Screed is generally a sand and cement mix that is finished to a desired height or fall/slope. It’s used instead of concrete because of it’s work-ability and strength in thinner thickness(no less than 10mm thick for bonded).
Depending on whether it is laid directly on a supporting structure (for example a reinforced concrete floor slab), on an isolating layer (for example a vapor barrier ie. polythene) or on a layer of thermal insulation and/or soundproofing material, it is known as a “bonded”, “unbonded” or “floating” screed.
A screed may also have an underfloor heating/cooling system incorporated within its structure and, in such cases, is known as a “heated” screed.
The most common type of screed design in New Zealand is Bonded screed.
Bonded screed – a bonding mixture is applied to the substrate (called slurry) and whilst still fresh the pre-mixed sand and cement is placed, compacted and troweled to the desired finish.
It is important to mix all components of the system correctly to manufacturers specifications, especially the slurry. Failing to do so, will most definitely result in hollow screed and ultimately a failure.
A critical point that is to often overlooked is ensuring the screed has cured enough before adding the next layer of material, ie. waterproofing or floor coverings.
Depending on the covering the screed should have a residual moisture level lower than 3%. The reason being, laying a barrier over a screed, with too much moisture, with the screed continuing to hydrate, that moisture will force its way upwards and cause the barrier to debond.
There are solutions to quicken cure times.
Special cements that hydrate faster, Topcem or Mapecem are two Regal Tiling often use.
A 2nd option is a vapor barrier primer/sealer.
What ever is chosen, be aware of the requirements, and check when getting a quote, the type of cement being used for your screeded showerbase. If regular Portland cement is being used, depending on its thickness, a wait of weeks may be necessary before applying other layers.
To sum up, when choosing which product to use to make the screed, be it a special binder, a pre-blended mortar or traditional site-prepared mortar, you must take into consideration the final use of the screed, site conditions (internal or external, the thickness to be laid, etc.), the type of flooring to be installed, the time to wait before installing the flooring and the time required before putting the flooring into service.