A guide to sub floor preparation

Self-levelling floor compounds help level up small surface irregularities on new and existing bases and usually come in one-part or two-part forms. Easily mixed and simple to apply, they find their own level giving a smooth finish for laying tiles or other floor coverings.

Most levelling compounds can only be applied up to 30-40mm and will need to be used in more than one application or with other materials required. However there are some exceptions on the market such as BAL Level Max which can be applied from 2 mm up to 80 mm in one application.

It’s always important to remember that levelling compounds are never suitable as a finished, wearing surface, and that they should always be covered.

Typically; self-levelling compounds are suitable with most screeds, including sand:cement, anhydrite and flooring grade asphalt. However, when levelling timber floors, or screeds containing underfloor heating then only fibre-reinforced products are suitable i.e. BAL Level Max. Remember to check the products you’re using with the manufacturer to make sure they’re suitable.

Before laying a levelling compound it is essential to ensure that correct preparation of the sub floor is carried out.  Firstly ensure that the surface is clean, firm, dry and free from grease, dust and any other contamination which may be considered barriers to adhesion.

Any new concrete or screed must be left to fully cure according to the manufacturers’ instructions before applying any levelling compound. Ensure that any direct-to-earth sub-floors incorporate an effective damp proof membrane.

The majority of subfloors will also need priming. Floors such as concrete, cement:sand screeds, anhydrite screeds, cement:sand screeds with underfloor heated screeds and suitably constructed suspended timber floors.  Priming ensures that the levelling compound fully adheres to the substrate.  If levelling over timber boards where the joints are open, we would suggest using thin heavy duty tape to bridge the gaps.

When mixing your self-levelling compound always ensure you follow the manufacturer’s instructions. Typically, products like BAL Level Max are mixed by adding powder to clean water five parts powder to one part clean cold water by weight. We would always recommend mixing with an electric drill mixer at a slow speed i.e. under 300rpm.  Always mix until a lump-free mortar is achieved and allow the mixed leveller to stand for two minutes before re-mixing for 30 seconds.

Once the required consistency is achieved, pour the mix over the prepared substrate and spread to the required depth using a suitable steel finishing trowel or long handled blade. Work the mixture well into the corners, edges and around projections. When you’ve finished spreading the mixture on the floor, pour some more mixture onto the floor and spread that out and blend it in with the previous area.

Before allowing to cure, ensure that the surface is sufficiently smooth to receive floor coverings without any further treatment. If you are laying tiles or slabs, floors should be corrected to SR1 (Surface Regularity) – this is plus or minus 3mm or less over a two meter straight edge. We would not recommend the use of spiked rollers on a fibre-reinforced product (e.g. BAL Level Max) as this can pull fibres through to the surface. If any trowel marks do remain, rub down before the compound has hardened.

Always allow the new floor to set before laying your finished floor surface. Setting times can vary depending on your chosen product normally between 4 to 24 hours and is dependent upon your floor finish. For example, BAL Level Max can be tiled after 4 hours, but must be left to set for 24 hours for other finishes such as vinyl or carpet. This is to ensure that the covering is fully protected from any moisture in the screed which will diffuse through the screed and damage the surface if not left to fully cure.

In wet areas allow a minimum of 24 hours before waterproofing the levelled screed with a tanking system such as BAL Tank-it. Suitable for use internally or externally, BAL Tank-it is a rapid-dying cementitious waterproofing system. It can be tiled in 90 minutes and no priming or matting is required. Thanks to its EN Classification BS EN 14891, BAL Tank-it is suitable for a wide range of projects including total immersion such as swimming pools.

It can be easily applied with only a brush, roller or trowel and as well as providing outstanding performance, like BAL Level Max, BAL Tank-it comes with full back-up technical and on-site project support and a written 25 year guarantee for complete peace of mind.

One should always remember that self-levelling compounds, such as BAL Level Max, are not a substitute as a wearing surface.

Finally, we would always advise checking with the manufacturer or their technical advisors for before application to ensure project success.

At BAL our Technical Advisory Service is fully equipped to provide support to contractors, architects and tilers using levelling compounds – including NBS M40/M10/M20 specification assistance, on-site consultancy and expert technical advice.

Contact BAL Technical Advisory Service on 0845 600 1 222 or our Innovation and Technology Centre for training enquiries on 01782 591120.

Covering the whole of the UK, BAL Product Support Technicians offer practical knowledge and on-site consultancy and training to tiling and flooring contractors.

If you’ve experienced a problem on your installation whatever brand you’ve used, you can trust our PSTs to offer dependable solutions and remedies.  What’s more, we’ll even send samples away for independent analysis at our laboratories to find the cause of the problem.


By David Wilson, 

UK Head of Technical Standards and Information at BAL