Concrete, steel and timber: the structural carbon choice

On a typical new building the structural frame and substructure account for the largest share of upfront embodied carbon, usually well over half. So the frame material decision, taken at concept stage by a structural engineer and a cost consultant, does more for the whole life carbon result than every specification choice made afterwards.

That decision is rarely a clean comparison. A timber frame with piled foundations in soft ground can carry more carbon in the substructure than the superstructure saves, and a steel frame with high recycled content on a shallow raft can beat both. Which is why the answer has to be calculated for the actual site rather than taken from a general ranking of materials.

Why the frame dominates upfront carbon

Upfront carbon covers the product and construction stages: extraction, manufacture, transport to site and installation. In those stages the heavy, high-volume elements dominate, and the frame and substructure are the heavy elements.

Cement is the reason. Clinker production releases carbon dioxide both from fuel and from the chemical conversion of limestone, and the process emissions are the harder part to remove. Steel carries the emissions of iron reduction in a blast furnace, or of electricity in an electric arc furnace, which is a substantially different figure. Timber carries relatively low process emissions and a sequestration credit that has to be accounted carefully.

Two consequences for planning submissions. The frame comparison has to be done early, because by the time the structural scheme is fixed the outcome is fixed with it. And the comparison has to include the substructure, because ground conditions can invert the ranking.

Cement replacement and low carbon mixes

The largest available reduction in a concrete frame comes from replacing part of the cement content, using ground granulated blast furnace slag or fly ash, and from specifying by performance rather than by mix.

Four practical levers, in rough order of effect:

1. Cement replacement, at the highest percentage the programme and the exposure class allow. Higher replacement slows early strength gain, which affects formwork cycles, so this is a programme conversation as much as a material one.
2. Concrete strength specified where it is needed rather than uniformly. A single high grade poured throughout is common and wasteful.
3. Structural optimisation, reducing volume through post-tensioning, voided slabs or simply a more efficient grid.
4. Reuse of existing substructure, which avoids the material entirely and is often the single biggest saving available on a redevelopment.

The constraint on cement replacement is usually the contractor's programme and the exposure conditions rather than the standards. State the replacement percentage assumed in the assessment and be honest that it is an assumption, because a value-engineered mix delivered at a lower percentage makes the submitted figure wrong.

Steel recycled content and reuse

Steel's carbon figure depends almost entirely on the production route. Electric arc furnace steel using scrap has a much lower figure than blast furnace steel from ore, and the difference is large enough to change a frame comparison on its own.

The difficulty is that recycled content is not something the designer controls at specification stage. Sections are bought from whatever mill supplies them, and a general claim that steel is recyclable is not the same as evidence about the carbon of the steel actually delivered. Use the supply chain's environmental product declarations for the specific product where they exist, and where they do not, use a conservative figure and say so.

Reuse is different and more interesting. Reclaimed steel sections, taken from a demolition and re-certified, carry a very low figure because the production emissions have already occurred. The barriers are practical rather than carbon-related: traceability, testing, tolerance and the programme risk of depending on a salvage stream. Where a scheme can use reclaimed steel, it is worth saying so in the submission because the carbon saving is large and the circular economy benefit is a separate planning benefit, as covered in circular economy statements and what the assessor checks.

Timber, sequestration and how it is accounted

Timber's advantage in upfront carbon is real, and the way sequestration is reported is where assessments go wrong.

Biogenic carbon stored in the timber is reported separately from the fossil carbon of production, and under the RICS methodology the stored carbon is recorded as a negative figure in the product stage and then released at end of life, so the net effect across the full life cycle depends on what happens to the material in sixty years. An assessment that claims the sequestration credit in upfront carbon and ignores the end-of-life release is overstating the benefit, and a reviewer will look for both entries.

Report three figures for a timber scheme: upfront carbon excluding biogenic storage, the biogenic storage separately, and the end-of-life position. That presentation is defensible. A single headline number with the credit folded in is not.

Sourcing matters as well. Certification, transport distance and whether the product is glue-laminated or cross-laminated all affect the figure, and cross-laminated timber carries more adhesive and more processing than glue-laminated beams and columns.

Fire and insurance constraints on timber

This is the part that changes schemes after the carbon case has been made.

For residential buildings above a threshold height, the rules restricting combustible materials in external walls limit what can be done with exposed timber at the facade, and the structural frame interacts with that where the frame forms part of the external wall. Structural timber inside the building is a different question, governed by the fire strategy, compartmentation and the charring calculation.

Then there is insurance, which is not a planning matter and is frequently the binding constraint. Insurers and warranty providers have their own positions on mass timber, particularly during construction when the frame is exposed and unprotected, and a scheme can be consented and then redesigned in structural steel because it cannot be insured on acceptable terms.

The honest position for a planning submission is to set out the carbon comparison, state the fire and insurance constraints as constraints, and avoid committing to a frame material in a condition. A condition requiring a specific structural material is a condition that may have to be varied, and a section 73 application to change a frame is an expensive way to discover this.

Foundation carbon and ground conditions

Substructure carbon is the variable most often left out of frame comparisons, and it can decide them.

A lighter frame reduces foundation loads, which on a site needing piles reduces pile length or number and can allow a raft instead. That is a genuine compounding saving. But a timber frame on soft alluvium may still need the same piled solution as a concrete frame because settlement, not load, governs, in which case the substructure saving is zero.

Three situations worth modelling explicitly. A site with existing foundations that could be reused, where retention beats any new-build material choice and feeds straight into the Sustainability Statement. A site with shallow competent ground, where frame weight has limited effect on foundation carbon. And a site needing deep piles or ground improvement, where frame weight matters a great deal.

Get the ground information early enough to do this properly. The comparison depends on a ground model, which means a site investigation before the structural scheme is fixed, and the interpretation belongs in a ground investigation report rather than in an assumption.

Evidencing the comparison in the submission

Present the comparison as an option appraisal with a common boundary and stated assumptions, not as a table of material carbon factors.

Then state which option was selected and why, including where a lower-carbon option was rejected for fire, insurance, programme or cost reasons. A submission that says the timber option carried the lowest upfront carbon and was not pursued because of warranty constraints is credible. One that presents the chosen option as the lowest-carbon without showing the alternatives is not.

The figures belong in the Whole Life Carbon Assessment, prepared to the RICS methodology, with the energy side reported consistently in the Energy Statement so the two documents agree on the reference study period and the operational boundary.

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This is general guidance rather than legal advice. Check the position with your local planning authority, a chartered structural engineer or a qualified carbon assessor before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.