Machine-laid cement screed

It looks like quick work. An experienced crew can often screed the ground floor of an ordinary family house in a single day. The result, though, depends on details that are settled before the first batch is mixed.

1. Checking the base and the levels. The crew checks the insulation, the membrane and the edge strips. They transfer the finished screed height into every room from the datum line (a level mark on the walls, usually one metre above finished floor) or with a laser.
2. Setting level points. Small pads or strips of mix are placed on the floor as reference points for levelling the surface later. On larger areas the crew lays screed rails, narrow guide strips set to the exact height.
3. Mixing. Sand, cement, water and, depending on the recipe, a plasticiser and polypropylene fibres go into the pump. The mix is semi-dry, closer to damp earth than to liquid. If you squeeze it in your hand, it holds its shape and does not run.
4. Pumping and spreading. The mix travels through the hose for tens of metres and can easily go up to an upper floor. In the room it is spread with shovels and rakes and roughly levelled.
5. Compacting and levelling. The mix is tamped and then drawn off with an aluminium straightedge along the level points. Compaction matters especially at the edges and over underfloor heating pipes.
6. Power floating. Once the mix has stiffened slightly, the surface is worked with a power float fitted with a rotating disc or blades. This gives a closed, flat and fairly smooth surface. Corners and areas around pipes are finished by hand.
7. Movement joints. Joints are cut or inserted according to a plan agreed in advance, mainly in door openings and on large areas.

Screeding comes fairly late in the building sequence. Anything that has to be built in underneath it, or any wet work done above it, should be finished first. Otherwise the floor gets broken open or damaged later.

• Windows and external doors are fitted. Screed needs a protected space with no draughts and no frost.
• Internal plastering is done. Wet work on the walls after screeding means dirt, moisture and a real risk of damaging the fresh surface.
• Water, drainage and electrical runs in the floor are laid, fixed and checked. Waste branches and floor drains sit at their final height and are capped.
• Underfloor heating is laid and pressure-tested. It should stay under pressure while the screed goes in, so that any damaged pipe shows up straight away.
• Thermal and impact sound insulation covers the whole floor, with no gaps, and the boards do not rock. Pipes and cables running through the insulation are levelled with an extra layer, so the screed does not rest on the insulation only at a few points.
• The membrane and edge strips are in place. The membrane overlaps by roughly 10 cm, is taped at the joints and turned up the walls. A foam edge strip runs around every wall, column, door frame and pipe penetration.

Think about logistics too. Sand is tipped and cement unloaded next to the house, so you need a firm area close to the entrance that a lorry can reach. The crew also needs water and usually electricity. Some screed pumps have their own diesel engine, others plug into a three-phase socket. If the site does not have a supply yet, a qualified electrician should install it. In a block of flats, agree in advance with the building management or owners' association on working hours, protection of the staircase and lift, and the route for the hose.

Floating screed

This is the standard choice in homes. The screed sits on insulation, separated by a membrane, and never touches the walls or the structure. That way it dampens footstep noise and can move freely as temperatures change. Thickness is usually around 45 to 60 mm. Over underfloor heating pipes, at least roughly 45 mm of cover is normally required, but the exact figures come from the heating system manufacturer and the designer.

Screed on a separating layer

This screed lies directly on the concrete slab, separated only by a membrane. It is used for example in basements, garages or workshops where no insulation is needed. Thickness usually starts at roughly 35 mm.

Bonded screed

This one is firmly bonded to the base with a bonding bridge. It can be thinner, typically around 25 to 40 mm, but the base must be sound, clean and free of dust. In renovations it is sometimes used to level old concrete. A single greasy or dusty patch is enough for the screed to come away from the base at that spot.

Cement or anhydrite screed

On many projects the choice is between cement screed and anhydrite (calcium sulphate, flowing) screed. Cement screed copes with moisture, suits bathrooms, plant rooms and garages, and tolerates rougher site conditions. It is usually the less costly option. It dries more slowly and shrinks more, though, so it needs movement joints and careful curing.

Anhydrite screed is poured as a liquid, levels itself and tends to end up flatter. Over underfloor heating it can be thinner and transfers heat well. It does not tolerate long-term moisture, usually has to be sanded before the floor covering goes down, and is not suitable outdoors or in wet areas. If you are unsure, decide based on where the floor is and which covering you plan.

Strength, fibres and mesh

Screed is classified under EN 13813, for example CT-C20-F4. CT means cement screed, C is compressive strength and F is flexural strength. Ordinary living spaces use lower classes, garages and workshops higher ones. Polypropylene fibres in the mix limit fine shrinkage cracks. Steel mesh is mainly used for heavier loads, over underfloor heating where the manufacturer requires it, or where the design specifies it. The mesh must sit within the screed, not at the bottom beneath it, otherwise it does nothing.

Screed is priced per square metre, but that rate always assumes a base thickness, and every extra centimetre means more material and more work. When comparing quotes, check which thickness they are based on.

Things that make the work easier:

• A large continuous area without many small rooms
• Flat, well-prepared insulation and accurate levels
• A short hose route and good lorry access for the sand
• Water and electricity available on site
• Indoor temperature above roughly +5 °C and all openings closed

Things that make the work harder:

• A small area, where setting up the machine and crew is spread over only a few square metres
• Many small rooms, bathrooms, corridors and alcoves
• Underfloor heating, which calls for careful compaction and close attention to pipe cover
• An uneven base that leads to varying thickness
• Upper floors, a long hose route or a flat in an apartment block
• Falls towards floor drains and level-access showers
• Winter work, when the space has to be heated

Many screed problems arise not during laying but in the weeks afterwards. Fresh cement screed needs moisture in the first days so the cement can harden properly. Draughts, direct sun or heating straight after laying make the surface dry faster than the bottom. The result is cracking and edges lifting, known as curling.

