Liquid screed vs sand and cement for underfloor heating: which to choose

Once the pipe is clipped down, the screed decides much of how the finished floor behaves: how thick the build-up is, how fast the floor responds, how long you wait before tiling, and what the pour costs. The two main choices are traditional sand and cement, mixed semi-dry and levelled by hand, and liquid flowing screeds, pumped in wet, which are either calcium sulphate (anhydrite) or cement-based flowing screeds.

This guide is the head-to-head: depth, speed, drying, laitance, cost and the cases where each one wins. For how deep your particular floor needs to be and the full drying and commissioning sequence, see our guide to screed depth and drying time over underfloor heating.

What is the difference between liquid screed and sand and cement?

Sand and cement screed is a semi-dry mix placed by hand with a straight edge; liquid screed is a wet, self-levelling mix pumped through a hose and left to find its own level. That one difference drives almost everything else.

	Sand and cement	Liquid flowing screed
Typical depth over UFH	65mm minimum, 65 to 75mm ideal (about 50mm with fibres)	About 25mm cover over the pipe, so floors from roughly 45mm
Placement	By hand, barrow or small pump	Pumped; a team lays hundreds of m² a day
Contact with pipe	Compacted by hand, voids possible	Flows around the pipe completely
Foot traffic	Longer wait	24 to 48 hours
Heating on	About 21 days, raised gradually	Force-drying with the UFH from about 7 days
Falls and wet areas	Yes	No falls; calcium sulphate unsuitable where permanently wet

Within liquid screeds, calcium sulphate (anhydrite) is the long-established option, and newer cement-based flowing screeds give the same pumped, self-levelling pour with cement chemistry, which removes the laitance and adhesive-compatibility issues covered below. One cement-based flowing screed quotes 25mm cover over water UFH pipe in line with BS 8204-7 and a thermal conductivity of up to 2.9W/mK.

Which costs less?

Sand and cement, per square metre. Published UK supply-and-lay rates put floating sand and cement screed at 65 to 75mm at about £14 to £20 per m², liquid anhydrite screed at about £18 to £28 per m², and liquid screed over underfloor heating pipes at about £20 to £35 per m², all excluding VAT, with pump hire typically £300 to £600 a day on top and minimum charges of £300 to £500 on jobs under about 20m².

Read those numbers with the depth in mind. A liquid screed floor is roughly half the thickness, so part of the premium comes back in material, and more of it comes back in programme: a pumped pour that is walkable in a day or two and force-dryable from day 7 can take weeks off a renovation compared with a 65mm sand and cement screed that wants about 21 days before the heating goes on. On a whole-house job the labour saving is real; on one small room, the pump and minimum load usually make sand and cement the sensible buy.

Which works better with underfloor heating?

Liquid screed, in three ways. First, full contact: it flows around every pipe, so there are no air voids insulating the pipe from the screed. Second, thickness: with about 25mm of cover instead of 65 to 75mm of total depth, there is far less material between the water and your feet, so the floor responds faster when the thermostat calls. Third, some cement-based flowing screeds publish high thermal conductivity, up to 2.9W/mK, which helps the heat spread from the pipe.

A thick sand and cement screed is not a bad partner, it is just a slower one: it stores more heat and smooths output over hours, which suits heat pumps running long and low, but it responds sluggishly to setbacks. Our guide to thermal mass and underfloor heating covers that trade-off. Either way, pressure test the loops before any screed goes in, see pressure testing underfloor heating, and fit edge insulation strip around the perimeter so the screed can expand.

The pipe is the same in both: PERT-EVOH oxygen barrier pipe, held by staples, clip rail or castellated panels. Note one pairing rule: check your panel system suits a liquid pour, covered in castellated panels with liquid screed, because a liquid screed will find any gap a semi-dry mix would bridge.

How long until you can turn the heating on and lay the floor?

This is where the two part company.

