"Timber has a lower carbon footprint than steel" is repeated often enough on site to sound settled. The published research says something more specific, and more useful, than that.
What the studies actually show
A widely cited life cycle assessment by Allan and Phillips, comparing mid and low-rise buildings, found CLT structures showed a 30 to 40 percent reduction in global warming potential compared to equivalent structural steel buildings. That's a genuine, meaningful difference, and it's the figure behind most of the general claims made about timber versus steel.
A 30 to 40 percent reduction is real. It is also an average across a body of studies, not a guarantee for any specific building.
The finding that complicates the simple story
A UK life cycle assessment published in 2026, studying a modular light-gauge steel social housing scheme, found a normalised embodied carbon intensity of 366 kgCO2e per square metre across modules A1 to A5, describing this as comparable to recent high-performing CLT buildings. In other words, a well-optimised, fabric-efficient steel scheme closed most of the gap the broader literature reports, not through a different material, but through better design and specification within the material it already used.
This matters more than it might first seem. The 30 to 40 percent figure is a comparison of typical outcomes, not a fixed physical law. A poorly specified CLT building and a well-optimised steel one can land closer together than the headline percentage suggests, and the reverse is just as true.
What this actually means for a real decision
Material choice is a genuine, significant lever, and the general pattern in the literature does favour CLT over steel by a meaningful margin on average. But the honest conclusion isn't "always specify timber," it's that the specification quality within either material matters enough to change the outcome substantially. A frame system your team already builds well, executed carefully, can outperform a lower-carbon material poorly specified. The only way to know which applies to a specific project is to actually model both options against the real design, not assume the answer from a general industry statistic.