Section E Screeds Plastering and Rainwater Goods
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 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.
Diagram E4 - Bonded screed
Slab surface must be roughened (scabbling), have all dust removed, be soaked in water, and have a bonding agent or cement grout brushed well in before screed is laid to ensure the screed is properly bonded. Lay screed directly after wetting and grouting of slab and compact it thoroughly. Bonded screed laid to a depth greater than 40 mm is more susceptible to curling or becoming hollow. To minimise the risk, lay bonded screed to a depth between 20 mm and 40 mm.
Diagram E5 - Unbonded screed
The unroughened base or damp proof membrane in unbonded screed prevents bonding, allowing some risk of curling and hollowness. By increasing the thickness of the screed, therefore, these risks can be minimised. The minimum thickness for unbonded screeds is 50 mm.
The addition of 10 mm or some small-sized aggregate to replace some sand in the screed mix can be useful in reducing the risk of curling. A higher workability to a regular stiff consistency will be required in this case or approximately 50 mm slump.
Curing of screeds
After laying screeds, cover with plastic sheeting directly after to cure them. Lap plastic sheets well and leave in place for 7 days. This sheeting is useful as it protects against foot traffic and aids in curing the screed. The risk of curling and cracking is increased in rapid curing screeds.
65 mm screed thickness is required where screed is laid directly on insulation.
Diagram E6 - Floating screed
Ensure screeds are laid to the appropriate thickness to avoid cracking and lifting.
Services in floors
Diagram E7 - Services with chase in concrete slab
Services should preferably be incorporated in the floor slab where possible. The guidance in the diagram below should be adhered to when it is not possible to incorporate services in the floor slab.
Diagram E8 - Services in screed