BS EN 13813 strength classes
Selecting a screed strength class: C and F values explained
Strength class designations such as CT-C25-F5 describe compressive and flexural performance and should be chosen against actual loading, not picked as a default.
BS EN 13813 designates screed strength using compressive (C) and flexural (F) classes, for example CT-C25-F5 for a cementitious screed with defined compressive and flexural strength values.
Selecting the right class is about matching these figures to the anticipated loading and floor covering, rather than defaulting to whatever a supplier lists as standard.
Because C and F values describe two different failure modes, a specification that only quotes one of the pair leaves the installer guessing which binder and mix design will satisfy both, so both figures should always travel together.
Strength class selection — the points that decide it
- The C value relates to compressive strength and the F value to flexural strength; both matter, and a high C value doesn't automatically mean a high F value.
- Heavier floor coverings and point loads generally call for a higher flexural class, since bending resistance is what resists cracking under load.
- Domestic living areas typically need a lower strength class than commercial or industrial floors, though the actual figure should be confirmed against use.
- Depth and strength class are linked: a thinner screed usually needs to compensate with a higher class if the loading demands it, within the manufacturer's limits.
- The designer's specification should state the exact designation (for example CA-C30-F6) rather than leaving it as a general description.
Reading a strength class designation
A designation like CT-C25-F5 tells you the binder type (CT for cementitious, CA for calcium sulfate), followed by the compressive strength class and the flexural strength class. Each figure is tested to a defined standard method, so the designation is a reliable shorthand once understood, rather than a marketing term.
Matching class to covering and use
Lighter coverings such as carpet generally place less demand on the screed's surface strength than natural stone or heavy tiling, and areas with trolleys, machinery or vehicles need higher classes than typical domestic rooms. The final decision should sit with the designer, informed by both the loading and the intended finish.
Protecting a floor while strength class develops
Strength class figures describe the fully cured floor, not the surface a few days after pouring, so boarding protects the slab while that strength is still developing.
Around SR-critical junctions, agreeing a tolerance for making good keeps the final inspection focused on flatness rather than incidental marking.
Naming the formulation behind a strength class
A strength class such as C25 can be delivered by either binder type, but cementitious and anhydrite mixes reach that figure on different timescales.
Where a specific surface finish is wanted, naming it alongside the strength class prevents the installer defaulting to whichever finish is quickest to apply.
Need help choosing a strength class?
Send us the intended use and covering for each area and we can help you check the class being proposed.
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Strength class selection — your questions
In short
Strength class designations such as CT-C25-F5 describe compressive and flexural performance and should be chosen against actual loading, not picked as a default.
Need help choosing a strength class?
Send us the intended use and covering for each area and we can help you check the class being proposed.