A unbonded screed floor is where a screed has been installed over the top of the concrete slab but it has not been stuck to the slab using adhesive.
The screed is isolated from the slab normally using builder's plastic.
Because unbonded screed floors are not integrated with the underlying slab substrate they cannot rely on the slab for strength as much as a bonded screed and for this reason they must be installed at a greater minimum thickness, normally a minimum of 40mm for a traditional sand cement screed or 35mm for an engineered screed from the ScreedMax Pro range such as Mapei Topcem.
Unbonded screeds will also often be reinforced with steel mesh or fibres such as ScreedMax Fibre to better control cracking.
The advantage of unbonded screeds are that they can be installed to a much greater thickness than bonded screeds as the risk of delamination is eliminated as they are unbonded by design and also costs can be saved because adhesive is not required.
The other major advantage of an unbonded screed is that as they are isolated from the underlying slab, if there is movement or cracking in the slab, this may not translate through to and damage the screed.
Also, if the underlying slab is contaminated or it for any reason it is uncertain whether an adhesive will form a good quality bond to the slab, an unbonded installation can avoid this issue by isolating with builders plastic.
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