Understanding drying times for levelling compounds

Martin Cummins

Published by Martin Cummins - UK Technical Support Manager, Bostik

Martin has been involved in the industry for over 20 years. He’s passionate about flooring in all its guises, with his particular expertise being in subfloor preparation products and adhesives. Martin is an active member of the CFA manufacturers committee and he sits on the CFA council. He aims to keep abreast of all the changes in the industry and feeds this knowledge back to installers to make sure when floors are laid they perform to their maximum potential.

When considering flooring installations, levelling compounds play a pivotal role in preparing the surface ready for floorcoverings. Understanding the factors that influence drying and curing times can therefore help to reduce downtime on site, maximise productivity and ensure projects are completed to a high standard within the required timescales.

Here I’ll discuss some of the factors that need to be considered when working with levelling compounds to ensure they dry and cure properly.

Removing residual moisture
Once the subfloor has been prepared, the first thing that is usually applied, if required, is a surface damp proof membrane (DPM) system. The highest specification DPM products are normally curing systems, such as epoxy resin or PU systems. These products don’t dry out, they cure. The curing process does not need anywhere for moisture to dry into, so absorbent subfloors and the airflow above are not the critical factors. The critical factor here is warmth.

Next, levelling compounds are applied to provide a smooth surface ready for the installation of floorcoverings. Levelling compounds both dry and cure and the reactivity of the binders (normally cements) will be affected by both the temperature and humidity on site, along with how the products have been stored since manufacture.

Suitable storage
Cold, damp storage conditions will slow down even the fastest setting products to some degree. If you are relying on the fast setting nature of the product for a work time programme, then make sure you store bags and bottles in a warm environment (at least 24 hours for single bags and several days for pallets) before you plan to use them.

Site standards
The site conditions also have an important role to play and a nice, warm environment is the key to a successful flooring installation. It is important to note that chemical reactions are slower the colder it is.

For standard levelling compounds (i.e. not fast setting) the delay in drying/curing times due to cold, damp sites can be significant and mean waiting days before you can continue flooring works. If you can’t get conditions improved or haven’t the job size to allow waiting, then look to use the fast setting options, but bear in mind these will also be slower than their stated drying time if temperatures are low. In all instances, a temperature of below 5oc will be a serious problem, as it will affect the curing of the binders and may result in long-term issues.

Application thickness
Another factor affecting the drying rate is the thickness that the product is applied. Drying times are often based on a 3mm thickness, but if you’re applying thicker than 3mm there is more moisture to escape, so you may need to factor in more time.

It is very difficult to tell if a levelling compound has dried sufficiently and most contractors use their instinct based on how it looks, however the colour change can often be misleading and more of a surface effect. A pin meter is a good way of assessing if the compound is anywhere near dry. A reading up in the red or a surface very easily compressed by the pins shows it is not cured or dry enough to lay on.

The role of smoothing compounds on adhesives
As most floor coverings are relatively impervious, it is critical to provide somewhere for the moisture to dry into. This is where the levelling compound or substrate comes into play. A 3mm levelling compound that is laid onto an impervious base such as a DPM will provide enough absorbency for the adhesive to dry (once the initial open time is allowed). If you have less depth of levelling compound then it can get to a stage where the adhesive saturates the levelling compound and cannot continue to dry. This can result in a poorly bonded floor covering or bleed through of adhesives at joints, or even discoloration of the floor coverings. Bear in mind that even at 3mm thick, it is necessary that the levelling compound is also dry. If the substrate under the smoothing compound is absorbent then the criticality of levelling compound depth is less significant.

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