Choosing the correct screed installation technique
A common question we have with our installers is how best to install the screed having regard to the site conditions and project objectives.
A bonded screed is the most traditional and common technique in Australia. A bonded screed is appropriate for thin screeds (15mm sand cement or 10mm engineered).
The main risk with a bonded screed is the risk of delamination where the adhesive that bonds the screed to the concrete slab fails. There are a number of factors which can give rise to delamination which we have dealt with in depth in other articles, but the main issues to be aware of are:
Screeds over 40mm thick have an increased risk of delamination
Good quality latex based adhesives will always give a better bond than a cement power and water slurry
Un-bonded screeds as their name suggests are screeds installed without an adhesive to bond them to the slab. In fact, they are installed over a separating layer such as builders plastic.
Un-bonded screeds need to have a minimum thickness of 40mm (35mm for our engineered screeds) but no maximum thickness. Because they are isolated from the slab they can be less effected by movement in the slab and delamination is not an issue because by their nature they are de-bonded.
Un-bonded screeds are a good option for thick screeds or screeds where there is a suspended slab or slab that is suspected as contaminated (which can lead to the delamination of bonded screeds).
Issues to be aware of when installing an un-bonded screed are to ensure that there is a firm and even compaction throughout the screed, not just the top section (especially when the screed is thick) and to consider using mesh or ScreedMax Fibre reinforcement to add additional strength to the screed.
A floating screed is the same as an un-bonded screed however instead of being installed over plastic it is installed over a compressible substrate, normally foam insulation boards.
For a floating screed, it cannot rely on any strength from the substrate, the screed must be strong enough to fully support itself even if the substrate moves or compresses. Therefore a floating screed should always be an engineered screed from our ScreedMax Pro range at a minimum thickness of 55mm. Furthermore, it is strongly recommended to install mesh or ScreedMax Fibres to reinforce the screed.
All heated screeds should use an engineered screed from the ScreedMax Pro range. This will protect the pipes from point loading and the screed has the strength to be able to handle the flexing that comes with the heating and cooling cycles.
Bonded Heated Screed
Bonded heated screeds using ScreedMax Pro must have a minimum of 25mm screed above the top of the water pipes. Mesh or ScreedMax Fibres must be used to protect the water pipes.
Un-bonded Heated Screed
An un-bonded heated screed also require a minimum of 25mm of ScreedMax Pro engineered screed above the top of the water pipes and mesh or ScreedMax Fibres must be used to protect the water pipes.
Floating Heated Screed
This is the most common heated screed installation. The water pipes are installed over foam boards to provide energy efficiency in the heating system. ScreedMax Pro must be used with a minimum of 55mm thickness, 25mm above the water pipes plus either mesh or ScreedMax Fibres.
A lot of problems can be avoided by choosing the correct installation technique. If in doubt, call us or the project engineers or architects. If there are other issues that need to be considered, we strongly recommend to not cut corners but give us a call as there are other techniques or products that we can use to address specific circumstances.