On embodied carbon, mass timber (CLT) tends to be the lowest of the three structural systems, concrete tends to be the highest because of cement, and steel sits in between, swinging widely with recycled content and the steelmaking route. That ordering is a tendency, not a verdict. The real answer is project-specific, and the choice depends on more than carbon: spans, fire, acoustics, moisture, cost, and programme all pull in different directions.
This is an early-stage comparison for architects, structural engineers, and developers weighing CLT vs concrete vs steel before formal assessment. The figures behind any final number belong in a certified life-cycle assessment. What follows is the reasoning to help you frame the decision well.
The short version
- Embodied-carbon tendency: CLT typically lowest, concrete typically highest (cement is the driver), steel variable and highly dependent on recycled content and production route.
- It’s never carbon alone: spans, fire strategy, acoustics, moisture management, cost, and programme often decide the system.
- Timber’s biogenic carbon is real but contested in accounting terms, so treat “negative” headline figures with care.
- As operational energy decarbonises, embodied carbon becomes a larger share of whole-life carbon (World Green Building Council, “Bringing Embodied Carbon Upfront”, 2019) — making the structural choice more consequential.
- Numbers are project-specific. Use indicative comparison early, then commission a formal assessment.
Why does the structural system matter so much for embodied carbon?
The structure is usually the single largest material mass in a building, so the system you pick sets the baseline for upfront embodied carbon before any other decision. The buildings and construction sector accounts for around 37% of global energy- and process-related CO₂ emissions (UNEP, Global Status Report for Buildings and Construction 2025-2026), and a meaningful slice of that is embodied in materials like cement and steel.
The leverage is front-loaded. Most of a building’s embodied carbon is committed at the concept stage, when the structural system is chosen, long before a certified assessment runs. That’s exactly the moment when teams have the least structured information to compare options.
Operational energy is also falling as grids clean up. As that happens, embodied carbon becomes a larger share of whole-life carbon (World Green Building Council, “Bringing Embodied Carbon Upfront”, 2019). The structural decision, once a footnote next to heating and cooling, now sits near the centre of a building’s carbon story.
For the wider context on why upfront emissions deserve this attention, see Embodied carbon in construction.
How do CLT, concrete, and steel compare on embodied carbon?
On a like-for-like basis, mass timber (CLT) tends to carry the lowest embodied carbon of the three, concrete tends to carry the highest, and steel falls in between with the widest spread. The reasons trace back to how each material is made, not to any single published figure. Below is the qualitative logic, which is what early-stage decisions actually need.
Concrete: cement is the carbon
Concrete’s embodied carbon is dominated by cement, specifically the clinker in Portland cement, whose production releases CO₂ both from fuel and from the chemistry of calcination. Because concrete is used in large volumes, even a modest per-kilogram figure adds up. Mitigations exist: cement replacements like GGBS or fly ash, lower-clinker blends, and leaner mixes. They reduce the number; they don’t remove cement’s structural role as the main contributor.
Steel: it depends on the recycled content
Steel’s embodied carbon is the most variable of the three because it hinges on the production route. Steel made largely from recycled scrap in an electric arc furnace tends to be considerably lower than steel made from iron ore in a blast furnace. Two members that look identical on a drawing can carry very different embodied carbon depending on where and how the steel was produced. This is why steel resists a single headline number more than the other two systems.
CLT and mass timber: low, but read the accounting carefully
CLT typically shows the lowest embodied carbon of the three because manufacturing engineered timber is less energy-intensive than producing cement or virgin steel. Timber also stores biogenic carbon drawn down while the tree grew. That stored carbon is real, but how it’s counted is contested: some methods report it as a headline negative, others exclude or defer it, and end-of-life fate (reuse, landfill, or burning) changes the picture. Sustainable forestry and the timber’s eventual destination both matter. Treat any “carbon-negative” headline as a prompt to check the accounting boundary, not as a settled fact.
Indicative framing only. On embodied carbon, mass timber tends to sit lowest of the three structural systems and concrete highest, driven by cement, while steel varies widely with recycled content and production route. These are ordinal tendencies from the underlying material chemistry, not certified per-project figures; the actual result depends on the specific design, supply chain, and assessment boundary.
Embodied vs operational carbon sets out how upfront and in-use emissions trade off across a building’s life.
What are the practical trade-offs beyond carbon?
