The Ultimate Guide to Floor Screeds

Are we about to witness a revolution in the normally sedate world of floor screeding? Quite possibly, says Ian Rock

Ian Rock investigates the changing world of screeds, how to install a screeded floor and explains why liquid screeds could be the future

When a widely used building material or method of construction is banned in other parts of the world it’s probably time to sit up and take notice. So should we be worried that in the Netherlands the manual application of traditional sand cement floor screeds is now prohibited? Dutch screeds must be applied using powered machines operated by personnel in a walking position rather than being trowelled on in a hands and knees position. Clearly this is more of a health and safety issue rather than a concern about poorly performing materials. But I have yet to meet a floor screeder who enjoys this laborious work; most complain of aches and pains for days afterwards. And when a job is unpleasant there’s more chance of it being done badly.

The Rise of Liquid Screeds

Very occasionally in the world of house construction a time-honoured way of doing things suddenly changes. In the late 1990s, beam and block ground floors started to supersede traditional solid concrete in mainstream housing. Two decades earlier, prefabricated trussed rafters ousted old-fashioned cut timber roof structures in most new homes. But it takes a lot to uproot tried and tested methods. Builders aren’t keen on risking changing something that works well, occasionally citing self-justifying defences like: “We’ve being doing it like this for the last 20 years”. Even technically superior methods of construction such as

timber frame, SIPS (structural insulated panels) and ICF (insulating concrete formwork) are a minority interest and, amazingly, underfloor heating is still widely ignored by corporate housebuilders. 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. Either way, we may be about to witness one such ‘sea change’ in the normally sedate world of floor 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 the majority of 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 de- spite 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.

Ashley Sexton is the director of Express Liquid Screeds, one of the UK’S leading approved installers. “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,” he says. 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. But anhydrite isn’t the only game in town and cement 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?

Sand/cement: The Pros and Cons

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: The Pros and Cons

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 times as quick) with consequent savings on labour. Costs are also reduced because it can be applied at depths as thin as 35mm. With underfloor heating you typically need 30mm cover above UFH pipes, making about 45mm in total, still incredibly slim by conventional standards. One of the big advantages of pumped liquid screeds is that they’re effectively self-levelling and self-curing which translates into a nice level finish. While traditional sand/cement screeds aim to achieve the standard known as ‘SR3’, which permits a 10mm difference over 2m, liquid screeds aim for perfection (SR1 standard) but guarantee SR2 with a variation of plus or minus 5mm. Quality is further assured as liquid screeds are applied by specialist approved contractors.

Anhydrite is far less prone to shrinkage than cement and needs far fewer expansion joints ( just across some door- way thresholds). This makes it compatible with large stone tiles that can be vulnerable to cracking should the screed beneath them shrink. Best of all, liquid screeds are ideally suited to UFH because the liquid envelopes the pipes eliminating voids, thereby enhancing heat transfer between the pipes and the screed. Also the slimmer covering over pipes allows a quicker reaction time with better response/controllability.

One downside of anhydrites is that you need to prepare the surface by sanding prior to tiling.

Pumped Liquid Cement-based Screeds

Applied in the same way as anhydrite and laid to the same thickness, liquid cement screeds have similar properties to anhydrite: they are self-compacting, quick to lay and Ufh-compatible. Manufacturers claim they are quicker to dry, even in adverse conditions, and can be tiled with little or no surface preparation. However, they’re relatively new to the UK and so far there’s little hard evidence to substantiate performance, although they’ve achieved a successful track record abroad. They’re also about 20% dearer in terms of materials, although costs may fall as demand rises.

Preparing the Floor

With liquid screeds of all types it’s essential to get the preparation right. The floor needs to be ‘ tanked’ like a mini swimming pool using a polythene sheet membrane (typically 500 gauge). In most cases the surface to be screeded will consist of 50 to 100mm-thick layers of insulation boards over a DPM (damp-proof membrane) on top of the floor structure. It’s important these sheets are stable when walked on and laid so they don’t rock about on uneven subfloors. This base layer of insulation needs to be lined with polythene sheeting to stop the liquid escaping and, in a

worst-case scenario, causing the insulation boards to lift up and float around the room like plastic ducks in a big bath. Lining also helps prevent any potential chemical reaction with the insulation underneath. Any rogue plumbing or obstacles may need temporary formwork placed around them to ensure 100% watertightness. To contain the liquid, the polythene sheet membrane is then lapped up the walls in a sharp right angle and glued in place with spray adhesive. This is in addition to existing layers such as the damp-proof course/damp-proof membrane, insulation strips around wall edges, and thin strips of compressible foam designed to accommodate any expansion once the screed is dry.

The final job is to tape all the joints and scrape off any lumps of plaster or debris so they don’t float up to the surface once the screed is poured. If you’re installing UFH it’s essential the pipes are pressurised to weigh them down and they’re securely fixed over the polythene membrane to prevent them lifting up and floating as the screed is poured.

With liquid screeds, it’s easy to underestimate the volume of material required if the floor structure is uneven. For example, the camber in beam and block floors means that the point you measure from might be a bit higher or lower than the median. So you might order 50mm depth and find it actually ranges from 40mm to 70mm.

