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Technical Infromation

Rising Damp

Where resilient, wood and textile floorcoverings are adhered to a conventional screed or concrete subfloor there will always be a danger of damage caused by moisture present in the cement-based sub-floor.

Where the screed or concrete sub-floor are correctly installed and allowed to dry sufficiently to attain the moisture content appropriate for the floor being installed, the moisture present in the area should not adversely affect the subfloor and hence the floorcovering you install.

Floorings can only be laid on screeds that have dried sufficiently. When using a flooring hygrometer to test the relative humidity, the relative humidity readings must be below 75% for installation to properly occur.

If dampness is detected then it needs to be left to dry out, unless a damp-proof membrane is used as underlay. Initially the drying will occur from the wet surface and water will be drawn up to the surface by the capillaries in the concrete or screed mortar. However, after a few days, the surface dries and the tops of the capillaries are free of water. At this stage, further drying has to take place by moisture vapour diffusing.

The diffusion process depends on how far the moisture vapour has to travel in a series of short paths with random changes in direction until it reaches the surface and escapes into the atmosphere. This is the reason why sub-floors consisting of thick screed or concrete take longer to dry. Under good conditions one day of drying per millimetre should be allowed for the first 25mm of screed and two days per millimetre for the next 25mm of screed and even more time for screeds thicker than 50mm.

New concrete slabs can take many months or even over a year to dry out. The drying times normally given are based on good drying conditions – such as a temperature of 20 degrees C and atmospheric relative humidity of 65%. The performance of the flooring in tandem with moisture will also very much depend upon the adhesives used, particularly if different types of adhesives are used to install the same flooring.

Normally there are two types of sub-floor construction – direct to ground or suspended. Suspended sub-floor constructions normally only has to be allowed to dry to a sufficiently low moisture level before the flooring is installed. Where a subfloor is laid direct to the ground, a conventional ‘sandwich’ damp-proof membrane consisting of a plastic sheet or waterproof coating is installed below the screed or concrete base to subsequently protect the applied materials from the effects of rising damp.

Where the direct-to-ground constructed subfloor has no protection from rising damp, or where the damp-proof membrane has failed, there are two options available. The first option is to use unbonded screen, minimum thickness of 50mm, on top of a proprietary sheet of damp-proof membrane. If a conventional screed is used this will typically take approximately two months to dry. Rapid drying screed systems are available, with F Ball a recommended supplier.

Where there are height constraints, such as existing thresholds, then an unbonded screed may not be an option, in which case the second option is a surface damp-proof membrane, as only a few millimetres are required to accommodate the smoothing compound and epoxy DPM. The object of the surface damp-proof membrane is to restrict the amount of water-vapour reaching the underside of the flooring.