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Ensuring a perfectly smooth subfloor

John details steps for when a flawlessly smooth subfloor finish is required.

ENSURING a smooth base for the receipt of floorcoverings is essential for achieving a visually attractive flooring finish, especially when installing thin resilient floorcoverings. Here I’ll detail steps for when a flawlessly smooth subfloor finish is required.

Contractors should begin any flooring installation by ensuring the subfloor is suitably sound, smooth, and dry enough for the receipt of new floorcoverings. As part of this, F Ball recommends the application of a levelling compound to achieve a smooth base.

There are also situations where additional preparations will be required to achieve a perfectly smooth base for the receipt of floorcoverings; for example, where sand/cement screeds are mechanically damaged or when working over a plywood base.

Before they go any further, it’s recommended contractors undertake a moisture test to determine subfloor moisture levels. Where relative humidity (RH) level within the subfloor is above 75% (65% if wood floorcoverings are to be installed), the application of a waterproof surface membrane will be required before floorcoverings are installed. If not treated, high levels of humidity in the subfloor can lead to discolouration of the floorcovering and loss of bond strength between the layers above, ultimately resulting in floor failure.

Plywood subfloors
When working over plywood subfloors, gaps at joins between panels, as well as holes caused by screws used to affix them to the substrate, should be filled. Otherwise, imperfections can transfer to subsequently applied floorcoverings, which can compromise the appearance of the finished installation. This is no more important than when installing thin resilient floorcoverings, such as LVTs.

For this purpose, floor finishing compounds are available. F Ball has developed Stopgap Micro Rapid, a new ultra-smooth, flexible, rapid-drying floor finishing compound for creating a flawless subfloor finish. The product can be applied to a seamless micro feathered edge to fill indentations formed by screws used to secure plywood and hardboard sheets and gaps at joins.

It can also be used to repair minor surface defects in a wide range of other absorbent surfaces, including sand/cement and calcium sulphate screeds.

The latest addition to F Ball’s range of floor finishing and repair compounds utilises the same advanced cement technology as the company’s fast-track levelling compounds. It’s walk-on hard and ready to receive new floorcoverings from just 20 minutes after application, and priming isn’t required prior to use, making it perfect for use when working to tight time constraints and deadlines.

Repair compounds
In the case that a contractor finds that the subfloor has dormant, non-structural cracks or holes in it, which may be the case where sand/cement screeds have been mechanically damaged, a suitable repair compound should be used to fill in the gaps.

F Ball’s Stopgap 400 Repair is a fast-setting, rapid-drying repair compound that can be applied in thicknesses between 2-50mm to repair internal floors in light to heavy duty areas. The product can also be used to fill gaps in tiled areas where individual ceramic tiles are missing.

Where excess subfloor moisture is detected, contractors should use a repair compound that is designed for use in damp conditions, such as F Ball’s Stopgap 460 Exterior Repair, prior to the application of a waterproof surface membrane. Stopgap 460 is weather resistant, so it can be also used to repair external floors as part of refurbishment projects.

Next steps
Once a repair or finishing compound has dried, contractors can then proceed with the application of a levelling compound or installation of a new floorcovering in the usual way, remembering to prime the subfloor beforehand to promote the adhesion characteristics of levelling compounds.

When used over absorbent surfaces, primers also stop the unacceptably rapid drying of subsequently applied levelling compounds, maximising the working time of the product. Priming also prevents pinholing: tiny bubbles formed by the slow escape of air from the subfloor, which burst on reaching the surface, leaving small pinholes as the levelling compound cures.

Finally, levelling compound selection should be determined by the subfloor type and intended use. When working over flexible subfloors, including plywood, a flexible levelling compound, such as F. Ball’s Stopgap 700 Superflex, is recommended to accommodate movements in the subfloor, which can cause other levelling compounds to crack.

Heavy duty levelling compounds, such as F Ball’s Stopgap 300 HD, are available for areas that will be subject to heavy loads or high levels of foot traffic.

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