WPC vs timber decking: compare carbon footprints fairly

Define what the deck must do #

Start with a functional brief: usable area, loading, exposure, slip performance, accessibility and appearance requirements. A board that needs closer joist spacing may change the supporting structure. A hollow profile and a solid profile are not equivalent solely because their widths match. Compare compliant designs, not arbitrary samples. Establish whether railings, foundations and substructures are identical or must be modelled separately. If those components are excluded, state that limitation prominently so the result is not presented as the footprint of a complete outdoor space.

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Convert the unit with a visible calculation #

As an illustrative arithmetic example only, suppose a declaration reports 2 kg CO₂e per kilogram of board for a specified stage. If the installed board mass is 18 kg per square metre, that stage contributes 36 kg CO₂e per square metre before any separately modelled losses or accessories. These are invented teaching values, not Green Plank results or industry averages. Establish coverage from the installed layout, including the treatment of gaps. Do not multiply by the profile’s gross width when the chosen quantity convention uses a different effective coverage.

Align methods and system boundaries #

Check PCR versions, indicator definitions, declared geography, data periods and covered modules. A production-stage result cannot be ranked directly against a whole-life result. Use the same treatment of biogenic carbon on both sides and keep module D outside the product-system total. An EPD’s verification does not automatically make every comparison using it valid. If one product lacks the necessary data, state the gap or use an accepted conservative dataset under the project method; do not manufacture a precise result from incompatible documents.

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Model service life with evidence, not a sales slogan #

A warranty is a contractual promise with conditions; it is not automatically a reference service life for LCA. Consider exposure, drainage, fixings, installation quality, use intensity and maintenance. Timber species and treatments vary, as do composite formulations and profiles. Apply a consistent building or project study period and justify replacement assumptions. A useful sensitivity check might compare two plausible service-life scenarios for each design. If the ranking changes when one uncertain replacement is added, the conclusion should say so rather than hiding that dependence.

Include maintenance without exaggerating either option #

Composite decking still needs cleaning. Timber maintenance depends on the chosen appearance, species, treatment and performance expectations. Specify cleaning frequency, water, products and any coatings or replacement parts actually required. Avoid assuming that all timber must be repainted annually or that all WPC is maintenance-free. Keep aesthetic renewal separate from a safety-critical replacement. Document whether the client accepts natural colour change; that choice can materially alter a maintenance scenario without changing the basic structural function.

Test transport, disposal and other environmental indicators #

Compare transport routes by mode, distance, payload and packaging. A short road journey is not automatically lower impact than every longer mixed-mode route. At end of life, use realistic collection and treatment conditions for each material and location. Where recovery is modelled, avoid giving one product an optimistic future market and the other a worst-case disposal route without justification. Then inspect the other relevant EPD indicators. Climate performance is important, but a carbon ranking alone cannot establish overall environmental superiority.

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Write a conclusion that survives scrutiny #

Report the functional brief, data sources, boundaries, main assumptions and sensitivity results with the headline figure. Distinguish measured supplier data from estimates. For a Green Plank specification, request the relevant product evidence and compare the complete proposed system on the same basis as alternatives. Do not infer that a brand’s Nordic experience proves a particular carbon advantage. A defensible conclusion may be conditional: one design performs better under the stated service-life and disposal assumptions. That is more useful to an architect than an unsupported universal winner.

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Comparison controls for a project assessment, not a ranking of brands or materials.

Common questions