Liquid Screed Guide for Heswall: UFH, Depth, Drying and Cost

Liquid screed explained

Liquid screed goes by several names: liquid floor screed, flowing screed, flow screed or self-levelling screed. Whatever it is called, it is a pourable mix, pumped over insulation or a concrete slab, that spreads out to a level surface ready for tiles, timber or other finishes.

Because it is delivered through a hose rather than barrowed in, it can be placed over a large area in a short time and can reach rooms well back from the road. That makes it a good fit for the bigger floor plates typical of Heswall's detached houses, as well as for single extensions and refurbishments.

Liquid screed for underfloor heating

Underfloor heating is often the reason a project ends up with liquid screed. A fluid mix works its way under and around the pipes, so there are no air gaps to slow the heat down, and warmth spreads more evenly across the room.

There is no single minimum thickness for liquid screed over underfloor heating; the product and the heating system decide it. As a rough guide, anhydrite screeds in homes are often specified with about 25mm over the top of the pipes, which on insulation tends to mean somewhere around 40 to 50mm in total. Cement-based products normally need a bit more cover. The manufacturer's figures are the ones to follow.

A thinner layer means less mass to heat, so the floor can react more quickly when the heating comes on. Before the pour the system should be filled and pressure tested, and the pipes firmly clipped down, because a flowing screed can lift any loose pipework.

• Pipework filled and pressure tested ahead of the pour
• Pipes clipped down securely so they cannot float up
• Heating left off while the screed is poured and in its early days
• Warming the floor up for the first time done gradually, following the screed manufacturer

How thick should liquid screed be?

Depth comes down to what the screed sits on and what load it will carry. Laid floating on insulation in a house, anhydrite liquid screed usually starts from about 35mm, with extra depth for heavier use or commercial floors. Where it is bonded directly to a concrete slab it can generally be thinner. Cement-based liquid screed typically needs a little more depth than anhydrite.

With underfloor heating, think of the depth as two parts: the height of the pipes plus the cover over them. Fix the finished floor level early on, especially in an extension, because it decides where door thresholds sit and whether you end up with a step into the existing house.

Anhydrite or cement-based liquid screed?

The two main types differ in their binder. Anhydrite liquid screed, also known as calcium sulphate screed, is gypsum based. Cement-based liquid screed, sometimes called liquid cement screed, uses cement but still flows and levels itself.

Anhydrite is a common choice over underfloor heating because it can usually go down thinner and transfers heat well. It does not like moisture, so it is generally kept out of areas that stay wet, and it normally needs sanding and priming before some floor finishes are laid.

Cement-based liquid screed copes better with moisture and often works with cement-based tile adhesives without the same surface preparation. Which one suits your Heswall project depends on the room, the finish, the timescale and the manufacturer's guidance, and we are happy to talk it through before you book.

How long does liquid screed take to dry?

Being able to walk on a screed and it being dry enough for flooring are two very different things. Light foot traffic is often fine within a day or two, but full drying depends on the depth, the type of screed, the temperature, the airflow and the season.

For anhydrite liquid screed, a widely used rule of thumb is roughly a day for each millimetre up to 40mm, then slower beyond that. Keeping air moving through the room speeds things up, and with many anhydrite products the underfloor heating can be switched on to force dry the floor once an initial period has passed, using the manufacturer's schedule. Drying for cement-based liquid screed varies more from product to product, so the data sheet is the guide.

The calendar is only an estimate. A moisture test is what confirms the floor is ready, against the limits set by the screed and the flooring manufacturers. Putting flooring down too soon is one of the most frequent causes of trouble, particularly with timber, laminate and some vinyls.

Getting the floor ready for liquid screed

Most problems on pour day start with a floor that was not quite ready. When you enquire, we confirm the area, the access, the preparation, the heating and the timing. Work through this list before the date:

• Shell watertight, with windows and doors fitted or openings sheeted
• Insulation flat, tight and fully supported
• Edge strip fixed around every wall, pillar and fixed unit
• Membrane down with all joints taped so nothing can seep through
• UFH pipes clipped, filled and pressure tested
• Finished floor height marked so everyone works to the same level
• A clear path for the hose from where the vehicle parks to the room
• Other trades warned to keep off the floor while it sets

What liquid screed costs per m2 and per m3

We do not publish a price list, because a liquid screed price per m2 only means something once you know the depth, the access and the size of the job. These are the factors that move the cost of liquid screed most:

Material is sold by volume, so the price per m3 and the price per m2 are tied together by depth. To estimate volume, multiply the area in m2 by the depth in metres. An 80 m2 ground floor at 45mm needs about 3.6 m3; at 40mm it needs about 3.2 m3. A few millimetres across a big floor adds up quickly.

For an accurate figure, send us the approximate area, the depth if your designer has specified one, whether underfloor heating is going in and a note about access. A photo of the room and one of the route from the road are a real help.

• Area: bigger floors are usually more efficient per m2
• Depth: every extra millimetre means more cubic metres
• Screed type, anhydrite or cement-based, and any special requirements
• Underfloor heating, which can change the depth and the product
• Access, including the hose run from the vehicle to the room
• Timing and how the pour fits with other trades
• How far the build-up is from ready on the day

Liquid screed compared with traditional screed

A traditional screed is mixed on site, wheeled in and levelled by hand. On a large floor that takes time, and the result depends a great deal on the skill of whoever is levelling it.

Liquid screed is pumped and finds its own level, so a big area goes down quickly and the surface is typically very flat. It is usually thinner, especially over underfloor heating, and wraps fully around the pipes. The trade-off is that it needs a well-sealed build-up and proper attention to drying before the floor finish goes on.

Not sure which is right for your Heswall build? Tell us about the room, the finish you want and your timescale, and we will give you a straight view.

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