How to Screed a Floor: Depths, Mixes and Drying Times

Types of screed

Bonded: laid directly onto a prepared concrete slab and bonded to it with a primer or slurry. It can be thinner because it works together with the slab.

Unbonded: laid over a damp-proof membrane, so it isn't stuck to the slab. It needs to be thicker because it stands on its own.

Floating: laid over insulation boards, often with underfloor heating pipes. It needs to be thickest of all because it's supported by a soft layer.

Minimum depths for sand and cement screed

Because concrete slabs are never perfectly flat, specify a little more than the minimum so the thinnest point is still thick enough.

Liquid (flowing) screed

Calcium sulphate (anhydrite) liquid screeds are pumped in and level themselves. They can be laid much thinner – typically from about 25mm bonded, 30mm unbonded and 35mm floating – and are popular over underfloor heating because they surround the pipes completely. They're a job for specialist installers. The surface usually needs sanding before flooring goes down, and they aren't suitable for damp areas or with steel reinforcement.

Sand and cement mix

Traditional screed is usually 1 part cement to 3–5 parts sharp sand, with 1:4 the most common. It's mixed “semi-dry” – damp enough to hold its shape when squeezed in your hand, but not wet enough to drip. Polypropylene fibres are often added to reduce cracking.

How much do you need?

Multiply the floor area by the depth in metres to get the volume, then multiply by about 2.1 tonnes per m³ and add 10% for waste. For example, 20m² at 50mm is 1m³, or about 2.3 tonnes of dry materials with waste. At 1:4, that's roughly 19 bags of cement and 1.85 tonnes of sharp sand. The Floor Screed Calculator works this out for traditional, bagged, liquid and self-levelling options.

How to lay a traditional screed

• Clean the slab, lay a DPM if needed, and fit a compressible perimeter strip around the walls.
• Set screed rails or battens to the finished level across the room.
• Mix in manageable batches with a forced-action mixer if possible.
• Lay the screed between the rails, compact it firmly, and rule it off level with a straight edge.
• Fill the rail gaps and trowel to a smooth, closed finish.
• Cover with polythene for the first few days so it doesn't dry too fast and crack.

How long does screed take to dry?

The standard rule of thumb for sand and cement screed is about 1 day per mm of thickness up to 50mm, and slower beyond that. So a 50mm screed needs around seven weeks in good drying conditions. Cold, damp or unventilated rooms take longer. Fast-drying additives and screeds can cut this to days.

Before laying moisture-sensitive flooring such as wood, laminate or vinyl, test the screed's moisture level – most floor coverings need it to be below about 75% relative humidity. Laying on wet screed is one of the most common causes of flooring failures.