Liquid Floor Screed Explained: Types, UFH, Drying and Cost

What is liquid screed?

Liquid screed (you may also see it called flowing screed, self-levelling screed or liquid floor screed) is a fluid mix that is pumped into a room and levels itself out. It goes down over insulation or onto a concrete slab and gives a smooth, flat surface for the floor covering that follows.

Because it is delivered through a hose, it can cover a big area quickly and reach rooms well back from the road. That makes it a common choice for extensions, new builds, renovations and floors with underfloor heating.

In Birkenhead that might mean a back kitchen in a Tranmere terrace or the whole ground floor of a new house. The material is the same either way; what changes is the preparation and the timing.

How it compares with a hand-laid screed

A traditional screed is mixed on site, carried in and levelled by hand. On bigger floors that is slow, and how flat the result is depends on the person with the straight edge.

Liquid screed levels itself as it is pumped, so a large floor can be done in a fraction of the time and the surface usually comes out very flat. It can often be laid thinner, particularly over heating pipes, and it wraps fully around them. The trade-off is that it needs a well-sealed build-up underneath and patience while it dries before any flooring goes down.

Not sure which suits your project? Tell us about the room, the finish you want and your programme, and we will give you a straight answer.

Anhydrite or cement-based liquid screed?

Liquid screeds come in two main types. Anhydrite liquid screed, also known as calcium sulphate screed, is bound with a gypsum-based binder. Cement-based liquid screed, sometimes called liquid cement screed, uses cement instead but still flows and levels itself.

Anhydrite is a favourite over underfloor heating because it can usually go down thinner and passes heat through well. It does not like prolonged wetting, so it is generally kept out of areas that stay wet, and the surface often needs sanding and priming before some floor finishes.

Cement-based types cope better with moisture and often work with cement-based tile adhesives without the same surface preparation. Which is right comes down to the room, the floor finish, the timetable and the manufacturer's guidance, and we are happy to talk it through before you book.

Liquid screed thickness and depth

How thick liquid screed needs to be depends on what sits underneath and what the floor has to carry. Laid over insulation as a floating screed, anhydrite products in homes are commonly specified from about 35mm, going deeper for heavier loads and commercial use. Bonded straight to a concrete slab, they can usually be thinner. Cement-based products generally need a little more depth than anhydrite.

Agree the finished floor level early. In older Birkenhead houses the new floor often has to line up with existing door thresholds and the step into the next room, and the depth of the screed is a big part of that calculation.

Liquid screed for underfloor heating

Underfloor heating is one of the biggest reasons people pick a liquid floor screed. As it flows it closes around each pipe completely, so there are no pockets of trapped air and heat travels evenly up through the floor.

There is no single minimum thickness for underfloor heating; it is set by the screed product and the heating system. As a rough guide, anhydrite products in homes are commonly specified with about 25mm of cover above the pipes, which on insulation often works out at a total depth of around 40 to 50mm. Cement-based liquid screeds usually need a bit more cover. The manufacturer's figures are the ones to follow.

A thinner screed means less mass to warm up, so the heating can respond faster. The pipes should be secured firmly, because flowing screed can lift loose pipework, and filled and pressure tested before the pour so any leak shows up in time.

• Pipes clipped or fixed so they cannot float up
• System filled and pressure tested before pour day
• Heating kept off while the screed is poured and in the early curing period
• First heat-up done gradually, following the screed manufacturer's schedule

Liquid screed drying times

Being able to walk on a floor and the floor being dry are two different things. Most liquid screeds will take light foot traffic within a day or two, but how long it takes to dry enough for flooring 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 about one day per millimetre up to 40mm, with drying getting slower beyond that depth. Ventilation speeds things up, and many anhydrite products let you use the underfloor heating to help dry the floor after an initial period, following the manufacturer's schedule. Cement-based products vary more, so go by the data sheet.

The calendar is only a guide. A moisture test is what confirms the floor is ready, and both the screed and the flooring manufacturers set the readings they will accept. Putting flooring down too soon is a common cause of trouble, especially with timber, laminate and some vinyls.

Preparing a floor for liquid screed

Most problems on pour day start with the preparation. Before we book, we go through floor area, access, the build-up, any underfloor heating and delivery timing with you. This checklist covers the main points:

• Building watertight, with windows and doors fitted or openings sealed
• Insulation boards flat, tight together and fully supported
• Edge strip fixed around every wall, pillar and fixed item
• Membrane laid with joints and edges taped so the screed cannot escape
• Underfloor heating pipes secured, filled and pressure tested
• Finished floor level marked so everyone knows the target height
• A clear route for the hose from the street to the room
• Other trades warned to stay off the floor while it sets

Liquid screed cost per m2 and per m3

We do not publish a price list, because two floors of the same size can cost quite different amounts. Any liquid screed price per m2 quoted online depends heavily on the depth, the access and the size of the job. These are the factors that move the cost most:

Liquid screed is normally bought by the cubic metre, so the price per m3 and the price per m2 are tied together by the depth. Volume is the floor area multiplied by the depth in metres: a 30 m2 kitchen extension at 50mm needs about 1.5 m3, while the same room at 40mm needs about 1.2 m3. That is why the depth makes such a difference to the cost per m2.

For an accurate quote, send the approximate area, the depth if you know it, whether underfloor heating is fitted and a note on access. A couple of photos of the room and the way in from the street are a big help.

• Floor area: bigger floors tend to work out cheaper per square metre
• Depth: a deeper floor needs more cubic metres of material
• Anhydrite or cement-based, and any specific product the job calls for
• Underfloor heating, which can change the depth and the product
• How far the hose has to run from the vehicle to the room
• Timing, and how the pour fits around the rest of the build
• Whether the insulation, edge strip and membrane are already in

More questions