Machine-laid anhydrite self-levelling screed

Anhydrite screed is also called liquid screed, gypsum screed or calcium sulphate screed. Its binder is calcium sulphate, not cement. Under EN 13813 it is marked with the letters CA, followed by its compressive and flexural strength, for example CA-C20-F4 or CA-C25-F5. These classes are the usual choice for homes.

Its big advantage is how fluid it is. The mix flows around underfloor heating pipes without leaving air gaps, so heat reaches the room quickly and evenly. It shrinks very little as it hardens, so it can cover larger areas without movement joints than a cement screed. The surface comes out very flat, which floor layers appreciate, especially under vinyl and large-format tiles.

It has one weakness: it does not tolerate constant moisture. It is not suitable outdoors, in a garage where snow melts off the car, in an unventilated cellar with rising damp, or in rooms that are regularly hosed down. An ordinary bathroom is fine, as long as the screed later gets a primer and a waterproofing membrane as the manufacturer specifies.

Typical build-ups and thicknesses

• Floating screed on insulation: the screed sits on thermal or acoustic insulation, separated by a sheet of polythene. It is usually laid from roughly 35 mm. The manufacturer sets the exact minimum depending on the screed class and how compressible the insulation is.
• Screed over underfloor heating: you usually need roughly 30 to 35 mm above the top of the pipes, so the total depth often ends up at around 45 to 50 mm.
• Unbonded screed on a separating layer: the screed sits on a concrete slab with only a polythene sheet in between. It can be thinner, from roughly 30 mm.

Most manufacturers advise against pouring layers thicker than about 60 to 80 mm in one go, because they take a very long time to dry. If the base underneath is very uneven, it is usually more sensible to make up the level first with insulation or a lightweight fill.

The pour itself takes a few hours, but the preparation decides whether it works. On site it often happens that the crew turns up and the windows are still missing, or the service penetrations have not been sealed. Then everyone either waits, or the screed goes into an unprepared space, and that comes back to bite later.

• A closed building: windows and the front door fitted, so no draughts or direct sunlight reach the fresh screed.
• Wet trades on the walls finished: plastering is normally done first, so no mortar drops onto the finished screed and the room does not get extra moisture.
• Services in the floor: cables, water pipes, drainage and ventilation ducts laid, fixed and covered by insulation, so nothing sticks up above the planned top of the screed.
• Insulation: boards butted tightly together with no gaps, stable and not rocking.
• Edge strip: a foam strip with a plastic skirt around the whole perimeter of every room, and around columns, stairs and penetrations. It is usually roughly 8 to 10 mm thick.
• Polythene sheet: laps of roughly 10 cm, joints taped, edges turned up the wall. Liquid screed will find every hole.
• Underfloor heating: pipes firmly clipped down, pressure test done, and the system kept under pressure during the pour so any damage shows straight away.
• Access: space for the mixer truck or silo and pump, water and power as the crew requires, and a clear route for the hose.

In a block of flats, agree everything with the building management or owners' association. The hose will run up the stairwell or through a window, the truck will stand outside and the pump is noisy. Replacing a screed inside a building does not normally need planning or building permission on its own. If it is part of a larger renovation, check the local building regulations with your local building authority.

Setting the levels

The operative uses a laser to set the height and places levelling tripods or small level markers across the floor. Their tips mark the top of the screed. With underfloor heating, the operative also checks that the screed above the pipes is deep enough everywhere.

Mixing and checking the consistency

The mix either arrives ready-made from the plant in a mixer truck, or as a dry mix in a silo, from which a mixing pump adds the water. Before the pour, the crew does a flow test. They fill a ring with the mix, lift it and measure the diameter of the puddle. The manufacturer gives the target value. A mix that is too wet separates: a weak layer forms on top and the screed can lose strength. A mix that is too stiff does not flow out properly.

Pouring and removing air

The hose starts in the room furthest from the exit and works back towards the door. The mix goes down in strips up to the height of the tripods. Straight afterwards, the operative works over the surface with a dappling bar, in two directions at right angles. This knocks out the air, levels the surface and blends the strips together. If this step is rushed, you get ripples on the surface and air bubbles underneath.

Movement joints and finishing

With underfloor heating, movement joints go between separate heating circuits and usually in doorways, for example by setting in a joint profile. The screed manufacturer and the heating design give the maximum bay sizes. Once the pour is finished, the area is closed off and nobody walks in for several hours.

People often ask why they should choose anhydrite when a cement screed copes with anything. The answer depends on where the floor is and what goes on top.

• Flatness: anhydrite comes out much flatter than a hand-laid semi-dry cement screed. Under vinyl, a full levelling compound is often unnecessary or much thinner.
• Underfloor heating: anhydrite surrounds the pipes completely and can be thinner, so the heating responds faster. Commissioning heating can usually start earlier than with cement.
• Shrinkage and cracks: anhydrite shrinks less, so bays between joints can be larger.
• Moisture: cement screed handles water better. Outdoor areas, garages, plant rooms with a flood risk and damp cellars need cement.
• Preparing for the floor finish: anhydrite almost always needs sanding and a suitable primer. Cement-based adhesives and levelling compounds must not go directly onto unprimed anhydrite, because the cement and the sulphate can react and weaken the bond.
• Drying: a thin anhydrite layer dries fairly quickly, but a thick one dries slowly and copes badly with poor ventilation.

There are also flowing cement screeds, which combine the self-levelling behaviour with better resistance to moisture. If you are unsure, ask the supplier about both options for your specific floor build-up.

