Do i need to seal/prime a concrete floor b4 tiling?

5 Jul 2011
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United Kingdom

I am away to tile my washroom floor tihs weekend. The washroom is a lean to building that used to be an outside toilet and coal bunker. You step down out of the house to the concrete floor about 6" below.

It used to have lino from previous owners and when I lifted this there where a couple of damp patches but they dried and were invisable within the hour. So it appears the floor sweats. There certainly wont be a DPC.

I was thinking that tiling straight onto the concrete as is would keep it allow it to brath through the grout?

Or would you seal it?

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bonding agents dont seal the floor, they just either tie up dust etc or provide a seperation between two surfaces that dont like coming into contact with each or such as concrete and gypsum.

If you are just onto an old sand and cement screed that is sound and free from dirt and grease a normal concrete based adhesive should be fine.
Ok thanks. The floor is sound, so i get your advice.

What about from a DPC point of view though. There wont be anything with the age of this floor and i have seen a little moisture show face on very wet days but that I think is more to do with outside harling and only happens near the edges.

If a little moisture comes up through the floor under the tiles will the grout lines allow it to breath and dry?

With this in mind do you still think normal adhesive and grout will be fine?

TBH most modern adhesives state that they can only be used with a DPM. If you really have damp coming through as opposed to condensation then no bonding\sealing agent is going to help.

If the floor is staying dry then you may get away with using a purely cement based adhesive (non flexible, no polymer modification) - many do and have a floor for years without issues -- but if you have damp coming through its going to get behind your tiles - grout isnt going to allow evaporation\transfer of moisture.,
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