It is important to consider the relationship between the base, the levelling screed and the flooring, and the potential for relative movement at cracks or joints, which can adversely affect the performance of the flooring.

Where there are structural movement joints in the base slab, these should be continued at the same position and width through the levelling screed to the surface of the finished flooring. The detail at the flooring surface depends upon the joint width, the type of floor finish, the service conditions and the expected amount of movement and may comprise proprietary movement joints or sealant joints.

Where construction joints in the base slab have opened, or can open, they could reflect through an overlaid bonded levelling screed. Similarly, at positions of rigid supports in base constructions, which are subject to deflection (e.g. supports to precast planks), there is a risk of reflective cracking through an overlaid levelling screed. In both these cases, if the floor finish is to be bonded and rigid, a joint capable of accommodating the expected movement should be formed through the levelling screed and floor finish.

Within the constraints of the joints required for accommodating base slab movements, described in the two preceding paragraphs, the levelling screeds should be laid in as large an area as possible in one operation to minimize the number of joints, consistent with achieving the appropriate surface regularity. The areas may be laid as strips 3 m to 4 m wide for convenience of application, with special attention to bonding and compaction along the edges.

SR1 – 3mm
SR2 – 5mm
SR3 – 10mm

levelling screeds laid in large areas tend to crack randomly as they dry and shrink. These shrinkage cracks are more easily dealt with than the more pronounced curling, which can occur at vertical butt joints if levelling screeds are laid in small bays. They do not generally affect the performance of bonded flexible floor finishes or rigid types of floor finishes, which are laid on a separate bed over isolating membrane.
However, shrinkage cracks in the levelling screed can continue to move and can reflect through bonded rigid floor finishes such as ceramic and stone tiles, terrazzo and synthetic resins.
To minimize this risk, the levelling screed should be divided into bays with stress relief joints to ensure that any movements in the screed coincide with acceptable positions of joints in the floor finish. Such stress relief joints should generally be spaced along the length of the strip at about 5m to 6 m intervals, although in rigid tiled floors the spacing may be dedicated by the need to have intermediate joints between the positions required for stress relief in the floor finish, which may be up to 10 m apart. For in situ floor finishes such as resins, account should be taken of the effect of such jointing on appearance. The detailing of these joints in the floor finish depends on the type of finish and the service conditions and may include proprietary movement joint sections or sealant joints.

Stress relief joints in the levelling screed should be formed either by cutting through with a trowel during laying or by saw cutting after hardening. The latter method allows the intermediate tile joints to be set out prior to saw cutting, making coordination of joint positions simpler. The joints in the levelling screed should be straight, vertical and at least to mid-depth. Saw cutting should not be delayed too long as there is a risk of random cracking occurring before cutting. The optimum ‘window’ for saw cutting depends on a number of factors but should not generally extended beyond 3 weeks to 4 weeks.

Where the levelling screed is reinforced, the reinforcing steel fabric should generally be discontinuous at stress relief joints to provide maximum reduction of the risk of random cracking elsewhere. This should be weighed against the risk of differential curling, which would be increased by discontinuing the steel fabric.

Heated Levelling screeds to receive most types of rigid floorings and some types of resilient floorings. Should be divided by movement joints to provide screed bays up to 40 m2 with a maximum bay length of 8m. The position of these joints should be designed to coincide with the joints in the flooring. Any random cracks in the screed that might open and close with thermal cycling are to be avoided with rigid floorings and some form of screed reinforcement can be found to be necessary to minimize such cracks.

Interior Screed Ltd are the UK’s leading floor screeding contractors specialising in all types of floor screeding systems. Established in 2010 and working with many of the biggest and most prestigious names in the residential and commercial building sector’s we install screeds into building developments of all sizes. From complex large-scale commercial projects to residential and mixed-use contracts, we carry a comprehensive range of floor screeding products and services designed to offer our customers a one-stop floor screeding facility.

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