Subfloor Preparation to Avoid Floor Failure

Failing to undertake proper subfloor preparation before the application of new floorcoverings can result in an unsightly finish or even complete floor failure, leading to costly remedial work. Fortunately, this can be avoided by adhering to just a few basic principles of subfloor preparation, including the need to guard against excess subfloor moisture – the leading cause of subfloor failure. Stephen Boulton, Technical Service Manager at leading manufacturer of subfloor preparation products, F. Ball and Co., goes through these processes.


Although it should be standard practice, there is still a lack of awareness regarding the need to undertake certain procedures before installing floorcoverings and often this results in lost time and unforeseen expenditure. Ensuring a long-lasting, visually attractive finish involves following a few other basic steps, including making sure the subfloor is in a suitable condition before work begins, applying a levelling compound to ensure a perfectly smooth base for the receipt of floorcoverings and remembering to prime beforehand.

Subfloor preparation

The first step in any flooring installation is to make sure the subfloor is suitably sound and smooth. If the subfloor is a newly-installed screed, contractors will need to ensure that any laitance – the crust of cement and fine aggregates that form on the screed’s surface as it dries – is ground off. The typical method of removing laitance is using a rotary sanding or abrading machine.

If the project is part of a refurbishment, the subfloor’s condition will need to be assessed once old floorcoverings have been removed. You may discover that the subfloor is cracked or damaged, in which case, it will require extra preparation. In most cases, you can fill cracks as deep as 50mm with a floor repair compound.

Subfloors may also be contaminated with oil, grease or other chemicals, necessitating mechanical preparation by grinding or abrading before applying other floor preparation products. Old adhesive residues will need to be removed mechanically unless a levelling compound suitable for use over old adhesive residues is used.

Moisture test

Contractors need to be particularly aware of the problems posed by excess subfloor moisture in the base. Whether the result of rising damp or residual construction moisture, unmanaged subfloor moisture can attack adhesives and floorcoverings, possibly causing complete floor failure. For these reasons, F. Ball and Co. recommends that a moisture test is conducted to determine subfloor relative humidity (RH) levels as part of any flooring installation.

The only method of measuring subfloor RH levels with certainty and compliance with British Standards is to use a calibrated digital hygrometer. Where a moisture test indicates that subfloor RH levels are above 75% (65% if a wood floorcovering will be installed), a moisture management solution will be required to suppress excess subfloor moisture levels and prevent floor failure.

Dealing with damp

The application of a waterproof surface membrane is the typical solution for effectively controlling damp. The best-performing epoxy waterproof surface membranes will isolate excess subfloor moisture where relative humidity values are up to 98%, with a single-coat application, which will fully cure in as little as three hours.

Quicker, two-coat systems are available that will create an effective barrier against residual construction moisture where relative humidity values are up to 95%. The first coat cures in 15 to 20 minutes and a further 30 minutes curing time is required for the second coat.

Levelling compound selection

Once satisfied that the subfloor is dry – or an appropriate moisture management solution is in place – you should apply a suitable floor levelling compound to ensure the subfloor base is smooth and level, free from imperfections that could, otherwise, spoil the aesthetic appearance of the finished floor.

General-purpose levelling compounds are available for many situations, as well as products with a wide variety of specialist applications. When contractors are working to tight timescales, the fastest-drying levelling compounds on the market will be walk-on hard from just 30 minutes and ready to receive floorcoverings in as little as 45 minutes after application. Flexible levelling compounds that are fibre-reinforced are recommended over flexible subfloors, such as steel and plywood, to cope with these subfloors’ natural movements. Levelling compounds with high compressive strength should be used where floors will be subject to heavy loads or high foot traffic, while calcium sulphate-based levelling compounds provide optimum compatibility with calcium sulphate screeds.

In most cases, it will be essential to prime a surface before applying a levelling compound. This promotes the optimum performance characteristics of the levelling compound and, when used over absorbent subfloors, such as concrete, prevents moisture being drawn from the levelling compound, which can cause it to dry too quickly and result in floor failure. For time-saving purposes, levelling compounds are available that can be applied directly over old adhesive residues without the need to prime beforehand.

Compatibility check

Once the levelling compound has cured, contractors can proceed to install floorcoverings using an appropriate adhesive. At this stage, particular floorcoverings and adhesives’ compatibilities should be checked to further ensure against floor failure. To do this, contractors should consult the adhesive manufacturer’s recommended adhesives guide or see the floorcovering manufacturers’ instructions.

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