• Keep the rooms closed and draught-free for roughly the first 2 to 3 days
• In hot weather you can lightly dampen the surface or cover it with sheeting if the contractor advises it
• Careful foot traffic is usually possible after 24 to 48 hours, full loading after roughly 28 days
• After the first week, start ventilating gradually, ideally in short bursts several times a day

How long does screed take to dry? A common rule of thumb on site is about one week per centimetre for the first four centimetres, after which drying slows down considerably. For 5 to 6 cm, allow several weeks, and in cold, damp periods two months or more. Feel is not a reliable guide. Moisture is measured before the floor covering goes down, most often with the carbide (CM) method. The limits come from the covering manufacturer. For wood and vinyl floors the threshold is usually around 2 % CM, and lower still with underfloor heating.

With underfloor heating, the system is brought up to temperature following a heat-up protocol. For cement screed this usually starts no earlier than roughly 21 days after laying, sooner for accelerated mixes according to the manufacturer. The temperature is raised a few degrees a day, held at maximum for a while, then reduced gradually again. Ask your heating engineer for the protocol. Floor layers and tilers often want to see it.

Screed is the link between the shell and the finishing work, so it pays to coordinate dates early.

• The plumber and electrician finish the runs in the floor and hand them over checked. Cutting into the screed later weakens it and damages the membrane and insulation.
• The heating engineer carries out the pressure test, leaves the system under pressure and commissions the heating once the screed has dried.
• The insulation installer or main building crew lays the insulation and membrane. If the screed crew does this, make sure it is in the quote.
• The tiler and floor layer need to know the final screed height so that thresholds, transitions between rooms and door heights work out. Before they start, the surface is usually sanded to remove laitance, the thin weak layer on top that reduces adhesive bond.
• The joiner fits timber door linings and internal doors after the screed and floor covering. Steel door frames go in earlier, and with those the edge strip matters even more.

Movement joints in the screed must carry through into the floor covering. If the tiler tiles straight over a joint, the tiles will crack later. Have the joint plan drawn and pass it on to the following trades.

Screed on its own does not usually need building approval. If it is part of a larger build or renovation, check the local building regulations with your local building authority. In a block of flats, do not change the floor build-up without the consent of the building management or owners' association, because it affects the load on the floor slab and the transfer of noise to your neighbours.

Missing or trimmed edge strip

The screed touches a wall or door frame and creates a sound bridge. The neighbours hear footsteps, and cracks appear along the walls as temperatures change. A common error: the strip is trimmed flush right after screeding and the skirting is then fixed with adhesive, which creates the bridge anyway. The strip should only be trimmed after the floor covering is laid.

Leaking membrane

Cement slurry runs between the insulation boards and sets. The impact sound insulation then loses its purpose and the screed is rigidly bonded to the slab in places.

Mix too wet

It is easier to work with, but it shrinks more, dries longer and leaves a weak, dusty surface. Scratch it with a key and it flakes and crumbles.

Screed too thin

Over heating pipes, over services or on uneven insulation, the screed is often thinner in places. That is exactly where cracks form, and under load it can break through.

Missing joints

On large areas, in L-shaped rooms or at doorways, screed will make its own crack, and in a place you do not want it. As a guide, bays of roughly 30 to 40 m² with a side ratio of up to about 1:2 are often quoted, and with underfloor heating frequently smaller, as specified by the manufacturer.

Floor covering on damp screed

Wood flooring cups, vinyl comes unstuck, and mould can grow underneath. This typically happens when a moving-in date is pushing and nobody measures the moisture.

Heating or drying too early

Full heating, hot air or a dehumidifier switched on straight after laying cause cracks and curling. Dehumidifiers are fine, but only after the first few days and used sensibly.

Laying the screed by machine is a job for a crew with the equipment and experience. You can, however, handle the preparation and the aftercare yourself and avoid a lot of trouble that way. Leave connecting the screed pump to the power supply to a qualified electrician.

1. Sweep and vacuum the base, and remove leftover plaster and building debris.
2. Check that the insulation boards lie flat over their whole area and do not rock, and fill any gaps.
3. Check the edge strip along all walls, columns, door frames and pipe penetrations, and fill in any missing sections.
4. Check the membrane overlaps and tape the joints.
5. Photograph the services in the floor with a tape measure in shot, so you know later where they run.
6. Cover and protect windows, sills, doors and the staircase from the mix.
7. Provide clear access to water and a space for the sand and cement.
8. After laying, close windows and doors and keep draughts out for the first few days.
9. In hot weather, dampen the surface or cover it with sheeting as the contractor instructs.
10. After about a week, start ventilating in short bursts, and keep heavy tools off the screed.
11. Have the moisture measured before the floor covering goes down, and write down the result.

Check the screed on the day it is finished and again a few days later, once you can walk on it. Have a two-metre straightedge ready for flatness. The permitted deviation depends on the agreed flatness class and the floor covering. Large-format tiles and vinyl are more demanding, so agree on this in advance.

• The surface is flat, with no gaps under the two-metre straightedge larger than the agreed tolerance
• The screed height matches the datum line and the planned height of the covering and thresholds
• The surface is closed, does not crumble and does not flake when scratched with a key
• There are no visible cracks, especially along walls, at doorways and in corners
• The edge strips are continuous and the screed touches no wall or door frame
• Movement joints follow the plan, in door openings and on large areas
• Floor drains and penetrations sit at the correct height and falls lead to the drains
• A bonded screed does not sound hollow when tapped
• With underfloor heating, the system stayed under pressure and the pressure did not drop
• You have the thickness, mix class and joint plan from the contractor in writing
• You have agreed on curing and on the date for moisture measurement