Is There A Self Levelling Screed?

I’m Andy Parkin, Managing Director of the Multi-Award Winning Speed Screed. I’m here to talk about self levelling screed.

I would say that I don’t believe there is a “self levelling screed” per say, yes a self-smoothing, but not self levelling, please don’t lay the screed and expect it to level itself.  However the term “self levelling screed” is used extensively in the industry and I will use it here.

The screed is a cement-based smoothing compound. Supplied generally in 25 kg bags, and mixed onsite with water (or latex for some products).

When laying vinyl, ceramic tiles, carpet and other floor coverings. the screed evens out substrates leaving a smooth surface to bond to.

Self Levelling Screed Construction Types

When we talk about self levelling screed, we need to consider the different types of screed construction.  There are three types:

• Bonded is very relevant to the self levelling screed, as it relies on the bond with the substrate. It would be suitable for this type of construction.
• Unbonded is where you have a membrane in between the substrate and the screed. This type of construction is not something that would generally not be be suitable.
• Floating construction is where you have the screed sat on insulation, be it acoustic or thermal insulation. This type of construction is not something that would be suitable for a self levelling screed.

Sound Substrate

When bonding I think it is important to highlight the process. When looking at the substrate, it needs to be sound. When we say “sound,” it needs to be solid. If you have crumbling, cracking, or other issues, then action needs to be taken before laying the screed.  There is no point in laying a self levelling screed onto something that isn’t stable, because it will only result in the screed cracking and/or de-laminating.

Contamination

Checks should be made to ensure that the substrate is not contaminated. Contamination can be a variety of things, oils, grease, dust, anything that may have gone into the pores of the screed (the capillaries of the screed). If there is contamination it may require some mechanical surface preparation, which may be grinding/scabbling or shot blasting.  If there is paint on the substrate then it will need to be removed, before laying the self-levelling screed.

The surface needs to be clean and vacuumed if there is dust/dirt on the surface. Before the floor screed is laid a primer needs to be applied to the substrate to assist in the bonding of the screed and the substrate.  Sometimes it can go down without the primer, always read the instructions before laying.  Either way you are going to need some penetration of the surface. It needs to bond, so it may be that you have to grind off the first couple of millimetres of the matrix to get down to something that is bond-able.

If the substrate is power floated concrete then the top of the matrix will need to be removed to get to the more open textured concrete that will allow the primer to penetrate. There are certain products that may go down directly onto it a power floated surface, but again, you would have to look at the requirement for each individual product. In general, you would be looking at mechanically altering the surface to allow for better adhesion between the screed and the substrate.

Screed Thickness

Self levelling screed thickness varies from product to product, however as a general guideline please find the depths below:

• Minimum depths are usually 2-3mm, however there are some products that can go down to “featheredge”
• Maximum depths are around 10-15mm, however with the addition of bulking aggregates (normally kiln dried sand) some screeds can go up to 50mm.

Self levelling screed is normally used to smooth the surface of concrete and screeds, when laying thin vinyl or thin floor coverings. Slight imperfections in substrate will show up in these types of floor coverings.  A self levelling screed will smooth out those slight imperfections.

I hope this helps with your “self levelling screed” questions, if you would like any further help please get in touch.