Screed uses – bonded, unbonded, and floating

A common method for a finishing layer on in-situ and precast concrete floor slabs is sand and cement screed. There are 3 principle ways, namely bonded, unbonded, and floating screed in which screed can be used.

To minimise the risk of curling for the standard tamped finish on domestic concrete floor slabs, screed at least 50 mm deep is required for unbonded screeds. For floating screed, i.e. screed laid directly on insulation, screed of at least 65 mm, ideally with light mesh reinforcement incorporated, is required to avoid curling.

Screed mixes

Screed should be mixed to the ratio 1:3 of cement:dry sand. The mix should be wetted enough to allow sufficient compaction; mix cannot be too dry as compaction of the mix will be difficult.

The use of proprietary bonding agents such as PVA (polyvinyl acetate) or SBR (styrene-butadiene rubber) in the screed to replace water in addition to ensuring screed is the correct depth can further minimise the risk of curling. These bonding agents should be used in accordance with the manufacturer’s recommendations.

Bonded screed

Diagram E8 - Services in screed