How to prepare the concrete slab before putting the tiles on top

To help a flooded property recovery after a flood, many people are advised to put in a solid concrete floor and to install ceramic tiles on top using waterproof tiles and adhesive.

To protect the new concrete slab from flood water in the house soaking down into it, and to protect internal living environment from dampness or water vapour rising from the slab following a flood, which could result in mould, condensation and damp issue, it is important that the following is undertaken before and during laying of the new slab:

• Ensure the new concrete slab is laid upon what is known as a “pre-applied fully bonded waterproofing membrane”, instead of upon the more typical polythene damp proof membrane – polythene cannot protect against flood water rising under pressure, it can only protect against capillary dampness rising from the ground under normal conditions. A pre-applied fully bonded waterproofing membrane will protect the slab from absorbing any flood water that may rise up from beneath it. These types of membranes usually have a fleecy upper surface, which allows the concrete to completely stick to it when it is poured, so the membrane and the concrete become one unit, with no pathway for water to get between them.

• After the new concrete slab has dried out, apply two layers of Crystaline Tanking Slurry over the top of the new slab and take this slurry up the internal faces of the existing solid walls for a small way to make sure the place where the walls join the floor is protected from water ingress too. These joints where the new slab will meet the existing walls are vulnerable areas for flood water to ingress. Therefore, before you take the slurry up the internal faces of the walls a short way, form a small concave curved fillet of sand & cement mortar at the wall and floor juncture, which will help to seal it and also ensure that the tanking slurry can transition from horizontal to vertical, without cracking. 

• Apply two coats of water-based epoxy resin floor paint fully over the tanked slab, over the curved sand & cement fillet and, and up the tanked internal faces of the walls a small way. The epoxy floor paint is a vapour barrier, as well as an additional waterproofing layer, so no water vapour will be able to escape up through it to cause condensation and mould later on.  

• If floor insulation is required upon the slab, use a “recoverable” and “resilient” type of floor insulation, like FoamGlass, and ensure it can be bonded to the surface of the slab with a suitable adhesive to stop water getting in between the insulation and the top of the slab. If this happens and the insulation is not bonded down to the slab, the insulation could float, and bacteria could live in the gap.

• “Recoverable” or “Resilient” materials and insulations are those which do not change their form or properties upon contact with or immersion in floor water, do not absorb water, and will not support bacterial growth.

• If insulation is required upon the slab, then a sand & cement screed will probably be needed over the insulation. Screeds are usually around 65mm thick, and provide a firm, finished walking surface over the insulation, to support floor finishes and furniture. If this is the case, a waterproofing additive can be added into the screed by the manufacturer when it is mixed to make it “hydrophobic” ie. to ensure the screed will not absorb water. For example, CEMEX can supply waterproof screeds.

• Whether the actual final upper floor finish will be just the surface of the new slab, or a new slab with insulation and screed over it, when the slab or screed is coated with 2 coats of epoxy floor paint, it is a good idea to sprinkle dried sand between the two coats of epoxy floor paint while the lower coat is still tacky. This will stay in place while the second coat of epoxy is applied over it, and the trapped sand will provide a better key for the tile grout to grab onto, so that the tiles will remain in place without slipping, 

• Finally use waterproof grout between the tiles, to stick them down and point them in.

It is important to realise that every property is different, and although the advice suggested above is sound in principle, it is of a general nature and may not work for every property. Therefore, it is essential to seek initial advice from a suitably experienced product manufacturer or a company that specialises in this type of work, so that a bespoke solution can be recommended. However, following receipt of said recommended advice, it should be possible for you to undertake this kind of work yourself.

This blog has been written with technical support from Michael Vernon BA Hons Architecture CSSW CRDS

Copyright: Mary Long-Dhonau March 2025