Virtually all flooring manufacturers specify that the screed must be sound, dry, level and free of contaminants when installing all types of floor coverings. Whether it’s carpeting, vinyl, LVTs, epoxy coatings or engineered wood in a hotel, office building, shopping centre or carpark, these are the elements that need to be examined. In this article, we investigate these four factors and what inferences can be drawn from the words used. We will then see if there are pre-existing standards available to guide specifiers with their projects.

1. Sound
A screed is a river sand / cement layer that has been applied on top of a concrete slab to provide a suitable finish onto which flooring can be laid. The strength of this topping or screed is based on the ratio of cement to sand in the mix. This screed topping should be no less than 20 MPa for residential and commercial applications. In specialised commercial and industrial applications, the engineer will specify the hardness to meet his “fit for purpose” requirements.

Based on Concrete Institute guidelines, this screed should be thicker than 40mm if no special actions are required to promote adhesion between the slab and the screed. If thinner screeds were specified, special arrangements would typically include the priming of the slab with a suitable bonding agent, and possibly the addition of the bonding agent into the river sand / cement mix to promote adhesion and hardness. When thinner than 40mm screeds are applied without the special arrangements, the outcome is often delamination between the screed and the slab, which compromises the screed.

Cracks in the screed, also known as “map cracking” is indicative of delamination between the slab and screed, and if this is the case, the screed is unsound. Fine cracks may be a result of shrinkage occurring during the drying process of the screed. These are generally harmless and need minimal rectification. Often cracks are induced during the drying process to eliminate the fine cracks caused by shrinkage. This is done by cutting straight lines into the screed at regular intervals (say 3m x 3m or 4m x 4m squares) in large floor areas. The shrinkage cracks then occur in the saw cut joins.

“So, ‘sound’ means that the screed is at the required strength or hardness; is firmly bonded to the slab and is not going to fail over time. Other substrates are also encountered, for example, power-floated slabs. These substrates are cementitious in composition and are generally harder and smoother than conventional screed toppings,” says Alistair Mc Dougall, Sales Manager at iTe Products.

2. Dry
When slabs are cast directly onto the ground, there should be a plastic sheet, a damp proof membrane onto which the concrete is poured. This membrane has been designed to prevent the ground moisture being drawn into the concrete and causing rising damp.

“Before installing an underlayment, moisture testing of the substrate should be done. If there is no damp proof membrane below a surface bed, or if moisture levels do not attain 5% (75% RH) or less, a suitable moisture, or vapour barrier should be installed. This will prevent the passage of water vapour and moisture through the concrete slab into the underlayment compound and final floor covering,” comments Sharon Margon, Technical Advice Supervisor at TAL.

Concrete and a river sand / cement screed toppings have prescribed curing rates. A rule of thumb guide for the rate of moisture loss is as follows: for the concrete/screed system to dry sufficiently for the installation of vinyl flooring, 28 days of drying time is needed for every 25mm thickness of slab/screed, or one day for every millimetre of screed thickness up to 50mm (BS 8203). A 100mm slab will therefore need approximately four months to dry sufficiently under normal conditions.

Alternatively a rapid-setting, self-levelling and smoothing compound can be installed to achieve a compressive strength of 25-30 MPa in 28 days.

“For commercial and industrial resin flooring applications such as hospital, hotel and restaurant kitchens, factory floors, parkades and even aircraft hangers, the preferred substrate is a concrete topping or concrete slab. In instances where the subfloor is not of quality, or at the required build or fails to receive the resin flooring, only compounds with significant compressive and flexural strengths should be used to achieve a suitable subfloor and ensure durability and longevity of the resin flooring installation,” says Sharon.

The most commonly accepted norm is that fully flexible and composite (heterogenous) vinyl are tolerant of screed moisture levels less than 3% MC. Semi-flexibles are generally accepted to be tolerant of 3,5% MC while carpeting, being porous and allowing moisture to evaporate through it, will have a higher tolerance. Wood and cork flooring are very moisture sensitive and generally demand that special measures be taken.

The maximum screed moisture tolerance is determined by the flooring manufacturer and should always be adhered to. If these specifications aren’t clear, refer to the relevant SABS code of practice (SANS 10070:2012 for vinyl, and 10186 for carpeting).

When screed moisture rises through the screed to encounter an impervious layer such as the floor covering, that moisture will accumulate underneath the flooring. If this moisture exceeds the flooring’s tolerance levels, the adhesive will typically soften and break down, resulting in lifting and bubbling in the flooring.

Screeds must therefore be dry enough to install the flooring. Testing for dryness is critical both before and after installation of the screed to ensure that the specified moisture levels have been achieved, and the installer must be competent to use the equipment.

3. Level
Various variations of this word are used, including “level”, “flat” and “smooth”, each having their own implication. “Level” implies that the screed has no angle from the horizontal, i.e. that it is perfectly level if a laser or spirit level is used to confirm its levelness. “Flat” implies that it shows no deviations when a straight edge or laser is used to confirm that the screed is flat. “Smooth” implies that the screed has no surface imperfections, graininess or lumps or divots. Vinyl flooring is very sensitive to any, even minute surface deviations, which will mar the floor covering’s appearance.

The SABS Code of practice SANS 10070:2012 states that the maximum deviation permissible for a class 1 screed is 1 x 3mm deviation vertically, either positive or negative from the horizontal plane when a 3m2 straight edge is laid over the screed in an arc around the pivot point. What this specification doesn’t cover, however, is the levelness of that flat screed.

“This levelness only really becomes critical when the horizontal plane is obviously visible as not being level against a reference point or line. It also poses a threat to safety in circumstances where warehousing reach trucks need to extend to very high vertical heights and the load then makes the reach truck unstable,” says Alistair.

Uneven screeds can impair the click systems used in some floor coverings, such as wood laminates, engineered wood and click vinyls. The manufacturers of these products set out their screed requirements to ensure successful installation of their floor coverings.

“We are therefore applying aesthetic norms for the screed to be acceptable in respect of flatness, levelness or smoothness,” adds Alistair.

4. Free of contaminants
Contamination of screeds is very common in renovations and applications where old premises are converted to new use areas. For example, a garage may be converted into a flat for residential use.

Motor vehicles frequently drop oil, brake fluid or other chemicals onto the screed. Many chemicals that contaminate screeds may have an adverse effect on the flooring to be installed above, resulting in discolouration, swelling or even softening. The adhesives may be prevented from bonding properly to the screed, or break down due to chemical incompatibility. It is therefore pivotal to ensure that no substance is present in or on the screed that may cause failure of the new flooring system.

All the above variables play a role in having a screed that is fit for purpose, and specialist flooring contractors who have years of experience tend to identify these as critical elements when doing professional flooring installations.

Full thanks and acknowledgement are given to and for some of the information contained in this article.

Photos courtesy of TAL.

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