Ask the expert: Can a Floor Levelling Compound be applied underneath a WSM?

The nature of a subfloor impacts whether a WSM or levelling compound should be applied first.

In most situations, it is advised that if a Waterproof Surface Membrane (WSM) is required, it is applied directly to a subfloor prior to the application of a levelling compound. However, where the subfloor is very uneven, a moisture tolerant levelling compound can be used prior to the WSM to provide an even subfloor and improve the coverage of the WSM.

Smooth subfloors

In most circumstances a WSM, such as Stopgap F77 or F78, can be applied directly to the subfloor, prior to a levelling compound, in order to suppress any excess moisture in the subfloor. This is with the exception of power floated concrete which should be mechanically prepared prior to the application of a WSM and in the instance of anhydrite subfloors we would recommend speaking to our Technical Service Department for advice.

The WSM works to protect the subsequent levelling compound and adhesive from the damaging effects of moisture, which could cause complete failure of the installation.

It is important that concrete and sand/cement screed surfaces are mechanically prepared to produce a clean, sound and dust-free before the application of a WSM.

Unsmooth subfloors

If the subfloor is uneven, the WSM could ‘pool’ in any undulations on the surface and will require a greater quantity of WSM to achieve full coverage, meaning more product is needed, adding more expense to the project.

By applying a levelling compound over the uneven subfloor, the WSM can then be applied at a more even and consistent thickness and will achieve better coverage rates.

Selecting the correct product

It is important to be aware that only certain levelling compounds can be used underneath a WSM; F. Ball only recommend the use of Stopgap Green Bag/114, Stopgap 850 and Stopgap 1200 Pro prior to a WSM as they have a high degree of moisture tolerance, meaning that they can cope with higher levels of moisture in the subfloor. Stopgap 460 Exterior Repair compound can also be used to repair smaller areas prior to the application of the WSM. Prior to application of these compounds, absorbent subfloors must be primed with Stopgap P131 diluted 1:7 with water.

Once the WSM is fully cured, the flooring installation can then continue as normal. This means applying a primer, followed by an appropriate levelling compound to create an absorbent base for adhesives in order to install decorative floorcoverings. Stopgap 1200 Pro can be applied directly over a WSM without priming, providing it is applied within 24 hours of the WSM curing.

Specialist adhesives

There are certain adhesives, such as Styccobond F40, F41, F46, F49 or B95, which can be applied directly to a fully cured WSM, without the need for a levelling compound. Refer to the relevant technical data sheets for full instructions.