Mass Timber Fire Safety: What Actually Happens, and How We Design for It

Timber burns. It's the first thing most people say about mass timber, and it's a fair place to start. Anyone who has watched a campfire knows wood burns.

But mass timber fire safety isn't about whether timber can burn. It's about how thick timber behaves when it does, and whether engineers can predict that well enough to design for it. With care, they can, and the details matter. Here's what we know, where the limits are, and what we'd ask your fire engineer.

What happens when thick timber burns

A cross-laminated timber (CLT) panel or glulam beam doesn't go up like kindling. When fire reaches its surface, the outer layer chars. That char is a layer of carbon that insulates the timber behind it. The burn slows as the char deepens, and the unburnt core keeps its strength. WoodSolutions, Australia's timber industry guidance, calls the char layer the key contributor to timber's fire resistance and notes that charring is fast at first, then slows.

Engineers turn that into numbers. A char rate is how quickly timber is lost to charring, measured as depth over time. Design guidance models burning at a constant rate and adds a thin "zero-strength layer" behind the char line to allow for heated timber that has lost strength. In Australia, AS/NZS 1720.4:2019 sets out charring rates for solid timber. The designer then adds enough extra section so the timber still carries its load for the fire resistance level (FRL) the building needs. The FRL is the number of minutes an element must keep doing its job in a standard fire test.

Predictable, but not simple

"Predictable" doesn't mean identical every time. A 2018 meta-analysis of the fire-resistance literature found that charring rates shift by up to about 18% across the density and moisture ranges expected in design. It also found that delamination, where glued layers separate, and failure of protective linings can change charring in ways that are hard to predict.

Large-scale tests sharpen the picture. In six compartment fire tests at University College London, using CLT rooms with different amounts of exposed timber, a fire died out on its own only when the char stayed in place longer than the fuel took to burn out, and the heat left in the room afterwards was low enough. A 2024 study of CLT residential rooms found that exposed timber walls add heat in proportion to the area exposed. It also found that a partial lining that falls off mid-fire may make self-extinguishment less likely, not more.

So the lesson isn't "timber is fine" or "timber is dangerous". How much timber is exposed, how linings behave and how connections are detailed all change the outcome. In tests by the US Forest Products Laboratory, encapsulated floor-to-wall connections developed less char than exposed ones. That detail work is engineering.

Exposed or encapsulated?

Encapsulation means covering timber with a protective lining, usually fire-rated plasterboard, so the timber takes longer to become involved in a fire. If the lining fails, charring starts and then slows as char builds. Exposed timber has no lining, so its surface is part of the fire from the start. Designers allow for that in the section size and the amount of timber on show.

What the National Construction Code says, in plain terms

The National Construction Code (NCC) offers two routes. A deemed-to-satisfy (DTS) pathway is a prescriptive recipe: follow it and you comply. A performance solution is an alternative in which a fire engineer shows the design meets the code's performance requirements.

For timber, the DTS pathway for fire-protected timber, meaning timber covered with fire-protective linings, was introduced in 2016 for apartments, hotels and offices. The 2019 edition extended it to all building classes, up to 25 metres effective height, about eight storeys. NCC 2022 sets out the requirements in Specification 10, including alternative protection for massive timber. The pathway pairs the linings with compliant sprinklers.

Victoria adopted NCC 2025 on 1 May 2026, so the clause details your project follows depend on the edition in force and your building surveyor. Leaving timber exposed, or going beyond those limits, typically means a performance solution and a fire engineer. WoodSolutions Technical Design Guides 17 and 18 explain how both pathways work.

Timber is one layer of the fire strategy

Mass timber fire safety never rests on char alone. Sprinklers, protective linings and the timber's own charring work together, and a fire engineer weighs how. The code work behind Australia's timber pathway modelled what happens if sprinklers fail, to check people could still get out.

Construction needs its own plan too. Timber is often unprotected until the linings go in, so fire precautions during construction matter. WoodSolutions Technical Design Guide 37R includes a section on them.

Questions to ask your fire engineer

Fire design is project-specific. These are questions to ask, not answers. Your fire engineer and building surveyor decide what applies to your building.

• Are we on the deemed-to-satisfy pathway or a performance solution?

• Which timber surfaces are exposed, and how much?

• What char rate and char fall-off assumptions are in the design?

• How are connections and joints protected?

• How do sprinklers and the rest of the fire strategy work with the timber?

• What fire precautions apply during construction, before linings go on?

• Who signs this off: the fire engineer, the building surveyor, or both?

See it for yourself

Mass timber is easier to judge when you can see it. Mass Timber Central, Australia's first dedicated mass timber collaboration and education space, is at 8 Bank Place in Melbourne's CBD. It's open Monday, Tuesday and Thursday, 10am to 3pm. Come and look at build-ups from our display partners, and bring your questions for our engineers. For any fire rating, check the manufacturer's own test reports.

Frequently asked questions

Does mass timber burn? Yes, it can. Thick timber chars on the outside, which insulates the core and slows the burn. Engineers design for the timber lost to charring.

Is mass timber safe in a fire? It depends on the design, as with any building. Research shows charring behaviour is predictable enough to design for, but it's sensitive to details such as exposed surface area, linings and connections. A fire engineer and building surveyor decide what suits each project.

Does mass timber have to be covered in plasterboard? Under the deemed-to-satisfy pathway, timber is protected with fire-protective coverings such as fire-grade plasterboard. Leaving timber exposed typically calls for a performance solution designed by a fire engineer.

Who decides how a timber building is designed for fire? The design team, with a fire engineer where needed, and the building surveyor who approves the design against the NCC.