Liquid Screed Guide: Cost, Drying Times and Underfloor Heating

What is liquid screed?

Liquid screed, also called liquid floor screed, flowing screed or self-levelling screed, is a fluid mix that is pumped onto the floor and finds its own level. It is laid over insulation or a concrete slab to create a flat, smooth base for the final floor covering.

Because it is pumped through hoses, it can be placed quickly over large areas and can reach floors well away from the road. It is widely used in extensions, new builds, renovations and over underfloor heating.

In St Helens that covers everything from a rear extension on a semi in Eccleston to a whole ground floor on a new build plot. The principle is the same; the preparation and timing change with the job.

Anhydrite vs cement-based liquid screed

There are two main families of liquid screed. Anhydrite liquid screed (also called calcium sulphate screed) uses a gypsum-based binder. Cement-based liquid screed, sometimes called liquid cement screed, uses cement as the binder but is still flowing and self-levelling.

Anhydrite screeds are popular over underfloor heating because they can usually be laid thinner and conduct heat well. They are sensitive to moisture, so they are generally not used in areas that will stay wet, and they normally need the surface sanded and primed before certain floor finishes go down.

Cement-based liquid screeds are more tolerant of moisture and are often compatible with cement-based adhesives without the same preparation. The right choice depends on the room, the floor finish, the programme and the manufacturer's guidance, and we can talk it through with you before booking.

Liquid screed for underfloor heating

Underfloor heating is one of the main reasons people choose liquid screed. As it flows, it surrounds the pipes completely, leaving no air pockets, which helps heat move evenly through the floor.

The minimum liquid screed thickness for underfloor heating is set by the product and the system, not by a single rule. As a guide, anhydrite products are commonly specified with around 25mm of cover over the top of the pipes in homes, which often means a total depth of roughly 40 to 50mm on insulation. Cement-based liquid screeds usually need a little more. Always work to the manufacturer's figures.

Because it can go down thinner than a hand-laid screed, there is less material to warm up and the system can respond more quickly. Before the pour, the pipes are usually filled and pressure tested so any leak shows up first, and they need to be properly secured, as the screed can lift loose pipework as it flows.

• Pipes filled and pressure tested before the pour
• Pipework fixed down so it cannot float
• Heating switched off during the pour and early curing
• Commissioning (bringing the heating up gently) follows the screed manufacturer's guidance

Liquid screed drying times

How long does liquid screed take to dry? Most liquid screeds can take light foot traffic within a day or two, but walking on a floor is not the same as it being dry enough for flooring. Drying time depends on depth, the type of screed, temperature, ventilation and the time of year.

A common rule of thumb for anhydrite liquid screed is around one day per millimetre for the first 40mm, with drying slowing down beyond that. Good ventilation helps, and many anhydrite products allow the underfloor heating to be used to help force-dry the floor after an initial period, in line with the manufacturer's schedule. Cement-based liquid screeds vary more by product, so check the data sheet.

Do not rely on the calendar alone. Moisture testing tells you when the floor is ready, and the screed and flooring manufacturers both set the moisture levels they accept. Laying flooring too early is one of the most common causes of problems with finished floors, especially timber, laminate and some vinyls.

Liquid screed thickness and depth

The right liquid screed thickness depends on what it is laid over and what the floor will carry. On insulation (a floating screed), anhydrite liquid screeds are commonly laid from around 35mm in homes, with more depth for heavier loads and commercial floors. Bonded to a concrete slab, they can usually go thinner. Cement-based liquid screeds tend to need slightly more depth than anhydrite.

Over underfloor heating, the depth is measured as cover above the pipes, so the total depth is the pipe height plus that cover. The product data sheet sets the minimum, and it is worth agreeing the finished floor level early, as it affects door thresholds and the step into an existing room.

Liquid screed cost per m2 and per m3

We do not publish prices because every floor is different, and any liquid screed price per m2 you see online depends heavily on depth, access and job size. These are the things that make the biggest difference to how much liquid screed costs:

Liquid screed material is usually measured by the cubic metre, so the cost per m3 and the cost per m2 are linked by depth. To work out the volume, multiply the floor area by the depth in metres: a 50 m2 floor at 50mm deep needs about 2.5 m3, while the same floor at 40mm needs about 2 m3. That is why depth makes such a difference to the price per m2.

The quickest way to an accurate figure is to send us the rough area, the depth if you know it, whether UFH is fitted and a note on access. A couple of photos of the floor and the route in from the road help too.

• Floor area: larger floors usually work out more efficient per square metre
• Depth: deeper floors need more material, so more cubic metres
• Type of liquid screed (anhydrite or cement-based) and any special product requirements
• Underfloor heating, which can affect the depth and product used
• Access and hose distance from where the vehicle parks to the floor
• Timing, and whether the pour has to fit around other trades
• How ready the floor is, including insulation, edging and membrane

Preparing a floor for liquid screed

A good pour starts with good preparation. Before booking, we confirm floor area and access, floor preparation, UFH and delivery timing. Use this checklist to get the floor ready for liquid screed:

• Building weathertight, with windows and doors in or openings covered
• Insulation laid flat and fully supported
• Perimeter edging strip fitted around all walls and fixed items
• Membrane laid and taped so screed cannot leak through joints
• UFH pipework fixed down, filled and pressure tested
• Level datum marked so the finished height is agreed
• Clear route for the hose from the road to the floor
• Other trades told so nobody needs to walk on the floor while it sets

Why choose liquid screed over traditional screed

Traditional screed is mixed, barrowed in and levelled by hand, which takes longer on larger floors and depends heavily on the skill of the person levelling it.

Liquid screed is pumped in and levels itself, so large areas can be done quickly and the finish tends to be very flat. It is usually laid thinner, especially over UFH, and it fully surrounds heating pipes. In return, it needs a well-sealed floor build-up and careful attention to drying before flooring goes down.

If you are not sure liquid screed suits your St Helens project, tell us about the room, the floor finish and the programme, and we will give you a straight answer.

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