How To Prepare A Subfloor Before Screeding

Floor screed is only as good as the surface it is laid on. No matter how well a screed is mixed, pumped and finished, if the subfloor beneath it has not been properly prepared, the finished result will be compromised.

Poor subfloor preparation is one of the most common causes of screed failure, leading to problems such as cracking, delamination, uneven surfaces and poor adhesion that can be both costly and disruptive to put right.

Here is what proper subfloor preparation involves and why each stage matters.

Why Subfloor Preparation Matters

Screed is applied as a relatively thin layer over the subfloor, typically between 40mm and 75mm depending on the application and whether underfloor heating is present.

At these depths, any irregularities, contaminants or weaknesses in the surface below will directly affect the performance of the screed above.

A subfloor that is not clean, sound and correctly prepared will prevent the screed from bonding or curing correctly, regardless of the quality of the screed itself.

Taking the time to prepare the subfloor thoroughly before any screed is laid is not an optional extra. It is a fundamental requirement of any well-executed floor screeding installation.

Assessing The Subfloor

Before any preparation work begins, the subfloor needs to be assessed to establish its condition and identify any issues that need to be addressed.

The key things to look for include structural cracks or movement, areas of loose or friable material, contamination from oil, grease or other substances, high moisture levels and any existing coatings or adhesive residues that could prevent bonding.

On new build projects, the concrete slab will typically need to be inspected to confirm it has reached sufficient strength and that the surface is suitable for screeding.

On refurbishment projects, the existing substrate may be more varied and may require extensive remedial work before screeding can take place.

Cleaning The Subfloor

Removing Debris / Contamination

The subfloor must be thoroughly cleaned before any further preparation work is carried out. All dust, debris, loose material and construction waste must be removed, and any areas of contamination from oil, grease or other substances need to be treated appropriately.

Contaminated areas that are not properly addressed will prevent the screed or any bonding agent from adhering correctly, creating weak spots in the finished floor.

Grinding & Scarifying

Where the surface of the subfloor has a smooth or glazed finish, or where there are residues of old adhesive or coatings that cannot be fully removed by cleaning alone, mechanical preparation using grinding or scarifying equipment may be required.

This process removes the surface layer of the concrete to expose a clean, open texture that will bond far more effectively with the screed or bonding agent applied above.

Repairing Cracks & Defects

Any structural cracks, holes or areas of defective concrete in the subfloor must be repaired before screeding begins. Cracks that are left unaddressed can reflect through the screed above, particularly in thinner applications, and voids or weak areas in the subfloor can cause the screed to crack or delaminate under load over time.

Crack repairs should be carried out using appropriate repair mortars or resins that are compatible with the screed system being used.

For moving cracks, where there is evidence of ongoing structural movement, the underlying cause of the movement should be investigated and addressed before any surface repairs are made.

Moisture Testing

Moisture in the subfloor is one of the most significant risks to a successful screed installation. Excessive moisture rising from the concrete slab can affect the curing of the screed, cause the screed to delaminate and, ultimately, damage any floor coverings applied above.

Before screeding, the moisture content of the subfloor should be measured using a calibrated hygrometer or carbide bomb test to confirm it falls within the acceptable range for the screed system being used.

Where moisture levels are too high, a surface damp proof membrane or moisture barrier may need to be applied before screeding can proceed, in addition to any structural damp proof membrane that forms part of the floor build-up.

Installing Perimeter Expansion Strips

Before the screed is laid, a perimeter expansion strip should be fixed around the edges of the floor area, against all walls, columns and any other fixed structures that penetrate the floor.

This strip accommodates the natural expansion and contraction of the screed as it cures and as temperatures change, preventing the screed from cracking at its edges where it would otherwise be restrained by the surrounding structure.

On larger floor areas, additional expansion joints may need to be incorporated within the screed itself, at intervals determined by the screed manufacturer's guidance and the specific requirements of the project.

Priming The Subfloor

In many screed installations (particularly those using liquid or self-levelling screeds), a primer is applied to the prepared subfloor before the screed is laid.

The primer has several purposes: it seals the surface of the subfloor to prevent it from drawing moisture out of the screed too quickly during curing, it improves adhesion between the screed and the subfloor, and in some cases it provides a degree of dust suppression on friable concrete surfaces.

The correct primer for the application will depend on the screed system being used and the condition of the subfloor, and should always be selected in accordance with the screed manufacturer's recommendations.

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