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Question on moisture in old concrete floors

I am renovating a house with concrete floors constructed in the 1950s. The kitchen floor suffers from dampness and is lifting the old vinyl tiles. If I remove these tiles, can I seal the concrete surface or should I replace the floor slab?

Answer

Moisture in floors is a complex problem. It can affect the tile adhesive so that tiles debond and lift. Moisture can be rising damp, i.e. moisture capillary rise or moisture vapour slowly diffusing through a slab from wet ground below. Whether or not floor should be broken out depends on its condition.

Old floor in good condition

This can be left in place However, a surface damp proof membrane (DPM) can be applied to the floor before laying new tiles. It may be necessary to cover the surface DPM with a latex smoothing underlayment before laying the tiles. Alternatively, the tiles could be fixed with an adhesive that is not affected by moisture e.g. epoxy based.

Old floor in poor condition

Replace the slab. The new concrete floor should have a polythene sheet DPM below it to prevent moisture from below migrating or diffusing upwards into the slab. A new concrete slab will contain inherent moisture, which could affect the tile adhesive.

The options are:
(i) Allow the slab to dry out for approximately 1month per 25 mm of concrete thickness
(ii) Use a tile adhesive not affected by moisture.
(iii) Use a surface DPM.

 
Acknowledgement: The Concrete Society
 
Other references:BS 8203 ´Code of practice for installation of resilient floor´
BS 8204 ´Screeds bases and insitu floorings´
BS 8102 ´Code of practicefor protection of buildings against water from the ground´
 

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