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Frequently Asked Questions

Can you apply smoothing / levelling compound over expansion joints?

No, the expansion joint should be taken up through the floor finish and a suitable expansion joint / cover fitted.

Do you need to prime smoothing / levelling compound prior to application of adhesives?

No, most smoothing compounds are polymer modified and have a controlled level of absorbency.

Which types of subfloor pose most risk of moisture?

The installation of a damp-proof membrane in new buildings became mandatory in 1965 but, if a building has been compromised by a leak at some point, the floor should be tested, even if it is above ground floor level.

New buildings still remain susceptible to moisture as the drying time of a new sand/cement screed is estimated to be 1mm per day up to a thickness of 50mm. A 150mm thick concrete subfloor can take up to a year to dry out naturally. Therefore, to ensure that a concrete subfloor is sufficiently dry to receive a floorcovering, all new build projects should be tested for moisture.

I’m installing new floorcoverings in a converted garage with ceramic tiles. Can I go straight to applying a levelling compound?

If the ceramic tiles are well bonded and clean over a permanently dry base, there’s no need to remove them. You will need to apply a primer beforehand, to promote adhesion between the levelling compound and the subfloor. Primers for use on non-absorbent surfaces, such as ceramic tiles, are the most appropriate choice as they are specially formulated to create a textured finish and enhance bond performance.

Are there any non-mechanical methods that can be used to repair a concrete base that is weak and friable?

In many cases, a weak or friable surface can be reinforced using a surface reinforcement system (SRS), a two-component epoxy resin reinforcement material designed to quickly stabilise and reinforce weak sand/cement or calcium sulphate screeds. You’ll first need to remove any existing floorcoverings and adhesive and check the likelihood of success of a surface reinforcement system according to manufacturer’s instructions. This is typically done using In Situ Crushing Resistance (ISCR) tests, involving weights being dropped from specified heights to test the soundness of the screed. After mixing together the two components, pour over the weak subfloor and spread out with a rubber squeegee, working into the surface until no more liquid is absorbed. The product penetrates the weak/friable surface, filling voids and static cracks and binding loose particles to strengthen the screed. The reinforcement system works overnight to provide a suitable base for the subsequent installation of subfloor preparation products.

I’m installing floorcoverings over a newly-installed concrete screed. Will a waterproof surface membrane be required?

Almost certainly. A recently-installed concrete screed is likely to contain a large amount of residual construction moisture, which can potentially cause complete floor failure if no waterproof surface membrane is in place to stop it rising up to the level of floorcoverings. You will need to leave at least seven days between the screed being installed, grind off any laitance and make sure it is completely clean, smooth and dust free. Then conduct a moisture test to determine subfloor relative humidity (RH) levels. A waterproof surface membrane will be required where subfloor RH levels are above 75% (65% where wood floors will be installed).

I’ve installed LVTs in a new conservatory. After less than a year, the floor has failed. The adhesive appears damp. What could have caused this?

This is very likely a result of residual construction moisture. Where subfloor relative humidity (RH) levels are above 75% (65% where wood floorcoverings will be installed), a waterproof surface membrane should be installed to stop excess subfloor moisture rising up to the level of floorcoverings and potentially causing complete floor failure. In this instance, you’ll have to remove the LVTs and mechanically prepare and clean the subfloor. Where subfloor relative humidity levels are below 98%, you can use an epoxy waterproof surface membrane to create an effective barrier against rising damp and residual construction moisture. You can then proceed to using a levelling compound to create a smooth base ready for the receipt of LVTs. It’s recommended that a temperature tolerant adhesive is used to install LVTs in a conservatory, to stop the movement of floorcoverings caused by extreme temperature fluctuations as a result of solar gain.

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