonly used in practice.

Method 3: Advanced Calculation Methods

Finite element thermal and structural analysis for complex geometries or non-standard fire exposures. Used for research and specialist applications.

Charring Rates (EN 1995-1-2)

One-Dimensional Charring (β₀)

One-dimensional charring applies to wide surfaces where charring proceeds uniformly inward:

Notional Charring (βn)

Notional charring rates account for corner rounding and fissuring effects on real cross-sections:

Charring Depth Calculation

The char depth at time t (minutes) is:

dchar,n = βn × t

For a 60-minute fire resistance requirement using solid softwood:

• dchar,n = 0.80 × 60 = 48 mm per exposed face

The effective cross-section reduction (including zero-strength layer) is:

• def = dchar,n + d₀ = 48 + 7 = 55 mm per exposed face

Worked Example: Glulam Beam Fire Design

Design Brief

• Beam span: 6.0 m
• Loading (fire condition): 8.5 kN/m (reduced from ambient 12.0 kN/m using ψ factors)
• Required fire resistance: REI 60
• Material: GL24h glulam
• Exposure: 3 sides (bottom and both sides; top protected by floor)

Step 1: Determine Charring Depth

Using notional charring rate for glulam: βn = 0.70 mm/min

Char depth at 60 minutes:

• dchar,n = 0.70 × 60 = 42 mm

Effective depth (including zero-strength layer):

• def = 42 + 7 = 49 mm per exposed face

Step 2: Calculate Residual Cross-Section

Starting section: 180 × 450 mm (b × h)

Residual section after 60 minutes:

• Residual width: 180 – (2 × 49) = 82 mm (both sides exposed)
• Residual depth: 450 – 49 = 401 mm (bottom only exposed)

Residual cross-section: 82 × 401 mm

Step 3: Verify Structural Capacity

Section modulus of residual section:

• W = (82 × 401²) / 6 = 2,197,000 mm³ = 2.197 × 10⁶ mm³

Fire-condition bending moment:

• M = (8.5 × 6.0²) / 8 = 38.25 kNm

Bending stress in residual section:

• σ = M / W = 38,250,000 / 2,197,000 = 17.4 N/mm²

GL24h characteristic bending strength: 24 N/mm² Fire-condition design strength (with kmod,fi = 1.0 and kfi = 1.15):

• fd,fi = 24 × 1.15 / 1.0 = 27.6 N/mm²

Utilisation ratio: 17.4 / 27.6 = 0.63 — PASSES

Step 4: Consider Oversizing

If the ambient design required a 140 × 400 mm section, the fire design requires oversizing to 180 × 450 mm — an increase of approximately 40 mm per exposed face plus the zero-strength layer.

Comparison with Steel and Concrete

Fire Performance Summary

The Steel Comparison in Detail

The comparison between timber and unprotected steel is particularly instructive:

Unprotected steel beam (IPE 300, S355):

• Ambient capacity: Adequate for design loads
• At 15 minutes: Steel temperature ~600°C, strength reduced to ~40%
• At 20 minutes: Steel temperature ~700°C, strength reduced to ~20%
• Failure: Sudden buckling or plastic hinge formation without warning

Timber beam (GL24h, 180 × 450 mm, 3-sided exposure):

• At 15 minutes: 10.5 mm charred per face, 97% of section intact
• At 30 minutes: 21 mm charred per face, 93% of section intact
• At 60 minutes: 42 mm charred per face, 82 × 401 mm residual — still carrying load
• Failure: Gradual deflection increase with audible warning

Fire Design Strategies

Strategy 1: Oversized Exposed Timber (Charring Method)

The most architecturally expressive approach — structural timber is left exposed and oversized to account for charring.

Advantages:

• No applied fire protection needed
• Timber aesthetic is maintained
• Simple, robust design approach
• No maintenance of fire protection systems

Typical applications:

• Glulam portal frames in sports halls and churches
• Exposed BSH glulam beams in commercial interiors
• Heavy timber roof structures
• Timber bridges and infrastructure

Strategy 2: Encapsulation (Plasterboard Protection)

Timber is protected by layers of fire-resistant plasterboard, preventing charring from starting.

