Why Floors Fail: When and how to screed

Laminate & vinyl flooring can fail if the surface onto which it is laid is not level. Screeds are used to form a flat and level surface, with adequate strength and resistance to indentation, on which other flooring is laid.Traditionally, screed is laid around 65mm thick onto a concrete subfloor. It’s a skilled job, usually undertaken by plasterers when the walls and ceiling are plastered. A screed mix is relatively strong but not intended as a wearing surface, the screeds primary purpose, using one part cement to three parts sharp sand, and when done well it gives a smooth and level floor on which to lay your chosen floor finish. A screed is also the preferred medium when laying underfloor heating pipes.

When laying tiles or laminated or vinyl flooring it is necessary to use a screed if the structural floor in not level. As a general rule the floor surface should not vary by more than 3mm per metre.

There’s nothing structural about screeds and some designs do away with one altogether. However, the floor beneath the screed is usually not laid to nearly such a high standard. Also, suspended precast concrete flooring systems, which are now increasingly being used instead of solid concrete slabs, have a noticeable camber which causes problems when it comes to laying most floor finishes. So although a screed can be expensive most builders will lay one when a top floor is planned.

The traditional way to lay screed is to mix the sand and cement on site with a mixer. Screeders are always looking for the perfect mix. This is pretty dry, almost powdery, when compared to brick and render mortars. The trouble is that mixing this by shovelling sand and cement into a mixer, and delivering to the screeders, keeps a labourer fully occupied — in itself expensive. So there has been a marked shift towards using ready-mixed screeds, delivered by lorry at the beginning of the day. Ready-mixed screeds come with added retardants to delay the set, so that you can be working all day with one load.

To standardise the process, there is a new trend towards pumped screeds, supplied and fixed by specialist crews. These pumped screeds use gypsum as a binder; they are also known as calcium sulphate or anhydrite screeds. They cost about 50% more per cubic metre than a cement mix, but they are much faster to lay and can be successfully laid at 50mm, or even 35mm depths where there are no underfloor heating pipes to cover, whereas a conventional screed needs to be at least 65mm deep.

The process of laying is very different to what we have grown used to with cement screeds. The biggest variable is the volume of screed used due to increased depths, especially on irregular sub-bases. It is not uncommon to for the depth of the screed to vary from 40mm to 80mm across a floor. Typical costs for, say, 100m² with an average depth of 50mm depth would be between $2800 and $4800.

The actual pumping process is very quick. The screed has to be carefully separated (via a polythene sheet) from any insulation sheeting placed below, because chemical reactions can take place although this is very rare.

Whichever system you use, ensure ample time for drying out. The rule of thumb is to allow a day for every millimetre depth, so that 70 days is the standard time recommended. If you have underfloor heating pipes buried in the screed, you can speed the process up but only very carefully: experts recommend doing nothing for a month, then putting the heating on at its lowest setting, turning it up by 3°C per day until the working temperature is reached.

Finally, there is the issue of underfloor heating. Best practice recommends that, to avoid movement issues, screeds should not cover an area of more than 40m2 or a distance in any one direction longer than 8m. Expansion joints should subdivide larger screed areas. Alternatively, you can place an anti-crack mesh in the screed or add fibres to the mix itself.

Levelling screeds can provide sufficient depth to incorporate ducts to contain services such as electrical power and telephone cables. Increasingly levelling screeds are being installed on Underfloor Heating Systems as they provide a thermal mass for the heating to dissipate into giving a consistent temperature across an area with a gradual rise and fall over time.

The thickness of the screed allows it to take up normal variations in flatness and levelness of the base on which it is laid. The common types of flooring laid over levelling screeds include carpet and carpet tiles, linoleum, laminate and vinyl flooring and tiles, wooden blocks, and ceramic tiles. Levelling screeds are not intended to act as a wearing surface and should always be covered with a final floor finish.

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