Screed depths and drying times for underfloor heating

Screed depth and drying time set the programme for the whole floor. Depth fixes your floor levels and the pipe cover. Drying time decides when the floor finish can go down, and it is the stage most often rushed.

This guide covers depths, drying times by thickness, how to test for dryness and how to use the heating to dry a screed safely. For the first heat-up schedule after laying, see how to install water underfloor heating.

How thick should screed be over underfloor heating?

It depends on the screed type and how it sits on the layer below. Over UFH the screed is almost always floating on insulation, which needs the most depth.

The sand and cement depths are from trade guidance (MPA Mortar). The cover figure is from one flooring maker's screed checklist. The liquid figures are from a screed maker and a UFH system maker. Your screed contractor's data sheet is the final word.

Worked example. 16mm pipe in a 65mm sand and cement screed leaves 65 - 16 = 49mm above the pipe, well over the 30 to 35mm minimum. The same pipe in a 45mm liquid screed leaves 29mm, which meets a 25mm minimum but not a 35mm one. Check before you set the finished floor level.

What is the difference between curing and drying?

Curing is the screed gaining strength. Drying is the spare water leaving it. A screed can be fully cured but still too wet for a floor finish.

• Sand and cement curing. Trade guidance is to cover the screed with plastic sheeting or similar to keep moisture in for at least 5 days. Drying out too fast causes cracking and a weak surface.
• Drying. Only starts in earnest once the covers come off. Warmth, low humidity and moving air speed it up.
• Heating. One UFH system maker says not to switch on until at least 21 days after laying a sand and cement screed, and not to use the system just to dry it.

How long does screed take to dry?

Use these rules of thumb for good drying conditions. BS 8204-1 allows one day per mm for the first 50mm of a sand and cement screed, with longer per mm beyond that. Liquid calcium sulphate screed is quoted at 1mm a day to 40mm, then 0.5mm a day.

The 75mm sand and cement figure is one screed additive maker's estimate. The 65mm figure assumes about 2 days per mm after the first 50mm, which is the same maker's rough rule. Liquid screed figures are 40 days for the first 40mm, plus 2 days per mm after that.

One adhesive maker warns that quoted drying times assume about 20°C, low humidity and an open surface, so real sites are often slower. Plan with these figures, then test before you lay the finish.

What slows screed drying down?

Cold, damp air and thick screed are the main causes. A screed in a closed, unheated building in winter can take far longer than the rules of thumb.

• Thickness. Each extra mm takes longer than the one before.
• Low temperature and high humidity. Both slow the rate water leaves the surface.
• No ventilation. Moist air has to be removed. Open windows and doors when the weather allows.
• Additives. One maker warns that retarders added at the plant or mixer severely affect drying.
• Gas or paraffin heaters. One screed maker says to avoid fossil fuel heaters because they add moisture to the air.
• Covering the surface. Stored materials, boards or plastic left on the screed after curing trap moisture.

Can I use the underfloor heating to dry the screed?

For many liquid calcium sulphate screeds, yes: the maker allows force drying with the UFH from as early as 7 days. For sand and cement screed, follow your screed maker. One UFH system maker says not to use the heating to dry it.

One calcium sulphate screed maker's force drying schedule is:

1. From day 7, start at about 15°C flow (or the lowest the manifold allows) and hold for 24 hours.
2. Raise the flow by no more than 5°C every 24 hours up to the normal running temperature, with a maximum of 50°C.
3. Hold at that temperature for 7 days.
4. Reduce by 5°C a day back to the starting temperature.
5. Switch off and let the floor cool for 48 hours before you test the moisture.

Worked example. For a system designed to run at 40°C: hold 15°C on day 7, rise 5°C a day to reach 40°C on day 12, hold to day 19, come down 5°C a day to 15°C by day 24, switch off, and test from day 26 or 27. That is about 4 weeks from the pour to the first moisture test, against about 60 days for a 50mm screed left to dry on its own. A test is still needed: the schedule speeds drying but doesn't prove it.

