With more than 50 years’ experience in the local and international flooring industry, Denver Coleman, Chairman of Polyflor SA, answers questions posed by installers, architects and readers on their flooring challenges.* In this issue, he discusses self-levelling screed.
There is much talk in the industry about using self-levelling screed. How does it work, and how should it be applied? Patrick Furlong, Natal Midlands, KwaZulu-Natal
As resilient flooring products have developed and construction skills have deteriorated, the need for top-quality self-levelling products has become a necessity in order to achieve a good flooring installation. A smooth and level floor preparation allows for easier maintenance as well as an improved flooring appearance and overall performance.
It is important to note that smoothing and patching compounds should not be confused with modern, good-quality self-levelling products and that the correct product needs to be chosen for the correct floor area.
This can be done by determining the hardness of the product (the minimum hardness to give adequate performance for commercial use should be at least 28 to 35MPa).
Prior to applying a self-levelling compound, the unevenness of the substrate needs to be established. This is easily done by placing a 3m steel straightedge on the floor, or using a “level light meter” to indicate any high spots, hollows or unevenness in the substrate.
According to “Accuracy in Buildings”, SANS 0155 1980, a Class 1 screed should not vary by more than 3mm. Most self-levelling compounds, however, require a minimum of 3mm to be effective.
Most smoothing and self-levelling products require a dry sub-base and will be susceptible to delamination or failure if not hardened sufficiently, due to the presence of moisture. Moisture levels should therefore always be tested ahead of time. These compounds also require a primer on the substrate to prevent any delamination and air bubbles or pinholes forming.
Some good-quality primers will dry in less than an hour (thereby speeding up the application of the self-leveller) although others may take 12-24 hours to dry sufficiently.
Steel or power float screeds or any substrate with a very dense surface has a tendency to trap moisture, giving unreliable moisture readings on the surface. Drilling is therefore always recommended and if moisture is found, a liquid damp-proof membrane should be applied.
It is vitally important to remember that a damp-proof membrane should always be applied before the self-levelling or smoothing compounds. Primer application should always be mixed and applied according to the specific manufacturer’s instructions.
All levelling, smoothing and patching compounds are generally susceptible to cold temperatures and should definitely not be used below 5oC and used with care below 8oC of both ambient and screed temperature. Screed temperature can be measured using an infra-red digital thermometer.
Self-levelling products can be applied using either a rake with large teeth or a gauging rake. When using a rake, the depth of the teeth will determine the thickness applied to the floor, whilst using a smooth scraper with skids at either end will result in a smoother application and allow you to determine the required thickness. Always check with the manufacturer regarding the preferred application method and apply a pin roller treatment over the area before the material starts to harden.
There are several feather-finish compounds available to quickly repair any minor imperfections that may appear in your self-levelling product. These can also be used for very thin applications to smooth, level or ramp down floor surfaces.
If you have any flooring question relating to design, installation, problems or commentary you wish to share with Denver, please e-mail him at firstname.lastname@example.org with ASK DENVER in the subject line or phone Blythe at Polyflor on 011 609 3500.
*The views expressed in this article are not necessarily the views or opinions of FLOORS in Africa and may not be applicable to all resilient floorcoverings. This article has not been solicited or sponsored by FLOORS in Africa.