There are four main types of screed that are used:

Sand & Cement (with added fibres)*most common use

Sand and cement screeds are a mixture of sand and cement generally in a 4 to 1 mixture ratio with water. We recommend the use of added fibres which reduce micro cracking to the screed surface and surface abrasion.

Sand and Cement Enhanced Screeds

Enhanced screeds are sand & cement screeds with added chemicals that improve the properties of the standard screed. The improved properties include faster drying times and/or extra strength. The additives are made by various manufacturers and are sold under their own trade name. E.g. Flexidry. The screeds can be obtained ready mixed or the additives can be site mixed. We recommend Flexidry products.

Anhydrite ( Calcium Sulphite) Screeds

Anhydrite screeds are a different type of screed from sand & cement. The screed is made from calcium sulphate, sand, water and other chemicals to form a liquid screed. Unlike sand and cement screeds (which are spread, compacted and levelled), the screed is poured onto the floor through a delivery hose and levelled with a dappling bar. Large areas can be covered much quicker with this screed type. It is essential however, that the floor and edge insulation must be fully waterproofed by taping and sealing all joints in the floor and edge insulation. These screeds are sold under manufacturers trade names and delivered to site ready mixed. Whilst a large area of this screed can be installed quickly it has the disadvantage of a long drying time and it cannot be laid in wet areas or laid to falls.

Liquid Cementitious Screeds

This type of screed is similar to an Anhydrite screed but uses cement instead of calcium sulphate, it is generally much stronger and can be rapid drying, although available it is not as common in use.

Expansion Joints-Crack Inducers

All screeds expand and contract to some degree so allowance has to be made for this. The perimeter insulation fitted with underfloor heating allows for some of this, however large areas will need to have expansion joints or crack inducer cuts in the screed itself to allow movement and avoid screed cracking. Expansion joints will also be required to mirror any expansion joints in the floor slab. These should be as recommended by the screeder or architect.

Day Joints

Day joints are positions where the screed has been finished on one day and will be carried on the next day. In these positions, to avoid cracking the joint needs to be reinforced with mesh to bond the screed together.
Alternatively an expansion joint could be fitted in this position.

Floor Tiles, Marble Floors, Stone Floors

Where the screed is to be finished with a rigid tile, marble or stone topping the expansion provisions are very important as screed movement can crack the floor finish. The flooring should be designed by the floor installer detailing expansion provision in the floor tiles themselves. This can then be mirrored with a separate bedding layer with expansion joints or in the screed below.
Alternatively, a de-bonded bedding layer for the floor finishes can be installed above the screed.

Screed level and surface flatness

All screeds have to be installed to a British Standard of level and flatness. Flatness –the variation in gap under a straightedge placed anywhere on the surface to be not more than the following.
- 10 mm under a 2mtr straightedge--SR3
- 5 mm under a 2mtr straightedge—SR2 #
- 3 mm under a 2mtr straightedge—SR1
# In general use

Permitted level tolerance from floor datum +/– 15 mm

Reinforcing Mesh

In sand cement type screeds the use of D49 mesh or chicken wire or an SBR slurry consisting of polymer, cement and water can be used to reinforce the screed to avoid cracking. In areas where a number of underfloor circuits can come together such as some manifold locations, or where the screed is below the recommended thickness, the screed should be reinforced by the installation of D49 mesh, at mid point in the screed, over the underfloor pipework.

Screed Treatment for finishes

If an adhesive or other finish is to be applied to the screed, it may not be able to be applied directly; a sealer may be required first. Always check with adhesive/finish manufacturer.

Remember

No concrete floor will be flat and level on a building site, so it is best do a level floor survey to ensure enough depth above the floor slab is available for the underfloor heating and screed. When specifying screed depths, ensure the minimum specified depth allows for some intolerance in the floor slab.

Protection

Screeds are not a wearing floor finish and should always be protected until floor covering is fitted

Screed Moisture Testing