Specifying and working with self-levelling screeds

by Darren
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In this feature, we take a look at the considerations that need to be kept in mind when specifying self-levelling screeds.

“When talking about screeds, it is important to remember that these products are alternatives to conventional methods,” says Ramielle De Jager from Saint-Gobain Weber. “They have been developed to address issues and improve conventional methods, making screed an extremely necessary alternative. It saves time and money by being rapid setting and offers better control on the outcome of the floor as the recipe is engineered to perform at a specified level for the specified purpose,” says Ramielle.

Before you consider screeds, it’s important to understand how subfloors are made up and how they are prepared for the installation of floorcoverings. Screeds are a sand/cement compound which is mixed on a building site and applied over a concrete slab to provide a suitable base onto which floorcoverings can be installed. How well this is done will determine the next steps and success of the flooring installation.

“Sometimes levelling compounds are referred to as a screed. Technically this is incorrect,” says Sales Manager at iTe Products, Alistair Mac Dougall, before explaining that a levelling compound is an underlayment that is part of the system used to create the ideal surface finish prior to the installation of the flooring.

Most older-generation smoothing compounds have been called self-levelling, which is a misnomer, as they function as a smoothing compound rather than as a self-levelling compound. For the purpose of this article, we will examine true self-levelling compounds and how they function within a system used in the preparation of the substrate ready for the flooring installation.

The reason for using a self-levelling compound arises when the choice of floorcovering to be installed is made. Resilient floorcoverings tend to mould to the substrate tightly, which reveals any (even very minor) substrate imperfections, while a carpet will hide defects much better. This then requires the substrate to be a flat, level and smooth as possible to ensure a good-looking floor.

The use of the area to be covered also plays a role in the requirements for the substrate.

“At one extreme, a warehouse floor where reach trucks operate, lifting heavy loads to great heights, will need an absolutely flat and level surface to operate on, otherwise they could topple. On the other end of the scale, a wavy substrate which is flat will not inconvenience people pushing trolleys, and if aesthetics are not important, little attention to detail may be necessary. In-between we have those areas which are sensitive to aesthetics, need to be hard enough to carry substantial weight and need to be durable, such as a hospital,” says Alistair.

Keep the following in mind when choosing self-levelling screeds
Andrew Dekker, National Operations Manager – Construction Products Division at TAL says that there are a number of things that should be kept in mind when specifying self-levelling screeds.

“Firstly, one needs to determine if you are looking for an underlayment or an overlayment screed because there are fundamental differences. An underlayment provides a perfectly smooth surface on which to install your floorcoverings, especially resilient coverings and laminates to ensure a good aesthetic appearance. An overlayment screed is a much more durable product designed as a final floorcovering,” says Andrew.

For underlayment screed, ensure that it provides sufficient strength for the anticipated traffic and service conditions. It should have enhanced flow characteristics and be rapid-setting to enable the contractor to continue with the flooring installation after minimal waiting time.

Why vinyl and similar floorcoverings fail: moisture
Moisture in the substrate is a very complex issue and it is one of the most common cause of failure in vinyl and similar floorcoverings. Moisture levels below the tolerances of the components that comprise the system do not pose a problem. However, when moisture levels exceed these tolerances, problems arise.

“There is always moisture within substrates, however it is the rate at which the moisture vapour is emitting that will determine the type and WFT (Wet Film Thickness) of the moisture barrier necessary. High moisture levels in the substrate, or more accurately, high rates of moisture vapour emissions, can adversely affect the performance and integrity of the screed, which could easily result in an installation failure i.e. debonding of the screed, ‘bubbling’ of the vinyl or resin. Be sure to use a 100% Solids Epoxy System for best results,” says Andrew.

Using trained installers and professional equipment is key
Application of self-levelling compounds requires professional equipment and training of the applicators. Although a rare occurrence with high quality products, if a roughened or uneven surface is experienced, the contractor can apply a second skim coat in strict accordance with the manufacturer’s specifications.

“The correct application of a self-levelling screed is imperative to a successful finished floor. A sound and prepared sub floor is required before applying the screed as this will ensure a robust screed which will provide an ideal base for your tiles, vinyl or carpeted floor to perform at its best,” says Ramielle.

Modern, third generation high performance self-levelling compounds provide a truly reliable cost effective solution. They dry rapidly, enabling traffic after two hours, and can have vinyl flooring installed the next day. This shortens the installation process. The formulations have a much better water temperature range, are resistant to swelling due to moisture and do not need grinding, sanding or post application work to obtain a great surface.

Andrew says there are a number of benefits of self-levelling screeds.

“Reduced labour costs make a self-levelling screed a cost effective choice. Using a factory produced product rather than site-mixing a mortar also ensures consistent quality, delivering a mirror smooth surface with minimal sanding and less resultant dust. A rapid setting screed further ensures quicker return to service times for the final floorcovering being installed,” says Andrew.

“Self-levelling screeds are a perfect solution for renovations and are the best way to prepare a floor on a new build,” comments Ramielle.

“When all factors are taken into account, they will be more cost effective than earlier versions. These self-levelling compounds are designed to enable large, continuous surfaces to be prepared to the highest specifications,” concludes Alistair.

Full thanks and acknowledgement are given to www.iteproducts.co.za, www.weber-tylon.co.za and www.tal.co.za for the information contained in this article.

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