• Sand and cement: allow about 21 days before turning the heating on, then raise the temperature gradually. Additives can shorten this.
• Calcium sulphate liquid: foot traffic after 24 to 48 hours, ventilate the building properly after 48 hours, and the underfloor heating can be used to force-dry the screed from about 7 days.
• Drying for floor finishes: a common rule for calcium sulphate screed is 1 day per mm up to 40mm and 2 days per mm beyond, at around 1mm per day in good conditions, and residual moisture must be below 0.5% before tiling. Force-drying with the heating shortens this substantially.
• Cement-based liquid: one maker quotes tiling from 7 to 10 days with its matched adhesive and force-drying with UFH from 7 days after the pour.

Whatever the screed, follow the first-heat cycle the screed maker specifies before the floor finish goes down, and record it for the handover. The commissioning sequence is in our screed depth and drying time guide.

What is laitance and why does it matter on anhydrite screed?

Laitance is a weak, dusty layer that forms on the surface of a calcium sulphate screed as it cures. It is too weak to tile onto, so it must be removed, typically by sanding, around 2 to 6 days after the pour depending on the product, which also helps the screed dry.

There is a chemistry problem underneath it too. The cement in a standard cement-based tile adhesive reacts with the gypsum in a calcium sulphate screed to form ettringite at the interface, which can debond the adhesive completely. So tiling onto anhydrite means laitance off, moisture below 0.5%, then a proper anhydrite primer applied until the surface stops absorbing it, before a cement-based adhesive goes anywhere near the floor, and an uncoupling membrane adds further protection.

Sand and cement screed and cement-based flowing screeds avoid the gypsum issue entirely, and one cement-based flowing screed states there is no surface laitance after curing. If your tiler has never worked on anhydrite before, that alone is a reason some specifiers choose a cement-based pour.

When is sand and cement still the right choice?

Often. Choose sand and cement when:

• The floor needs falls: wet rooms and floors draining to a gully cannot be done in a self-levelling liquid.
• The area is small: one bathroom or a porch rarely justifies a pump, a tanker and a minimum charge.
• The area may get permanently wet: calcium sulphate screeds are unsuitable there.
• Budget beats programme: at £14 to £20 per m² laid it is the cheapest screed, and any competent general builder can place it with clip rail or staples holding the pipe.
• Door thresholds allow the depth: 65 to 75mm plus insulation needs to fit under the doors; where it will not, a thin liquid pour or an overlay board system is the answer.

Frequently asked questions

Is liquid screed better than sand and cement for underfloor heating?

Technically, usually yes: about 25mm of cover instead of 65 to 75mm of depth, no voids around the pipe, faster response and force-drying from about 7 days. Sand and cement remains the right call for falls, wet areas, small areas and tight budgets.

How thick does each screed need to be over the pipe?

Liquid flowing screeds need about 25mm of cover over the pipe, so total depths from roughly 45mm. Sand and cement wants 65mm minimum and ideally 65 to 75mm overall, or around 50mm with fibres. Exact minimums are set by the screed maker's specification.

How soon can the heating go on after screeding?

Sand and cement: about 21 days, then raise the temperature gradually. Calcium sulphate liquid screed: the heating can be used to force-dry from about 7 days after the pour. Always follow the screed maker's first-heat procedure.

Can you tile straight onto anhydrite screed?

Not straight onto it. The laitance must be removed, residual moisture must be below 0.5%, and the surface must be primed with an anhydrite primer before a cement-based adhesive is used, otherwise the gypsum-cement reaction (ettringite) can debond the tiles.

Is liquid screed more expensive?

Per square metre, yes: around £18 to £28 against £14 to £20 for sand and cement, plus pump costs. Per project it is often closer than it looks, because it is thinner, far faster to lay and quicker to dry.

Which screed is best with a heat pump?

Both work. A thin liquid screed responds faster; a thicker sand and cement screed stores more heat and suits long, low heat pump running. Design the pipe spacing for low flow temperatures either way.

Can I lay liquid screed myself?

No, it arrives by tanker and is pumped by a specialist team. Sand and cement is the DIY-feasible option, and the pipework beneath, clipped or on castellated panels, is the same either way.

Sources: Designing Buildings: Screed (BS 8204, thicknesses and drying)