Australian Standard AS1884-2012 outlines the requirements of a subfloor, the implications of which are set out.
Clean. If the subfloor is contaminated with any substance which will prevent the adhesive sticking to it, then that contaminant must be removed. This includes marker lines and paint. The surface should, of course, be thoroughly swept of all small loose debris and dust.
Smooth. Any nibs of concrete, lumps of mortar, nail or screw heads, peaks of floorboards and wood knots must be smoothed off. Failure to do this will result in the ‘telegraphing’ of these to the floorcovering surface with subsequent premature wear. Similarly, all cracks, holes, etc. should be filled and made good.
Even. Apart from being smooth, a subfloor must be even, otherwise, a wave appearance will result in the finished floor, and, in the case of tiling, difficulties will be experienced in keeping the tiles in bond (i.e. in line with each other).
Sound construction. A subfloor must be rigid and not subject to flexing, otherwise cracking of the floorcovering will ultimately result. With solid subfloors, the screed should be of good quality. Failure of adhesion will result if floorcoverings are laid over weak or powdery screeds. Permanently dry. Before any floorcovering is installed on a concrete floor the floor installer must be satisfied that the subfloor is dry and that there is no possibility of rising damp occurring. There are several danger areas, covered below.
In floors incorporating underfloor heating systems, to eliminate the possibility of moisture being trapped beneath the warming system, the heating should be turned off for at least 48 hours before moisture testing is carried out.
Some floors may be heated by steam/hot water pipes. These should be treated with great caution. Difficulties with vinyl decorative floorcoverings can be experienced due to these systems which, if overheating, cause degradation of the covering. Similarly, steam pipes and/or hot water pipes laid too close to the surface and unlagged, may also cause damage to both adhesive and decorative finish. Clauses 3.1.3 of AS 1884-2012 refer further to the requirements and conditions for the installation of resilient floorcoverings on subfloors incorporating underfloor heating or hot water pipes. After the adhesive has dried gradually increase the heat.
Some moisture will always remain in the concrete and does no harm as it is static. If, however, the moisture level is still such that, in the right conditions, it will move rapidly upwards and break the adhesive bond with the floorcovering, causing bubbles and blisters or mould growth then a very expensive relaying of the floor will become necessary.
The problem may not show itself until the building is in commission with hot central heating pulling the moisture quickly up through the slab - perhaps a year or more after installation.
A correctly calibrated and installed hygrometer gives an accurate reading when the small volume of air trapped inside it is in equilibrium with the subfloor, i.e. when a series of readings show the same figure and it is not rising. If the readings show 70 percent relative humidity or less then they meet the acceptable dryness level suggested in AS 1884-2012.
This standard suggests that on normal concrete equilibrium will be reached in not less than 72 hrs. Experience shows that the vapour movement from power floated slabs is much slower, so that it can be even 2-3 weeks before equilibrium is reached and a true reading shown. When artificial drying aids have been used to accelerate drying out these should be turned off four days before taking final readings.
Calibration is done by placing the hygrometer over a saturated salt solution in a desiccator for four hours, after which the instrument should be adjusted to read 70°. In use the hygrometer should be sealed to the floor, either by using a ring of putty round the edges or by covering it with a 50-100cm square of transparent polythene, taped to the floor round the edges. Poor sealing will result in wrong readings.
The 70 percent figure set by Australian Standards realistically takes note of the margin of error still likely in a correctly used hygrometer. However, at above 70% RH fungus spores can grow. Note that the standard is the same for linoleum and vinyl.