Depth ranges explained
Minimum and maximum depths by liquid screed system type
There is no single minimum or maximum depth for liquid screed; the practical range depends on the build-up type, the product and the loading involved.
One of the more common specification errors is quoting a single 'standard' depth figure without reference to the build-up type it applies to.
Bonded, unbonded and floating systems each have their own practical depth ranges, and the correct figure for a project should come from the manufacturer's datasheet for the specific application, cross-checked against the designer's loading assumptions.
Because depth ranges differ so much by build-up, a schedule that lists depth against build-up type and product name, rather than a single blanket figure, is the clearest way to avoid a contractor defaulting to whatever they have on the van.
Minimum and maximum depths — the points that decide it
- Bonded systems generally have the thinnest workable range, since the substrate contributes structurally.
- Unbonded systems typically require a somewhat greater minimum than bonded, to compensate for the lack of substrate composite action.
- Floating systems usually need the greatest depth of the three, particularly where underfloor heating pipes are embedded.
- Maximum depths are less commonly discussed but matter for deep-fill situations, drying time and potential shrinkage; very deep pours may need a different product or staged approach.
- Depth ranges quoted generically online should always be checked against the current datasheet for the specific screed being used on the project.
- Local thickening at service runs, manifolds or pattress boxes can push depth beyond the general range and should be shown separately on drawings.
Why ranges vary between products
Calcium sulfate and cementitious screeds have different flow characteristics, shrinkage behaviour and strength development, which is why their datasheets specify different minimum and maximum depths even for the same build-up type.
A depth that is perfectly acceptable for one manufacturer's product may fall outside another's stated range, so depth cannot be treated as a fixed industry number.
Deep-fill situations
Where a substantial depth is unavoidable, for example over deep service voids or significant level changes, it's worth discussing with the screed supplier whether the standard product is appropriate at that depth or whether a deep-fill variant or a staged fill using a different material beneath the screed is more suitable.
Naming the formulation within a depth range
A depth range quoted generically for 'screed' can be met by either a cementitious or a calcium sulfate product, yet the two dry and shrink differently at the same thickness.
Where a specific surface finish is also wanted, that should sit in the same clause as the depth figure rather than being left to the installer's default practice.
Vapour control across different build-up types
The depth ranges discussed here assume the correct damp-proof membrane or vapour check layer is already in place beneath the chosen build-up, which should be confirmed rather than assumed.
Where insulation with limited compressive strength forms part of the build-up, its interaction with the vapour layer needs resolving on the drawing before ordering, not on site.
In short
There is no single minimum or maximum depth for liquid screed; the practical range depends on the build-up type, the product and the loading involved.
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Minimum and maximum depths — your questions
Need a depth range confirmed for your project?
Share your build-up type and loading requirements and we'll help confirm a workable depth range.