Machine pouring is efficient on larger areas. The following factors have the biggest effect on the price per square metre:

• Size of the area: the truck, pump and crew have to travel the same way for a small job as for a large one, so a small area is relatively more demanding. For a single bathroom, machine pouring often does not pay off.
• Depth and how much it varies: the thicker the layer and the more uneven the base, the more material and the longer the drying.
• Access: distance from the truck, the floor level, hose length, narrow stairs. An upper floor in a block of flats with no parking near the entrance makes the job harder.
• Layout: lots of small rooms, alcoves, stairs or columns mean more edge strip and more handwork.
• Underfloor heating: the depth above the pipes has to be watched and joints formed between circuits.
• State of preparation: if the crew has to tape polythene, level insulation or wait for other trades, the job takes longer.
• Weather and temperature: screed is not poured below roughly +5 °C. In winter the building needs heating, in summer heat the screed needs protecting from draughts and sun.

Walking on it and loading

You can usually walk carefully on the screed after roughly 24 to 48 hours. It takes light loads after a few days and full loads, according to the manufacturer, usually after two to four weeks. Heavy scaffolding, pallets of tiles or stacks of materials have no place on fresh screed.

Ventilation

Keep the windows shut for the first one to two days. The screed needs moisture to set, and draughts could cause cracks. After that, the opposite applies: ventilate hard, ideally by opening windows wide several times a day for short periods. Anhydrite will not dry on its own in a closed, humid room. A dehumidifier helps too, if the manufacturer allows it.

Sanding

As the screed hardens, a thin, softer layer forms on top, often called laitance. After roughly a few days to a week, it is removed with a sanding machine and the dust is vacuumed off. Sanding opens the pores, so the screed dries faster and the floor finish bonds better. Without it, the adhesive can tear off together with the surface layer.

How long it takes to dry

A common rule of thumb is roughly one week per centimetre of depth up to about 4 cm. Thicker layers dry much more slowly. Temperature, humidity and ventilation all play a part, so the calendar never replaces a moisture test.

Starting the underfloor heating

With anhydrite, commissioning of the heating can usually start after roughly seven days. It begins with a low water temperature of around 20 to 25 °C, which is raised step by step to the maximum the manufacturer sets, held for a while and then lowered again gradually. Record the whole process in a heating start-up log, because the floor layer will want to see it. Never turn the heating up full straight after the pour.

Moisture testing before the floor finish

Before any floor covering goes down, the residual moisture is measured. The carbide method (CM) is the most reliable. For anhydrite, commonly quoted limits are roughly 0.5 % CM without heating and roughly 0.3 % CM with underfloor heating. In the UK and some other countries, a hygrometer test measuring relative humidity is more common, and flooring manufacturers give their limits in that form. For wood floors and other sensitive finishes, follow the flooring manufacturer's instructions. Tests are taken at points marked in advance, so nobody drills into a pipe.

What comes next

Once the screed is dry, it gets a primer suitable for anhydrite, possibly a levelling compound, waterproofing in the bathroom, and then tiles or another floor finish. The edge strip is trimmed only after the floor finish is complete, not before.

• Leaking polythene: the mix runs under the insulation, the boards lift and the screed is thinner in places. Later this shows as cracks or a hollow sound.
• Missing edge strip: the screed presses against the wall, cracks form and footstep noise travels into the structure. In a block of flats, the neighbours will complain.
• Loose heating pipes: they float up in the liquid mix, leaving only a thin layer above them in places, which then cracks.
• Draughts or sun right after the pour: the surface dries too fast and develops a network of fine cracks.
• No ventilation in the following weeks: the screed stays wet, the flooring date keeps slipping and mould can appear under the floor finish.
• Floor finish laid on damp screed: wood floors lift, vinyl comes loose and tile adhesive bonds less well.
• Skipping sanding and primer: the adhesive tears off with the surface layer, and cement-based adhesives risk a chemical reaction at the bond.
• Tiles laid across doorway joints: if tiles run over a movement joint without a break, they crack along the line of the joint.

Leave the actual machine pour to a crew with a pump: it cannot be done without the equipment and experience. You can, however, take on much of the preparation and the aftercare, which saves work and helps the quality. Before you start, ask the supplier for their requirements for the base, edge strip and polythene.

1. Clean the base of mortar lumps, stones and dust.
2. Check that all services are fixed down and covered by insulation.
3. Lay the insulation boards tightly butted and pack or level any uneven spots.
4. Fix the edge strip around the whole perimeter and around door frames, columns and penetrations.
5. Lay the polythene with generous laps and tape all joints thoroughly.
6. Turn the polythene up the wall above the future top of the screed and tape it to the edge strip skirt.
7. Seal pipe penetrations and corners with tape so the mix cannot get underneath.
8. Keep the windows shut and stay out of the rooms for the first one to two days after the pour.
9. Then open the windows wide for short periods several times a day, and keep a reasonable temperature in winter.
10. Have the screed sanded and vacuumed as the supplier instructs.
11. With underfloor heating, run the start-up according to the log and record the temperatures.
12. Book a residual moisture test before the floor finish goes down.

At handover, walk the whole floor, ideally with a straightedge and possibly a laser. Small dips can be filled later with levelling compound, but problems with depth or cracks need sorting out with the contractor straight away.

• Flatness checked with a 2 m straightedge against the requirements of the planned floor finish
• Screed height matches the agreed level and lines up with thresholds and stairs
• Depth above the underfloor heating pipes checked and recorded
• Surface free of cracks, hollow spots and obvious ripples
• Edge strip intact around the whole perimeter
• Movement joints between heating circuits and in doorways in the agreed places
• No pressure drop in the underfloor heating after the pour
• Delivery note showing the screed class, plus written instructions for ventilation and heating start-up
• Sanding date agreed and moisture test points marked