Simplifying subfloors

Neil Sanders on levelling uneven subfloors before applying a waterproof surface membrane

FLOORING contractors are used to applying a waterproof surface membrane before a levelling compound when high subfloor relative humidity levels mean that a barrier against moisture is required to prevent floor failure. In some circumstances, it may be required to apply a floor levelling compound prior to the waterproof surface membrane. Neil Sanders, technical director at F Ball and Co talks us through when this is the case and other considerations when deciding if a levelling compound should be applied first.

When a waterproof surface membrane is required, it is typically advised that this is applied before a levelling compound, directly to a subfloor. However, where the subfloor is very uneven, a moisture tolerant levelling compound can be used before a waterproof surface membrane to provide an even subfloor and improve the coverage and evenness of the waterproof surface membrane. However, it is essential that contractors check the compatibility of the floor levelling compound first.

In most circumstances, a waterproof surface membrane, such as F Ball’s Stopgap F77 or F78, should be applied directly to the subfloor, before the levelling compound, in order to suppress any excess moisture in the subfloor.

The waterproof surface membrane works to protect the subsequently applied levelling compound and adhesive from the damaging effects of moisture, which could cause complete failure of the installation.

Subfloors should be properly prepared before the application of a waterproof surface membrane, and some subfloor types will require specialist preparation. For example, power floated concrete and sand/cement screeds should be smooth, mechanically prepared, free of dust or other contamination before the application of a waterproof surface membrane.

If the subfloor is uneven, a subsequently applied waterproof surface membrane can pool in any undulations on the surface. This will necessitate a greater quantity of product to achieve full coverage, adding more expense to the project. It can also result in an extended curing time where the membrane has pooled.

In this instance, applying a levelling compound over the uneven subfloor before the waterproof surface membrane is applied can help achieve a more even and consistent thickness, as well as better coverage rates; thereby saving on cost. However, contractors need to choose a compatible levelling compound.

Only certain levelling compounds can be used underneath a waterproof surface membrane. F Ball only recommends the use of Stopgap Green Bag/114, Stopgap 850 and Stopgap 1200 Pro prior to the application of a waterproof surface membrane. This is because they have a high degree of moisture tolerance, meaning that they can cope with higher levels of subfloor moisture. Stopgap 460 Exterior Repair compound can also be used to repair smaller areas prior to the application of the waterproof surface membrane.

Following correct preparation of the substrate, it is always recommended to prime subfloors before the application of a levelling compound in order to reduce the formation of pinholes in the surface. This can be achieved with a primer, such as Stopgap P131, which should be diluted to a ratio of one part primer for every seven parts water and allowed to dry.

Once a membrane has been applied and it has fully cured, the flooring installation can then continue as normal. Typically this means priming again and applying an appropriate levelling compound to create an absorbent base for adhesives in order to install decorative floorcoverings. F. Ball’s Stopgap 1200 Pro can be applied directly over a waterproof surface membrane without priming, providing it is applied within 24 hours of the waterproof surface membrane being fully cured.

There are also certain adhesives, such as Styccobond F40, F41, F46, F49 or B95, which can be applied directly to a fully cured waterproof surface membrane. This means there isn’t the need for another application of levelling compound.

To check that an adhesive is suitable for use with a particular floorcovering, flooring contractors can consult manufacturer’s guidelines, or F Ball’s recommended adhesives guide (RAG), which lists the adhesives recommended for use with floorcoverings produced by over 200 leading floorcovering manufacturers.
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