Depths & specification

Depth for bonded build-ups

Bonded build-ups rely on adhesion to the base, so depth selection is governed by substrate condition and finished-floor tolerance rather than insulation thickness.

Depth for unbonded build-ups

Unbonded screeds sit on a slip membrane rather than bonding to the base, so depth must be sufficient for the screed to perform as an independent structural layer.

Depth for floating build-ups

Floating screeds sit on a compressible or semi-rigid insulation layer, so their depth must account for point loads, insulation type and any embedded services.

Minimum and maximum depths

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.

Depth over UFH pipes

Cover depth over UFH pipes is a distinct figure from overall screed depth and must satisfy both the pipe manufacturer's and screed manufacturer's requirements.

Depth over insulation

The depth of screed needed above an insulation layer depends heavily on the insulation's compressive strength and how loads are expected to spread through it.

Domestic vs commercial loading

Loading regime, more than the label 'domestic' or 'commercial', is what should determine screed strength class and depth on a given project.

Point loads and racking

Concentrated point loads need specific attention in a screed specification, since a floor that copes with general foot traffic can still fail locally under a racking leg.

Strength class selection

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.

Surface regularity clauses

A surface regularity clause should state the SR class required and the covering it needs to suit, since flooring type drives how flat the finish needs to be.

Tolerances and datum

Overall level tolerance against a fixed datum is a separate specification item from surface regularity and needs its own clear reference point and figure.

Specifying to BS 8204

BS 8204 provides a recognised framework for screed specification, but the specification still needs project-specific clauses translating its principles into practice.

Depth by build-up type

A comparative look at how bonded, unbonded and floating build-ups pull depth in different directions, and how to reconcile them on one project.

Cover over UFH pipes

A focused look at minimum cover requirements over underfloor heating pipework, distinct from overall floor depth, and why the distinction matters.

Insulation thickness and depth

How the thickness of insulation chosen for thermal performance directly influences the screed depth needed above it.

Depth tolerances

A plain explanation of how depth tolerance works in practice, what causes deviation, and how it should be checked and recorded.

Load and depth

A guide to working backwards from expected floor loading to arrive at the correct screed depth, rather than assuming a generic domestic figure.

Deep build-ups & level changes

How to approach unusually deep screed build-ups and the level changes they often create at doorways, extensions and staircases.

Specifying strength class

A practical walk-through of how to word a strength class requirement so it is unambiguous and testable, rather than a vague reference to 'suitable strength'.

Wording an SR clause

Practical wording guidance for surface regularity clauses, so the requirement is checkable on site rather than a subjective description.

Specifying joints

A guide to specifying the different joint types a liquid screed floor may need, and how to word this clearly rather than leaving it to site judgement.

Specifying edge insulation

Why edge insulation deserves its own specification line, covering both its thermal role and its role as a movement isolation layer.

Writing a spec clause

How the individual elements covered across this section — depth, strength class, surface regularity, joints and edge detailing — come together into one written clause.

Screed for heat pump systems

Heat pumps run at lower flow temperatures than boilers, so screed depth, conductivity and commissioning need to be specified with that lower thermal driving force in mind.

Drying times and programme

Drying time for liquid screed depends on depth, ventilation and site conditions, so a specification should set a testing regime rather than rely on a single fixed number of days.

Reinforcement and crack control

Liquid screeds already carry a degree of crack resistance from their formulation, but larger areas, restrained edges and heavy point loads still need a considered joint and reinforcement strategy.

Screed over precast concrete

Precast concrete floors bring joints, camber and moisture considerations that differ from an in-situ slab, and the screed specification needs to address them directly.

Fire and compartment floor requirements

Liquid screed itself is generally non-combustible, but fire resistance is a property of the whole floor build-up, so the specification needs to reference the tested assembly rather than the screed in isolation.

Screed in refurbishment

Refurbishment screed specifications need to deal with unknowns that a new-build design rarely faces, including hidden services, variable existing levels and limited headroom for a new build-up.

Quality assurance and site records

A clear quality assurance clause sets out what records the screed contractor should provide, from batch and mix documentation to depth checks, so disputes later have an evidential basis.

Screed for large format tiling

Large format tiles are unforgiving of an uneven or insufficiently dry substrate, so the screed specification needs a tighter surface regularity class and a clear moisture and primer sequence.

Daywork joints and pour sequencing

Large floor areas often need more than one pour session, and the specification should set out where daywork joints sit and how sequencing is managed rather than leaving it to be decided on the day.

Substrate preparation responsibilities

Disputes over substrate readiness are common where a specification does not clearly state who is responsible for preparing the base before the screeding contractor arrives.

Common specification mistakes

Many screed disputes trace back to a small set of recurring specification gaps, such as vague depth wording, missing drying criteria and unclear preparation responsibility, that are straightforward to avoid once identified.

Moisture Testing

Moisture testing confirms a liquid screed has dried sufficiently for the specified floor finish, using recognised RH probe or carbide methods rather than guesswork.

Heating Commissioning

Commissioning underfloor heating gradually before floor finishes are laid protects the screed and the covering from thermal shock and premature installation.

Tolerance Disputes

Disputes over floor flatness are best resolved by referring back to the SR class specified and taking objective measurements, not by visual impression alone.

Sampling & Strength Checks

Verifying screed strength on site involves a combination of sampling at placement and in-situ checks, referenced against the compressive strength class specified.

Protection & Trades

Protection specifications set out how a liquid screed is safeguarded from follow-on trades once poured, covering access, coverings and site sequencing.

Primers & Adhesives

Primers and adhesives used over liquid screed must be compatible with both the screed type and the flooring finish, and this should be confirmed at specification stage.

Acoustic Performance

Acoustic performance in a liquid screed floor build-up is achieved through the wider system design, particularly resilient layers, and should be specified as a complete assembly.

Thermal Performance

Thermal performance over underfloor heating depends on screed conductivity, consistent coverage above the pipes and correctly specified depth.

Levels & Datums

Finished floor level and datum specification tells the screed installer exactly what level to achieve, and needs to be clear, consistent and coordinated across the whole project.

Handover Documentation

Complete handover documentation for a liquid screed floor provides a traceable record covering the mix supplied, testing carried out and any commissioning undertaken.