We often get asked as to whether you can install a screed over a polyurethane membrane. This is pretty common as the concrete slab will often be waterproofed and then the screed applied on top.
Where the question becomes a little more complex is whether the screed will be bonded to the PE membrane.
If the screed can be installed thick enough that it can be installed as an unbonded screed, this is ideal as the screed is effectively independent from the membrane and bonded is not the objective.
However, if the screed needs to be bonded to the membrane (either by design or because the screed is thin) then this can be much more complex.
The problem with bonding to PE membranes is that they are smooth and repel water and give the bonding slurry nothing to obtain a mechanical fix to nor can the slurry penetrate into the membrane in the same way it would into the surface of concrete to form the bond.
The solution to this is typically to sand seed the membrane to the point of refusal to create a rough surface texture that the bonding slurry can obtain a mechanical fix to.
Its important to note that this is primarily a question for the supplier of the slurry, its not strictly a screed question, but is a bonding slurry question, so it should always be confirmed with the supplier of the bonding slurry.
As a general rule, we would never recommend that a traditional cement powder and water slurry be used in an attempt to bond to PE membrane, even if it has been sand seeded. Only a high quality adhesive slurry should be used and only then once it has been confirmed as fit for purpose with the supplier of such slurry.
