Screed: Everything You Need to Know
When it comes to floor screed, liquid options are shaking up the normally sedate screed world. Hugh James explains why.
Pouring liquid screed.
There’s no law that says you have to use a floor screed to cover your ground floor structure before laying your chosen covering.
Floor screeds are not structural. For example, in a garage conversion, it’s common for floorboards to be placed over thick sheets of insulation instead.
One of the main reasons screeds are used in the vast majority of homes, despite the extra expense, is that they’re very useful for levelling uneven floor structures. This is particularly important with modern precast concrete beam and block floors, which have a distinct camber that can cause problems when it comes to laying floor finishes.
Screeds are also the preferred medium for underfloor heating pipes as the material excels at absorbing and radiating warmth, becoming an integral part of the heating process.
Floor Screed Mix
Conventional sand/cement screed uses a relatively strong mix (1:3 cement to sharp sand) with a fairly dry, almost powdery consistency.
Traditionally, it would be prepared on-site but shovelling sand and cement into a mixer is very labour intensive and prone to inconsistency, which is why most larger sites now use ready-mixed screeds delivered by lorry, in a similar way to foundations.
Ready-mixed screeds also come with added retardants to delay the set, so a single load can last all day. But despite such improvements in quality, poured liquid screeds installed by specialist firms are starting to make major inroads into a market dominated until now by traditional sand and cement.
Sally Pepper, the head estimator of flowscreed® Ltd explains: “There’s been a huge increase in the popularity of pumped anhydrite screeds over the last five years; it’s estimated they now account for nearly 20% of the market,” she says.
Understanding Floor Screed Terminology
The terminology can be a little confusing because the terms ‘anhydrite’, ‘calcium sulphate’ and ‘gypsum’ are used interchangeably.
The key ingredient is anhydrous (dry) calcium sulphate, typically comprising around 35% recycled content. When mixed with water this forms gypsum, the same material found in modern plasters and plasterboard. The gypsum replaces conventional cement as a binder.
Cement also seems to be making something of a comeback. Cement-based products like McGraths’ Cemfloor and Belitex claim to offer similar benefits to gypsum but without the downsides.
What's the Best Type of Floor Screed?
Old-fashioned sand/cement screed is relatively cheap to produce and place as it can be prepared by unskilled labour and trowelled on site. It also requires minimal preparation other than taping joints between insulation boards. And unlike poured liquid screeds, sand/cement readily lends itself to the formation of sloping floors in wetrooms.
The downside is often patchy quality, uneven finish and a propensity to develop cracks. Where floors are uneven, a self-levelling compound may need to be applied before floor tiling. To avoid cracking, manually applied screeds are commonly laid in 5m-long bays, sometimes with anti-crack mesh or fibres added.
Although it’s compatible with underfloor heating, hand-mixed sand/cement isn’t ideally suited as it’s relatively thick (typically laid 65mm to 75mm deep) and prone to harbouring pockets of air that can act as a barrier to heat transfer. Using the ready mix improves the consistency but adds to the cost and, unless skilfully applied, the quality can still be mediocre.