Hand-laid floor screed up to 7 cm

The job covers mixing, moving the mix around the site, placing it to the required level, ruling it flat and finishing the surface. With a floating screed it usually also includes the perimeter edge strip and the separating membrane. Thermal or impact sound insulation, reinforcement and removal of the old floor are normally priced separately. When you agree the work with a contractor, ask exactly what is included.

By hand or pumped

In large new builds, screed is often pumped. It is fast and the whole floor goes down in one pour. Laying by hand makes sense where a pump does not: in a smaller flat, in one or two rooms, on an upper floor where no hose can reach, in a building with no vehicle access, or when the floor is done room by room because you are still living in the others. Working by hand also gives better control over details, such as the fall towards a floor drain in a wet room.

Why up to 7 cm

Thickness directly drives the amount of material, the amount of work and the drying time. Seven centimetres is enough for a typical floating screed and for covering underfloor heating pipes. If you need to make up more height, fill the extra depth with insulation, lightweight concrete or a levelling fill, not with a thicker screed. A thick screed is heavy, loads the floor below unnecessarily and takes a very long time to dry.

One local point: in many older houses the upper floors are timber joists, not concrete. A wet screed adds considerable weight, so on a timber floor have the structure checked first, or consider a lighter dry system instead.

Cement screed

Sand and cement screed is the traditional choice. It copes with moisture, so it suits bathrooms, cellars, garages and ground floors. It shrinks as it hardens, so it needs movement joints and takes longer to dry. As a floating screed it is usually laid around 5 cm thick, and over underfloor heating pipes a cover of roughly 4.5 cm is commonly required. The exact figures come from the manufacturer of the mix. For hand laying, a semi-dry to plastic consistency is most common. It is compacted and then trowelled.

Anhydrite screed

Anhydrite (calcium sulphate) screed is a flowing screed that levels itself. It can be thinner, needs fewer joints and wraps tightly around underfloor heating pipes, which is why heating installers like it. It is sensitive to permanent moisture, though, so it does not belong outdoors, in a garage or in an unventilated cellar. It can be used in a bathroom if a proper waterproofing layer goes on top. A weak skin forms on the surface, which is sanded off before flooring is bonded down. It can be mixed by hand from bags, but on larger areas it is more often pumped so it can be poured continuously.

Bagged ready mix or site mixing

A bagged dry mix has consistent quality and the manufacturer states exactly how much water to add. When screed is mixed on site from sand and cement, the result depends on the crew's experience. The sand should be sharp and well graded, usually 0 to 4 mm, not soft plastering sand. The cement to sand ratio is usually around 1:3 to 1:4. Floating screeds often get polypropylene fibres, which reduce the risk of hairline cracks, and a plasticiser, which means less water is needed.

Bonded, unbonded or floating

• Bonded screed goes straight onto sound concrete over a primer or bonding bridge. It can be thinner, but the base must be clean, sound and dust-free.
• Unbonded screed sits on a membrane and is not bonded to the base. It is used on old or weaker substrates.
• Floating screed sits on thermal or acoustic insulation and does not touch the walls or pipes. It is the most common type in flats, because it reduces footstep noise for the neighbours below.

Steel mesh is sometimes added to a floating cement screed. It only works if it sits roughly in the middle of the thickness, not at the bottom. Mesh trodden flat onto the insulation does nothing. Whether you need it at all depends on the loads and on the recommendation of the mix manufacturer or the designer.

Most screed problems start before anyone opens a bag. Before the agreed date, check the following:

• The finished floor level: a datum line is marked on the walls, and you know how thick the tiles or wood flooring will be, including adhesive
• Services in the floor: electrics, water, drainage and heating are finished and fixed, and underfloor heating has passed its pressure test
• Plastering: it is usually done before the screed, so the new floor does not get dirty or damaged
• Windows and the entrance door are fitted, because draughts and frost harm fresh screed
• Room temperature: it should not fall below roughly 5 °C during laying and for several days afterwards
• A clean base, free of rubble, dust, oil and old adhesive residue
• Water and power on site, and a mixing area, preferably close to the entrance
• A route for carrying material: lift, stairs and protection of shared areas

In a block of flats, tell the building management or owners' association about the dates, the noisy work and the use of the lift. A new screed with insulation adds load to the floor structure. If you are changing the floor build-up significantly or altering the structure, check with your local building authority what local building regulations require. Do not assume it is a minor job that needs no approval.

1. Checking the base and levels. The crew surveys the floor with a laser, finds the highest and lowest points and confirms the planned thickness fits everywhere.
2. Insulation and membrane. For a floating screed, insulation boards are laid tightly butted, then covered with a membrane with overlaps, turned up the walls. The membrane stops cement slurry running into the joints and creating sound bridges.
3. Edge strip. A foam strip, usually around 8 to 10 mm thick, is fixed around the room, around pipes and at door frames. It lets the screed move without cracking at the walls.
4. Screed rails. Mortar spots or strips are formed at the right height, or steel screed rails are set. The straightedge is then pulled along them.
5. Mixing. The mix goes into the mixer with an exact amount of water. A wet screed spreads more easily, but it shrinks more and has a weaker surface.
6. Placing and compacting. The mix is placed bay by bay, compacted and ruled off with an aluminium straightedge along the rails. With anhydrite, it is poured and de-aired with a spiked roller or a dappling bar.
7. Finishing. Once the screed has stiffened slightly, it is trowelled by hand or with a power float. The finish depends on the flooring that will go on top.
8. Joints. At doorways and in large areas the screed is divided. Either a joint profile is set beforehand, or the fresh screed is cut to roughly a third of its depth.
9. Curing. For the first few days, cement screed is protected from fast drying with sheeting or by dampening. Windows stay closed, and direct draughts or sunlight must not reach the floor.

