Mass timber char depth calculator

What a timber char depth calculator does

Exposed mass timber resists fire by charring at a predictable rate. The char layer insulates the wood behind it, so a beam, column or CLT panel keeps a residual core that can still carry load. This calculator works out how deep the char goes after a given fire exposure, adds the allowance for heat-weakened wood, and reports the residual dimensions and how much of the area, section modulus and stiffness remains.

Two published methods are built in. NDS Chapter 16, explained in AWC Technical Report 10, uses a nominal char rate of 1.5 in/h, a non-linear char depth a_char = βn × t^0.813, and an effective char depth 20 % deeper; for CLT it lets each lamination fall off when the char front reaches its glueline. EN 1995-1-2 multiplies a design charring rate from its Table 3.1 by time and adds a 7 mm zero-strength layer. Imperial defaults to NDS and metric to EN 1995-1-2; you can use either in either unit system.

The result is a section, not a fire rating. Whether the residual member carries its load in fire depends on the design load, the fire design adjustment factors, connections and any protection, which the method's structural checks and the engineer of record cover.

How to use the mass timber char depth calculator

1. Choose Imperial or Metric. Imperial starts with the NDS method and an 8¾ × 24 in glulam beam; metric starts with EN 1995-1-2 and a 240 × 600 mm beam.
2. Pick the Method and the Member: a beam or column, a CLT panel, or a solid panel such as NLT, DLT or GLT charred from one face.
3. Enter the Fire exposure time (min), usually the required fire-resistance rating: 60, 90 or 120 minutes.
4. For a beam or column, choose the Exposed sides and enter the Member width and Member depth. For a panel, enter the Panel thickness, and for CLT the CLT lamination thickness.
5. For EN 1995-1-2, choose the Timber product (EN 1995-1-2 Table 3.1) or enter custom rates. For NDS, keep the Nominal char rate βn at 1.5 in/h unless your product data give another value.
6. Read the char depth, the effective char depth, the residual dimensions and the percentages retained, then download the PDF, CSV or the SVG section diagram.

Worked example

Imperial: An 8¾ × 24 in glulam beam exposed on three sides for one hour chars 1.5 in (a_eff 1.8 in) by NDS Chapter 16, leaving 5.15 × 22.2 in: 54.4% of the area and 50.4% of the section modulus.

Metric: A 240 × 600 mm softwood glulam beam exposed on three sides for 60 minutes chars 42 mm (d_ef 49 mm) by EN 1995-1-2, leaving 142 × 551 mm: 54.3% of the area and 49.9% of the section modulus.

Char depth and effective char depth (NDS Chapter 16)

NDS Chapter 16 uses a non-linear char model because charring slows as the insulating char layer thickens. With βn = 1.5 in/h for sawn lumber, glulam, structural composite lumber and CLT, the char depth after t hours is a_char = 1.5 × t^0.813 in. The effective char depth adds 20 % for the zone of heated, weakened wood and corner rounding: a_eff = 1.2 × a_char. Each exposed face loses a_eff, so a beam exposed on three sides loses 2 × a_eff from its width and a_eff from its depth.

IBC Section 722 points to NDS Chapter 16 for calculating the fire resistance of exposed wood members, so these are the depths most US mass timber designs start from.

Charring rates in EN 1995-1-2 (Eurocode 5)

EN 1995-1-2 gives two design charring rates. β0 is the one-dimensional rate for a face with no exposed corners, such as a slab soffit. βn is a larger notional rate that folds in the corner rounding of a beam or column exposed on three or four sides. The reduced cross-section method then adds a zero-strength layer k0 × d0, with d0 = 7 mm and k0 rising from 0 to 1 over the first 20 minutes, so d_ef = βn × t + 7 mm for any exposure of 20 minutes or more.

The second-generation revision of EN 1995-1-2 changes several of these values and adds rules for CLT; the calculator uses the 2004 edition.

CLT char depth and lamination fall-off

Full-scale fire tests of CLT showed char layers falling away as the char front approached a glueline, exposing fresh wood that then charred quickly again. AWC TR 10 models this with the time to reach each glueline, t_gi = (h_lam ÷ βt)^1.23 hours, the number of laminations that fall off, n_lam = t ÷ t_gi rounded down, and a_char = n_lam × h_lam + βt × (t − n_lam × t_gi)^0.813. Thinner laminations fall off sooner, so they char deeper.

PRG 320-2018 added an elevated-temperature adhesive test intended to stop this fall-off, but the stepped model remains the conservative NDS approach. The thickness retained is not a strength ratio for CLT: which layers remain, and whether they run parallel to the span, decides the residual capacity.

Worked example: a glulam beam exposed on three sides

Take an 8¾ × 24 in glulam beam exposed on its soffit and both sides for one hour under NDS. a_char = 1.5 × 1^0.813 = 1.5 in and a_eff = 1.8 in. The residual width is 8.75 − 2 × 1.8 = 5.15 in and the residual depth 24 − 1.8 = 22.2 in, so 54.4 % of the area, 50.4 % of the section modulus and 46.6 % of the moment of inertia remain.

In metric with EN 1995-1-2, a 240 × 600 mm softwood glulam beam for 60 minutes chars 0.70 × 60 = 42 mm, and d_ef = 42 + 7 = 49 mm. The residual section is 142 × 551 mm: 54.3 % of the area and 49.9 % of the section modulus. The structural fire check then compares that residual section, with fire-design strength factors, against the load present during the fire.

Who it is for and when to use it

Method, formulas & assumptions

NDS Chapter 16 (AWC TR 10): a_char = βn × t^0.813 (βn = 1.5 in/h, t in hours), a_eff = 1.2 × a_char; for CLT with equal laminations, t_gi = (h_lam ÷ βn)^1.23, n_lam = ⌊t ÷ t_gi⌋ and a_char = n_lam × h_lam + βn × (t − n_lam × t_gi)^0.813. EN 1995-1-2: d_char = βn × t with exposed corners (3 or 4 sides) or β0 × t without (Table 3.1 rates), d_ef = d_char + k0 × 7 mm, k0 = min(1, t ÷ 20 min). Each exposed face loses the effective depth; area, strong-axis section modulus and moment of inertia retained are ratios of the residual to the original rectangle.

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Using the char depth instead of the effective char depth. The extra heated-zone allowance (20 % in NDS, 7 mm in EN 1995-1-2) must come off the section too.
• Forgetting exposed faces. A beam that looks protected on top may be exposed if the deck above is not continuous or gaps are not sealed.
• Treating CLT thickness retained as strength retained. Cross layers carry little bending; model the remaining layup.
• Mixing methods: applying EN charring rates with NDS strength factors, or the reverse.
• Ignoring connections. Steel plates and fasteners usually need more cover or protection than the member itself.

Scope & limits

Section properties only: it does not check load capacity in fire, connections, gypsum or encapsulation protection, or the fire-resistance rating, which need the method's structural fire design or a tested assembly. EN 1995-1-2:2004 has no CLT fall-off rule, so CLT is charred one-dimensionally under that method. Rates are published design values for unprotected members; product approvals and the engineer of record govern.

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