Levelling subfloors before applying a waterproof surface membrane


John Green, technical service officer at F. Ball, on situations where it is beneficial to apply a smoothing compound before a waterproof surface membrane and what to consider when doing this.


Flooring contractors are used to applying a waterproof surface membrane before a smoothing compound when high subfloor relative humidity levels mean that a moisture management solution is required to protect against floor failure. However, we commonly receive questions about what to do where a moisture management solution is required and the subfloor is particularly rough or uneven.

In these situations, it is recommended to level the subfloor beforehand to ensure even coverage of the subsequently applied waterproof surface membrane. Contractors just need to make sure that they select a moisture tolerant smoothing compound and follow other recommended subfloor preparation processes.


Moisture management

Where subfloor relative humidity levels exceed 75% (65% if bonded wood floorcoverings will be installed), a moisture management solution will be required to prevent excess subfloor moisture attacking adhesives and damaging floorcoverings, potentially causing complete floor failure. For this reason, F. Ball recommends that a moisture test is conducted as part of any flooring installation.

The usual way of suppressing excess subfloor moisture is with the application of a liquid waterproof surface membrane, such as Stopgap F77 or Stopgap F78, to create a barrier between the subfloor and floorcoverings and adhesives, as well as subsequently applied subfloor preparation products.

In most circumstances, a liquid waterproof surface membrane can be applied directly to the subfloor once contractors have made sure that it is suitably sound and free of contaminants.


Rough or uneven subfloors

However, if the subfloor is rough or uneven, a waterproof surface membrane can pool in any undulations on the surface when applied, necessitating 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.

To avoid this, the application of a smoothing compound over the subfloor beforehand can save time and cost. However, it is important to select a product that can be indefinitely exposed to moisture. Moisture tolerant smoothing compounds include Stopgap Green Bag with Stopgap 114, Stopgap 850, Stopgap 1200 Pro and Stopgap 1500 AquaPro. Stopgap 460 Exterior Repair compound can also be used to repair small areas of a subfloor prior to the application of the waterproof surface membrane.


Prior preparations

Following correct preparation of the substrate, it is always recommended to prime subfloors before the application of a smoothing compound to reduce the possibility of pinholes and craters forming in the drying smoothing compound as a result of air escaping from the subfloor, which can be transmitted through the waterproof surface membrane and affect the appearance of a finished installation.

Once a waterproof surface membrane has been applied and it has fully cured, the flooring installation can then continue as normal. Typically, this means priming again and another application of an appropriate smoothing compound to create a suitably absorbent base for adhesives to bond to decorative floorcoverings. Stopgap 1200 smoothing compound or Stopgap 1500 high-performance, water-mix smoothing underlayment can be applied directly over a waterproof surface membrane without priming, providing this is done within 24 hours of the waterproof surface membrane curing.


Specialist adhesives

There are also certain adhesives, including Styccobond F40, F41, F46, F58 PLUS, F49 and B95, which can be applied directly to a fully cured waterproof surface membrane, eliminating the need for another application of smoothing compound. It is recommended that flooring contractors see manufacturer’s instructions to check the suitability of adhesives for application over particular kinds of waterproof surface membrane.

To check that an adhesive is compatible with chosen floorcoverings, flooring contractors can consult F. Ball’s industry-leading Recommended Adhesives Guide (RAG®). The 2023 edition of the guide lists adhesives recommended for use with over 6,000 floorcoverings, produced by over 200 manufacturers. A continually updated version of the guide is available on F. Ball’s website and as a free app. It is also available as an A5 printed booklet. Alternatively, F. Ball’s technical service department are on hand Monday to Friday, 8.30am – 5.00pm, to answer your questions about F. Ball products and how to use them.