02 Apr 2020

A beginner’s guide to subfloor preparation before tiling

Before floor tiling begins, it is critical that subfloors are prepared properly. This means that they are level, sound, dry, free from deflection, dust and any other contaminants which may be considered barriers to adhesion, says David Wilson, UK Technical Services Manager at Building Adhesives Ltd (BAL).


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It is important that any new structural concrete should be left to cure and dried in air for at least six weeks. Cure cement-sand screeds for at least seven days, before drying in air for a further two weeks. A longer period of time will be required in poorer drying conditions or when heating the floor.

Ensure that any direct-to-earth sub-floors incorporate an effective damp-proof membrane. If you are laying tiles or slabs directly to the floor using tile adhesive, the maximum permitted deviation underneath a 2m straight edge resting on the floor should be 3mm (SR1).

A suitable smoothing and levelling floor compound can correct surface irregularities and, therefore, achieve SR1 on new and existing floors. Available in one- or two-part forms, they are easily mixed and simple to apply; they find their own level giving a smooth finish for laying tiles or other floorcoverings.

There are a number of products on the market, each with different features, generally going from anything from 2mm up to 80mm in bed depth. In general, smoothing and levelling compounds are not designed to be used as a finished or wearing surface and will, therefore, require the application of a suitable floorcovering.

Smoothing and levelling compounds, can be used on a variety of substrates. However, when levelling timber floors, or screeds containing underfloor heating, use of fibre-reinforced products may be considered. Remember to check carefully with the manufacturer and any datasheets they produce to make sure they’re suitable for your subfloor.

The majority of subfloors will also need priming before levelling and tiling. Floors such as concrete, cement-sand screeds, anhydrite screeds, cement-sand screeds with underfloor heated screeds and suitably constructed suspended timber floors. Priming ensures good adhesion of the smoothing and levelling compound to the substrate.

Follow the manufacturer’s instructions when mixing (including water ratios etc.), ensuring that a lump-free mixture is achieved. 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.

Always allow the smoothing and levelling compound to set before laying your finished floor surface. Setting times can vary depending on your chosen product normally between four to 24 hours and is dependent upon your floor finish.

In wet areas, allow a minimum of 24 hours before waterproofing the levelled screed with a suitable tanking system.

If you are inexperienced in subfloor preparation, we always recommend seeking further assistance from the manufacturer, whether by phoning their technical team or getting someone to visit your site.

Further information....

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