Advantages:

• Smaller timber sections (no oversizing needed)
• Higher fire resistance achievable (90–120 minutes)
• Timber does not contribute to fire load
• Standard timber sections can be used

Typical applications:

• Timber frame walls (standard domestic construction)
• CLT panels with plasterboard lining
• Multi-storey timber buildings (regulatory compliance)
• Party walls and compartment boundaries

Plasterboard protection times:

Type F = fire-resistant plasterboard to EN 520

Strategy 3: Hybrid Approach

Combining exposed timber with localised encapsulation:

• Expose beams and columns (oversized for charring)
• Encapsulate CLT floor/ceiling panels (plasterboard soffit)
• Use sprinklers to extend permissible exposed timber area
• Apply intumescent coatings to specific high-risk zones

Strategy 4: Sprinkler Trade-Off

Active fire suppression (sprinklers) can be used in combination with timber fire design:

• Reduces the probability of fire reaching flashover
• May allow reduced fire resistance periods in some jurisdictions
• Enables more exposed timber in the design
• Provides redundancy in the fire safety strategy

Connections in Fire

Timber connections are often the critical element in fire design because:

• Steel plates and bolts conduct heat into the timber
• Reduced cross-sections at connections concentrate stress
• Connection failure can lead to progressive collapse

Connection Fire Design Principles

1. Recess steel plates: Embed steel plates within timber members with minimum 20 mm timber cover
2. Protect exposed steel: Cover exposed steel connectors with timber plugs or intumescent paste
3. Oversize connection timber: Provide additional sacrificial timber around connection zones
4. Use timber-to-timber connections: Dowelled or pegged connections avoid steel heat conduction
5. Design for fire-condition eccentricity: Charring may shift the neutral axis, creating additional moments at connections

Typical Connection Solutions

Regulatory Framework

Building Regulations (England & Wales)

Approved Document B sets out fire safety requirements including:

• B3: Internal fire spread (structure) — defines required fire resistance periods
• B4: External fire spread — controls external cladding and facade materials

Required fire resistance periods (load-bearing elements):

Timber structures can achieve all these ratings through appropriate design (charring method, encapsulation, or combination).

Eurocode National Annexes

Each country's National Annex to EN 1995-1-2 may modify:

• Charring rates for specific products (particularly CLT)
• Partial safety factors for fire design
• Permitted use of sprinkler trade-offs
• Requirements for connection fire design

Always check the relevant National Annex for the project jurisdiction.

Practical Specification Guidance

Specification Clause for Fire-Resistant Timber

Structural timber members shall be designed for fire resistance in
accordance with EN 1995-1-2 using the reduced cross-section method.
Minimum fire resistance period: REI 60. Charring rates shall be taken
from EN 1995-1-2 Table 3.1 or product-specific test data. All
connections shall be designed to achieve the same fire resistance
period as the connected members. Steel connection components shall be
recessed with minimum [X] mm timber cover or protected with
intumescent coating to achieve the required fire resistance.

Design Checklist

• Determine required fire resistance period from building regulations
• Select fire design strategy (charring, encapsulation, or hybrid)
• Calculate charring depths for all exposed faces
• Determine residual cross-sections
• Verify structural capacity under fire-condition loads (reduced load factors)
• Design connections for fire (recess steel, provide timber cover)
• Specify cavity barriers in all concealed spaces
• Consider sprinkler provision and its impact on design
• Coordinate with fire engineer on overall fire strategy
• Document fire design assumptions in the structural calculations

Post-Fire Assessment and Repair

One of timber's advantages is the ability to assess residual capacity after a fire event.

Assessment Methodology

1. Visual inspection: Identify extent of charring on all exposed members
2. Char depth measurement: Use a probe or knife to determine the depth of charred material
3. Residual section measurement: Calculate remaining uncharred cross-section
4. Strength verification: Compare residual section capacity against applied loads
5. Connection assessment: Check all connections for heat damage to steel components
6. Moisture assessment: Check for water damage from firefighting operations

Repair Options

Advantages Over Steel Post-Fire

After a fire, timber structures are often easier to assess and repair than steel:

• Visible damage: Char depth is directly measurable (vs hidden metallurgical changes in steel)
• Predictable residual capacity: Uncharred timber retains full strength (vs unknown residual strength in heated steel)
• Localised repair: Individual members can be replaced without affecting adjacent structure
• No hidden weakening: Steel can appear undamaged but have permanently reduced strength from heating above 600°C

Related Resources

• Structural Timber Specification Guide — comprehensive pillar guide
• Timber Frame vs CLT vs Glulam — structural system comparison including fire performance
• C24 Structural Construction Timber — standard structural softwood
• BSH Glued Laminated Beams — glulam for fire-resistant exposed structures
• Glulam Beam Span Tables — span and section data for glulam design
• C24 vs C16 vs C30 Timber Grading — strength class selection