Set the flow temperature at the blending valve or heat source, not with the room thermostats. Our guide to blending valves explains how.

How do I test if the screed is dry?

Use a hygrometer test, which measures the relative humidity (RH) of the air trapped over the screed surface. A reading below 75% RH is the usual target for most floor finishes.

1. Switch the heating off. One screed maker allows 48 hours for the floor to cool first.
2. Seal a hygrometer box or hood to the screed surface so no air leaks in.
3. Leave it to settle. One adhesive maker leaves a surface hygrometer on the unheated floor for about 4 days.
4. Take the reading and record it with the date and location. Test several spots, including the thickest areas and away from windows.

Other methods include the carbide (CM) test, which gives a moisture percentage. Some flooring makers give limits in CM % instead of RH, so check which one your flooring warranty uses.

How dry does the screed need to be for each floor finish?

Each floor finish has its own moisture limit, set by the flooring or adhesive maker. These figures from screed and flooring makers show the range:

Wood is the most sensitive finish, and one screed additive maker notes that many wood flooring suppliers want lower figures than the 75% RH in BS 8201. Read underfloor heating under tiles, LVT, laminate, wood and carpet for each finish's temperature limits.

How do I prepare the screed surface before the floor finish?

Remove the laitance, the weak, dusty top skin, then prime if the finish is bonded. Calcium sulphate screed needs this most.

• Calcium sulphate laitance. One screed maker recommends sanding with a 60 grit orbital sander with vacuum, just before the floor finish goes down. Sanding removes the laitance and gives a key. It doesn't change the screed depth.
• Primer. Bonded finishes on calcium sulphate need primer, usually 2 coats, to separate the screed from cement-based adhesives. Use the adhesive maker's own primer.
• Sand and cement. Clean off dust and loose material, then prime as the adhesive maker instructs.
• Flatness. Check for high and low spots. Self-levelling compound can correct small dips once the screed is dry.

Products for this job

• Edge Insulation Foam Roll
• Underfloor Heating Grid Membrane 1.2m
• Castellated Floor Panel 1220 x 820mm
• Hydraulic Pressure Test Pump
• Blending Valve Pump Pack 20 to 55°C
• Pressure and Temperature Gauge for UFH Mixing Unit
• Ultra Prime IT Multi Surface Primer 5L

Frequently asked questions

What is the minimum screed depth over underfloor heating pipes?

For liquid calcium sulphate screed, one maker's minimum is 25mm over the pipe, 30mm nominal. For sand and cement, one flooring maker asks for at least 30 to 35mm cover. Follow your screed's data sheet.

How long after screeding can I turn on underfloor heating?

Many liquid calcium sulphate screeds allow force drying with the UFH from 7 days. For sand and cement, one UFH maker says no earlier than 21 days. Always follow the screed maker.

How long does a 50mm screed take to dry?

About 50 days for sand and cement and about 60 days for liquid calcium sulphate, in good conditions. Force drying with the UFH can shorten the liquid screed time.

Is liquid screed better for underfloor heating?

It is thinner, flows around the pipe with no air pockets and can often be force dried, so it suits UFH well. It needs a sealed, taped base and careful surface preparation before tiling.

Can I tile onto screed that isn't fully dry?

Don't. Moisture trapped under tiles, vinyl or wood can cause debonding, cracking or movement. Test with a hygrometer and wait until it is under your flooring maker's limit.

Why has my new screed cracked?

Common causes are drying too fast in the first days, no edge insulation for expansion, heating up too early or too quickly, and bays that are too large. Ask your screed contractor to inspect it before the floor finish goes down.

Can I walk on screed the next day?

Light foot traffic is often possible within a day or two, depending on the product, but the screed is nowhere near dry. Keep heavy loads and stored materials off it.

Sources: MPA Mortar Data Sheet 22 (screeds), Gyvlon flowing screed data pack, Timoleon staple system data sheet, Ronacrete: drying of screeds (BS 8204-1), Instarmac: calcium sulphate screeds, Senso screed requirements checklist, Solid Wood Flooring Company UFH advice