Moisture in new concrete floors

Moisture in new concrete floors can affect the long-term serviceability of finishes and possibly the building maintenance and operating costs. Problems include damage to some types of flooring such as carpets, some types of adhesive, some types of self-levelling or smoothing underlay materials and some types of screed materials, wall or skirting materials. Although damage to finishes is usually the main concern, moisture leaving a floor could affect the building environment by increasing the relative humidity in the room and increasing the risk of condensation.

The preferred test for assessing floor moisture is the insulated hygrometer test procedure in BS 8203, Code of practice for installation of resilient floor coverings. This test effectively measures the moisture leaving the floor, rather than the moisture content of the floor. Resistance type moisture meters are not recommended for the assessment of floor moisture. Often flooring suppliers state that their materials can only be laid on floors where the surface relative humidity, measured by the BS 8203 test, is 75% or less. This relatively low value was determined many years ago and some now regard it as unrealistically low for modern site timescales and many modern flooring materials.

With a typical concrete floor about a quarter to a third of the original water content will evaporate, which could be 6 to 9 l/m² for a 150 thick slab. It will take many months for a slab to dry sufficiently to give a top surface RH of 75%, even in a well ventilated and weather tight building, particularly for ground slabs and slabs on permanent formwork. Drying out will be severely delayed if there are sources of external moisture or vapour migration through a slab from the ground below. In practice there is an extremely high probability that a normal slab will not have time to dry out sufficiently between construction and the laying of the flooring. Thus non-achievement of 75% is virtually inevitable on site and must be designed out, for example by using non-sensitive finishes or non-moisture sensitive adhesives.

For further information see Concrete Advice Sheet 22, Moisture in floors.

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