LEVELLING DAMP SUBFLOORS
Stephen Boulton, Technical Service Manager at F. Ball and Co. Ltd. explains why it’s sometimes necessary to apply a levelling compound before a waterproof surface membrane.
Contractors are often presented with the challenge of installing floorcoverings over an uneven substrate with excess subfloor moisture present. In such situations, a liquid waterproof surface membrane (WSM) is normally applied to provide a barrier against moisture, prior to the installation of a levelling compound to create a smooth, level surface onto which a floorcovering can be installed. Where surfaces are particularly rough or damaged, however, it is recommended that a levelling compound is applied before a WSM.
Rough or uneven subfloors are a common feature in refurbishments, particularly where old floorcoverings have been removed, causing damage to the subfloor. If excess moisture is detected in the subfloor, it is recommended that a WSM is applied to create a barrier to the moisture before a levelling compound is installed. However, if the subfloor is particularly rough or damaged, the application of a liquid WSM may result in the liquid pooling in undulations in the subfloor, necessitating a greater quantity of product to achieve full coverage and extending the curing time.
where the liquid has pooled, the integrity of the membrane, when cured, may also be compromised.
LEVELLING FIRST In these circumstances, to ensure even coverage of the waterproof surface membrane, it is recommended that a levelling compound is applied over the subfloor first.
Because the levelling compound will be exposed to damp, it is important to select a product that is moisture tolerant. F. Ball recommends the use of its Stopgap Green Bag/114, Stopgap 850, and Stopgap 1200 Pro levelling compounds prior to the application of a WSM because they have a high degree of moisture tolerance. Stopgap 460 Exterior Repair compound can also be used to repair non-structural cracks or holes or smaller areas of a subfloor before a waterproof surface membrane is applied.
PRIOR PREPARATION Following correct preparation of the substrate, it is 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 F. Ball’s Stopgap P131, which should be diluted to a ratio of one-part primer for every seven parts water before use. Alternatively, the substrate can be thoroughly wetted to a saturated, surface dry condition. Both methods reduce the possibility of pinholes and craters forming in the levelling compound, caused by the slow escape of air from the subfloor, which can transfer through a subsequently applied waterproof surface membrane.
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Once the levelling compound has cured, a waterproof surface membrane, such as F. Ball’s Stopgap F77 or F78, can be applied and the flooring installation can then continue as normal. Where Stopgap F77 has been applied, the cured membrane should be primed before applying a further levelling compound to provide a suitably absorbent base that will enable adhesives to create an effective bond between the subfloor and floorcoverings.
For Stopgap F78, in all cases, a further levelling compound can be applied without the need to prime and must be applied within 12 hours of the membrane curing. If application cannot be achieved within the timeframe, a further coat of Stopgap F78 must be applied and allowed to dry before proceeding with installation of the underlayment.
SPECIALIST ADHESIVES There are also certain adhesives which can be applied directly to a fully cured waterproof surface membrane under certain conditions, removing 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. The guide is available as a free app or a printed booklet and an interactive version can also be found on the F. Ball website.
www.f-ball.com
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