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Masterclass In Screed

Flow screeds are shaking up the normally sedate world of screeds. Hugh James explains why.

Very occasionally in the world of house construction a time-honoured way of doing things suddenly changes. But it takes a lot to uproot tried and tested methods.

Builders aren’t keen on risking changing something that works well.

For something new to catch on there needs to be a compelling reason, usually in terms of cost or time saving. Sometimes the impetus is driven by changes to the Building Regs, such as meeting stiff thermal insulation targets, or architects may start specifying something en-masse that’s of demonstrably improved quality or performance.

The increase market of flowing screeds

Either way, we may be about to witness one such ‘sea change’ in the normally sedate world of floor screeds with the rise of liquid screeds.

There’s no law that says you have to cover your ground floor structure with a screed before laying floor coverings. Screeds are not structural. For example, in garage conversions it’s common for floorboards to be placed over thick sheets of insulation.

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.

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.

Hugh James, director of flowscreed® Ltd, says: “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 30% of the market,” he says., says: “There’s been a huge increase in the popularity of pumped anhydrite screeds over the last five years.

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. Andy Vincent from Screed Giant says: “The market has been shaken up in last six months as calcium sulphate has been joined by new cement-based products like McGraths’ Cemfloor and Tarmac’s Belitex.” These claim to offer similar benefits to gypsum but without the downsides. So, what’s best?

The Pros and Cons

Sand/Cement

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 UFH, 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 ready mix improves the consistency but adds to the cost, and unless skilfully applied the quality can still be mediocre.

As a rough guide, laying a conventional screed costs around £15/m2, increasing by around £5/m2 when delivered ready-mixed.

Pumped Anhydrite Screeds

Anhydrite (liquid calcium sulphate) screeds offer a number of advantages:

  • They’re not much dearer than ready-mix sand/cement, costing around £25/m3 (laid to a 50mm depth).

  • Although the material itself costs about 50% more per m3 than sand/cement, being poured from a large hose makes it much quicker to lay (it’s claimed 20 t