Try lighting a log with a match. You can't. You need kindling, because a big piece of timber protects itself: the outside chars, and that char layer insulates the wood inside. Builders have known this for centuries. Structural engineers have turned it into a calculation.
That calculation is what lets you leave timber exposed in a fire-rated building, which is usually the reason clients want timber in the first place.
Charring in one paragraph
When timber burns, it chars at a fairly steady rate. For typical structural softwood that's roughly 0.6 to 0.7 mm a minute, depending on density and the standard used. Below the char there's a thin layer that's hot enough to have lost its strength, so we allow for that too. What's left in the middle is cool, sound timber that keeps carrying load.
So for a 120-minute fire resistance level (FRL), each exposed face loses somewhere around 85 to 95 mm once the weakened layer is included. Design the member so the core left after that loss can still carry the fire load, and you can leave it exposed.
What that looks like in practice
The rule of thumb: the longer the rating and the more faces exposed, the bigger the section.
- CLT floors exposed from below. On a recent aged care concept, a 140 mm CLT floor did the job structurally when its underside was lined with fire-rated plasterboard. To leave the soffit exposed in the common areas at 120 minutes, the panel went to 240 mm. That thickness was driven by vibration anyway, so the fire rating came almost for free.
- Glulam columns. A column exposed on four sides chars from every face, so it loses timber from all of them. On the same concept, the exposed balcony columns were sized at 300 mm square to stay bare at 120 minutes.
- Beams. Exposed on three sides, they char from the bottom and both sides, so they get deeper and wider.
The details that matter
Charring calculations assume the timber behaves. A few things can stop it:
- Adhesives. In CLT, the char can reach a glue line. Some adhesives soften with heat and let the charred layer fall away, exposing fresh timber and speeding things up. Ask your supplier about fire performance adhesives and test data. Not all products are equal, and some charge extra for it.
- Connections. Steel brackets and screws conduct heat into the timber. They need to be covered, recessed or sized for it.
- Gaps and joints. Fire gets into joints between panels. They need sealing and sometimes cover strips.
- The compliance pathway. In Australia, exposed mass timber in taller buildings usually needs a performance solution from a fire engineer rather than the NCC's deemed-to-satisfy route, which is built around encapsulating the timber. That's normal and well trodden, but it means involving the fire engineer early.
Pick your moments
Leaving everything exposed is expensive: thicker timber everywhere, more fire engineering, and more visible connections to detail. Most successful projects pick their moments. They expose the timber in lobbies, common lounges, dining rooms and feature stairs, where people gather and the effect is biggest. Bedrooms and corridors get plasterboard linings, where services need to run anyway.
That approach keeps most of the structure at its efficient size and spends the extra timber where it counts.
The opportunity people miss
Some building classes, aged care among them, default to 120 minutes under the deemed-to-satisfy provisions. On many timber projects, a fire engineer has been able to show that 90 minutes is appropriate for a sprinklered building. That can thin down elements or remove a layer of plasterboard across the whole building. It's worth asking the question early.
If exposed timber is part of your vision, tell us when you submit your project. We'll show you where it's achievable and what it costs in extra timber.