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Commercial Flooring News

Damp Subfloor

Peter Daulby on moisture in subfloors

MOISTURE in a subfloor can cause problems and call for costly replacement of resilient flooring. Here I explain some of the damage caused by damp subfloors and how the flooring on top is affected.
Problems caused by moisture in subfloors can be avoided by taking steps to ensure that the subfloor is dry and free from defects. Although the issues associated with damp subfloors are varied, the solutions tend to be similar.
The growth of mould is common when a subfloor is damp. This manifests itself as a blue, grey or black discolouration. The mould becomes trapped between the damp subfloor and newly installed covering and can give off a musty, unpleasant odour and cause the adhesive to fail and the flooring to lift.
Moisture can also lead to a pressure build up which causes bubbles or blisters to form. They can be very small or relatively large and often release considerable amounts of water onto the flooring when pierced.
Releasing the water can be a temporary solution, but in reality more extensive action is needed to remedy the cause of the blistering.
Bonding problems between the subfloor and the covering are also frequent where there are damp subfloors. Alkaline salts originating from the concrete can be carried by moisture from the subfloor and can chemically attack the adhesive, causing loss of adhesion between the subfloor and the covering.
Where there is a water-based adhesive, it will become emulsified and fail. These alkaline salts can also attack the plasticisers in the flooring making it brittle and shrink. Sometimes moisture from the subfloor can cause the latex screed to delaminate. This can often be seen on the surface of the flooring as a rippling effect.
Discolouration is mainly seen in heterogenous flooring, although this also occurs in some light-coloured homogenous products. The discolouration tends to appear as a pale yellow colour.

This is believed to be the result of substances originating from under the floor migrating into the covering.
This discolouration can fade over time as the subfloor dries out. However, as previously stated, a long term solution is required to prevent the discolouration from recurring.
In all the above cases, the permanent solution is the same. Moisture in the subfloor should be identified and dealt with accordingly.
At Altro we always recommend installations undertaken in accordance with BS 8203. The first thing is to carry out a moisture test using appropriate equipment. Sometimes a damp subfloor is due to leaky pipe work. Usually a pressure test can confirm this.
As always the cause of any leak must be identified and fixed. With new and existing buildings a damp subfloor should be treated with a suitable damp proof membrane, such as Altroproof Solo. Where problems have already occurred, the latex screed and the covering should be removed and replaced.
Of course you can also use a barrier such as Altro Everlay A or B. Consideration could also be given to using an adhesive-free safety flooring such as Altro XpressLay, which can be installed without using a damp proof membrane up to 97% relative humidity.
T: 01462 489405
Peter Daulby is technical services manager at Altro

This article has been reproduced from the Contract Flooring Journal website. You can find them at