The work per square metre depends on how much effort that area takes. These make it easier:

• A larger continuous area with a simple layout
• A ground floor or a lift, and a short distance from the mixer
• A flat base where the thickness is roughly the same everywhere
• A ready site where nothing is waiting on other trades

These make it harder:

• Many small rooms, narrow corridors and recesses
• An upper floor without a lift, where every bag and barrow goes up the stairs
• An uneven base with large differences in thickness
• Falls towards floor drains and walk-in showers
• Underfloor heating with closely spaced pipes that need careful work
• Working in the cold, when the room has to be heated and the screed protected for longer
• Laying in stages instead of one continuous job

Before the screed

The electrician, plumber and heating installer must have finished everything in the floor. Afterwards nothing can be chased in without damaging the screed and the insulation. If you plan underfloor heating, agree with the installer that the system stays under pressure while the screed is laid, so any leak shows up straight away. Remember door frames and thresholds too, so the doors do not catch once the flooring is down.

How long it takes and when you can walk on it

An experienced crew usually lays a typical flat by hand in one or two days. A cement screed can usually take careful foot traffic after 24 to 48 hours, light loads after a few days, and it reaches full strength after roughly 28 days. Anhydrite is often walkable sooner, usually within two days according to the manufacturer. A rough rule for drying: cement screed dries about a centimetre a week for the first four centimetres, then more slowly. A 5 cm screed therefore will not be ready for flooring for several weeks, and later still in cold weather without ventilation.

After the screed

With underfloor heating, the screed is commissioned with a heating-up protocol: the temperature is raised step by step, held, then lowered again. For cement screed this usually starts no earlier than three weeks after laying. For anhydrite it can start earlier, as the manufacturer specifies. Before flooring goes down, residual moisture is measured, reliably with the carbide (CM) method. Some flooring manufacturers specify relative humidity instead, so use the method they ask for. For cement screed under wood and vinyl, around 2 % or less is usually required, for anhydrite around 0.5 %, and lower still with underfloor heating. The flooring and adhesive manufacturers set the exact limits. Then come sanding where needed, priming, waterproofing in wet rooms, and finally the tiles or flooring.

• No edge strip. The screed presses against the wall, cracks around the edges, and the neighbours below hear your footsteps.
• No joint at the doorway. A crack eventually appears right at the threshold and carries through into the tiles.
• Too much water in the mix. The surface dusts and crumbles, and the screed shrinks more and cracks.
• Drying too fast. Open windows in summer or a dehumidifier straight after laying cause cracks and curling at the edges of bays.
• Flooring laid too early. Wood flooring cups, vinyl comes loose, and tiles can lift or crack.
• Too little cover over pipes. Long cracks appear above the pipes and the floor heats unevenly.
• A dusty base under a bonded screed. The screed does not bond, sounds hollow when tapped and can come loose under load.
• Mesh lying at the bottom. The reinforcement does nothing and cracks appear anyway.

In practice it often happens that the screed is fine, and the real mistake is impatience when laying the flooring. Measuring moisture costs far less than replacing a floor.

You can screed a storeroom, cellar or small room yourself if you have a mixer, a long straightedge and a helper. For larger areas, underfloor heating, or a floating screed in a flat above neighbours, call a contractor. For a first attempt, use a bagged ready mix.

1. Clean the base, vacuum the dust and fill any larger holes.
2. Mark the finished screed level on the walls, measured from the datum line.
3. For a bonded screed, prime the base as the mix manufacturer instructs. For a floating screed, lay the insulation and membrane.
4. Fix the edge strip around the whole room and around pipes.
5. Set screed rails or form mortar strips to the marked height and let them harden.
6. Mix with the exact amount of water stated on the bag, and only as much as you can lay in time.
7. Lay the mix from the corner furthest from the door, working towards it. Compact it and rule it off along the rails.
8. Remove the rails, fill the grooves with mix and trowel the surface smooth.
9. Cover the screed with sheeting, close the windows and keep loads off it for several days.
10. Before laying flooring, have the moisture measured or keep to the manufacturer's drying time.

Check flatness with a two-metre straightedge laid in different places and directions. Bonded flooring, large-format tiles and vinyl usually allow only small gaps of a few millimetres under a two-metre straightedge. The flooring manufacturer states the exact tolerance. Scratch the surface with a key or nail: it should only score, not crumble. Tapping with a small hammer reveals hollow spots.

• The screed is at the agreed level from the datum line in every room
• The edge strip runs all the way round and the screed touches no wall or pipe
• Joints are in the doorways and large areas as agreed
• No large gaps or humps under the straightedge
• The surface does not crumble or dust and does not sound hollow
• No major cracks, only occasional fine surface hairline cracks
• Falls in the bathroom lead to the drain and water does not pool
• You have in writing the screed type, the date it was laid and the recommended drying time
• For underfloor heating, the date and procedure for the heating-up protocol are agreed
• A residual moisture reading is recorded before the flooring goes down