Applying the Screed

The installers usually arrive on site a couple of hours in advance to set up their pump and check floor measurements to ensure the amount of screed ordered will be sufficient. Working from a level datum point, such as the base of the stairs or a doorway, a number of tripod levelling gauges are placed around the floor at various points and with the help of a laser adjusted to a consistent depth, thereby ensuring the finished screed surface is level throughout. Once the

Drying Out Times

For conventional sand/ cement and anhydrite screeds you can walk on the floor after about 48 hours once it becomes solid. Normal site traffic can resume after about a week and partition walls erected. Another week and a kitchen can be fitted. Unfortunately you can’t lay floor coverings until the screed is 100% dried out all the way through. Allow a day for every mm depth up to the first 40mm, then two days per mm. The optimum drying conditions would be an ambient temperature of around 20°C and well-ventilated rooms (windows must be closed for the first 72 hours to prevent draughts causing over-rapid drying). To be sure the floor has fully dried (less than 75% relative humidity or 0.5% moisture content) a special hydrometer can be used. In some cases it may be acceptable to lay impermeable floor tiles a little sooner.

However, drying times can be affected by several factors. With sand/ cement, specifying a modified admixture can speed things up so floor coverings can be down in about three weeks. Anhydrites are very sensitive to drying conditions — it helps if the walls are plastered and dried out first so that room moisture levels aren’t too high. If time is short the process can be speeded up using ‘force drying’ techniques, shrinking drying times to less than 40 days. Dehumidifiers can be used to accelerate drying 72 hours after placing. And after the first week UFH can be set to its lowest temperature and gradually turned up by 3°C per day until the working temperature is reached.

Some regard drying times as the achilles heel of anhydrites. But this may be down to lack of awareness about how they work. Andy Vincent from Screed Giant says: “Calcium sulphate liquid screeds have been misunderstood. In the UK, 95% of screeds get laid too early, hence the risk of poor drying. Elsewhere they regard it as a floor finish so it’s applied later in the process when the indoor atmosphere is suitably dry.”

The big advantage of cement liquid screeds is that their drying times are claimed to be as short as two to three weeks for a 40mm thick floor with good drying conditions, compared to around 40 days with calcium sulphate. They can also be force-dried and won’t re-absorb moisture, so are more reliable in poor drying conditions.

But being so new the jury is still out. Tramex will soon be conducting scientific comparison testing in Ireland under less than ideal ‘real world’ conditions. Watch this space.

mixer truck arrives the pre-mixed pumped screed is poured from a hefty-looking hose until the surface level reaches the marker tripods, which are then removed. It generally takes less than an hour to lay an entire ground floor in an average new house. To make sure the screed is smooth and level with no air bubbles, the freshly applied screed is briefly manually agitated by a bloke wading through using a ‘dappling bar’.

Tiling and Floor Coverings

Sand/cement screeds are usually a bit dusty or sandy on the surface due to precipitation or poor mixing so they tend to need a firm brushing before priming and tiling. Calcium sulphate screeds are more demanding in this respect, because as the anhydrite cures a fragile coating of fine particles known as ‘ laitance’ is deposited on the surface along with any bits of debris that may have floated up. This weak surface film is too friable and flaky to tile onto and needs to be abraded by sanding after about five to 10 days and the dust vacuumed off. Failure to do so is a major cause of failure with floor tiles as it can peel off. Having this sanding done professionally can add around £8/m2 to the cost.

However, there are some new ‘ low laitance’ varieties that only require a stiff brush or light sanding with 60 grit sandpaper. Problems can develop if the laitance is left for too long, as it becomes harder to remove and can also hinder the drying process. This isn’t such an issue if you plan to lay unbonded flooring without adhesive, such engineered timber boards. But before tiling it’s essential that floors are free of loose particles and primed to seal porous surfaces and prevent suction of moisture. You also need to use special anhydrite-compatible tile adhesives that don’t chemically react with gypsum, such as Anhyfix or Keracoll H40.

New Varieties

Manufacturers like Tarmac provide specially formulated products such as Topflow Soundbar — an acoustic screed originally designed to improve sound insulation between flats. These can be placed on top of timber floors, first having been strengthened with a layer of resilient foam under 24mm-thick chipboard.

For use with underfloor heating, specially formulated Thermio Plus only needs a super-thin 20mm covering over the pipes, making it more efficient at conducting heat into the room with quicker response times to temperature changes and reduced drying times. Other specialist screeds have been developed for use with Lewis Deck dovetail sheeting, for underfloor heating and acoustics on timber joisted upper floors.

The Future

Small developers have long wanted to use liquid screeds, especially with underfloor heating. But awareness of the benefits of anhydrite has been tempered by a certain amount of ‘ bad press’ about drying times and the need to abrade it to remove surface laitance.

Andy Vincent reckons “cement-based liquid screeds are likely to be the future for self-builders.” But some newer anhydrites are laitance-free and, being thinner, are quicker to dry. Ashley Sexton adds: “The only downside with cement-based products is they can be prone to shrinkage and cracking. If this can be overcome it will knock spots off the anhydrite market.” Time will tell how this plays out. Either way, good old sand/ cement will still be around because pumped screeds are only cost-effective on floors areas of at least 40m2. For many smaller home extensions, floors will continue to be laid in the traditional way for the foreseeable future. H