Carbon rarely decides a structural system on its own; spans, fire, acoustics, moisture, cost, and programme usually do. A system that looks best on embodied carbon can still be wrong for a given brief. The table below summarises the main trade-offs qualitatively, so you can sense-check a shortlist before any detailed engineering or assessment.
| Factor | CLT / mass timber | Concrete | Steel |
|---|---|---|---|
| Embodied-carbon tendency | Typically lowest | Typically highest (cement) | Variable; depends on recycled content |
| Fire | Combustible; relies on charring rate, encapsulation, and code limits | Inherently non-combustible | Non-combustible but loses strength when hot; needs protection |
| Span / structural reach | Good for moderate spans; long spans get heavy | Strong in compression; long spans via post-tensioning | Excellent for long, clear spans |
| Programme / speed | Fast: prefabricated, dry, light, quick to erect | Slower: wet trade, curing and formwork on site | Fast: prefabricated, rapid erection |
| Cost | Project-specific; competitive where speed and weight pay off | Often lowest base material cost; mature supply chain | Project-specific; sensitive to market prices |
| Other | Light (smaller foundations); moisture-sensitive; warm finish | Heavy; excellent thermal mass and acoustics | Light; long spans; usually needs fire protection |
Fire, acoustics, and moisture
These three quietly shape the decision. Timber is combustible, so a CLT design leans on predictable charring rates, encapsulation, and the limits set by local fire codes; concrete and steel are non-combustible, though steel still needs protection because it weakens at high temperatures. On acoustics, concrete’s mass is an asset, while timber floors often need added build-up to meet impact-sound targets. Moisture is timber’s discipline: it must stay dry in manufacture, transit, and service, which puts real demands on detailing and site protection.
Spans, weight, and foundations
Steel reaches the longest clear spans most efficiently, which matters for column-free floors. Concrete excels in compression and handles long spans through post-tensioning. CLT suits short-to-moderate spans well; push it long and the section grows. Weight then cascades downward: lighter timber and steel frames can shrink foundations, an embodied-carbon saving that lives below ground and is easy to overlook at concept stage.
Cost and programme
Concrete often wins on base material cost and a mature, local supply chain. Timber and steel are prefabricated and erect quickly, which can compress programme and reduce financing and preliminary costs even when material rates are higher. Whether that trade favours your project depends on site constraints, height, and local market prices, so treat cost as a range to compare, not a fixed ranking.
Indicative cost framing. Concrete frequently carries the lowest base material cost thanks to a mature supply chain, whereas CLT and steel can offset higher material rates through faster, prefabricated erection that shortens programme. Which system is cheaper overall is project-specific and depends on height, site, and local prices; this is orientation for early comparison, not a quotation.
How can you compare structural systems before a formal assessment?
You compare them indicatively, by class and tendency, then commission a certified assessment once a system is chosen. Early on, the goal isn’t a precise kilogram figure; it’s a defensible shortlist and a clear sense of which trade-offs matter for this brief. Ordinal comparison, paired with the practical factors above, is usually enough to narrow three systems to one or two.
This is where Elementa fits. It gives teams an indicative class comparison of material strategies, including structural systems, from quantities or surveys, before formal assessment. The output is for comparison and decision support, not certification, compliance, or official reporting. Underlying material data draws on the ICE Database (Inventory of Carbon & Energy), with cost orientation referencing ÖNORM B 2061. Think of it as a structured way to frame the CLT vs concrete vs steel embodied carbon conversation, then hand a clear shortlist to a qualified assessor.
A practical sequence: estimate quantities for each option, compare indicative impact classes and cost ranges side by side, layer in fire, acoustics, span, and programme, then validate the chosen system with a formal life-cycle assessment using a methodology such as the RICS Whole Life Carbon Assessment (RICS).
FAQ
Is CLT always lower carbon than concrete and steel?
No. CLT tends to show the lowest embodied carbon of the three, but it’s a tendency, not a rule. The result depends on the specific design, the steel’s recycled content, the concrete mix, transport distances, and how biogenic carbon and end-of-life are accounted for. Always confirm with a project-specific assessment rather than relying on the general ordering.
Does timber’s stored carbon make it carbon-negative?
Not straightforwardly. Timber stores biogenic carbon drawn down as the tree grew, which is real, but accounting methods treat it differently and the end-of-life fate matters. Some methods report a headline negative; others exclude or defer the credit. Treat “carbon-negative” claims as a reason to check the assessment boundary, not as a settled figure.
Why does steel’s embodied carbon vary so much?
Steel’s embodied carbon hinges on the production route. Steel made mostly from recycled scrap in an electric arc furnace tends to be considerably lower than steel made from iron ore in a blast furnace. Identical-looking members can carry very different embodied carbon depending on source and process, which is why steel resists a single headline number.
Can Elementa give me certified carbon figures for each system?
No. Elementa provides indicative impact classes and comparisons for early-stage decision support, based on the ICE Database. It is not a certified LCA or EPD tool, and its output should not be used for compliance, certification, or official reporting. Use it to shortlist and frame the decision, then commission a formal assessment from a qualified professional.
Bringing it together
The headline ordering is useful and limited: CLT vs concrete vs steel embodied carbon tends to run lowest for mass timber, highest for concrete, and variable for steel, but the real number is always project-specific. Carbon also shares the decision with fire, acoustics, moisture, spans, cost, and programme. The strongest early-stage move is to compare systems indicatively, by class and tendency, then let a certified assessment confirm the choice.
Elementa is built for exactly that moment before commitment, giving teams a structured, indicative way to compare structural strategies, not a certified figure. If that fits how you work, join the waitlist to be among the first to access it.