It sounds counterintuitive, but large timber beams are actually more fire-resistant than unprotected steel beams of equivalent load capacity. This is a well-established fact in fire engineering, and it's one of the reasons timber frame construction is experiencing a renaissance.
The science is straightforward. When a large timber beam is exposed to fire, the outer layer chars. This char layer is an excellent insulator — it slows heat penetration into the remaining wood. The interior of the beam remains cool and structurally sound for an extended period.
Steel, by contrast, conducts heat rapidly throughout its entire cross-section. At around 1,100°F (593°C), steel loses roughly half its load-bearing capacity. In a building fire, unprotected steel beams can fail catastrophically and without warning within 15-20 minutes.
A 12x12 timber beam, on the other hand, chars at a predictable rate of about 1.5 inches per hour on each exposed face. After an hour of fire exposure, the remaining cross-section is still roughly 9x9 — reduced, but still carrying significant load. The char rate is so predictable that fire engineers can calculate exactly how long a timber beam will maintain structural integrity.
This property is particularly relevant for reclaimed old-growth timbers, which tend to be denser than modern wood. Higher density means slightly slower char rates and higher residual strength in the uncharred core.
In practical terms, this means that a building framed with large reclaimed timbers provides excellent fire safety — often exceeding the performance of unprotected steel. Combined with modern fire suppression systems, timber frame buildings are among the safest structural types in a fire scenario.