"What you see is what you get" is only correct if you know exactly what you have under your feet. The base for it all!
The sub-floor normally consists of either suspended timber boards on wooden joists, usually in older buildings, or a concrete/cement and sand screed floor, usually in newer buildings.
It can, however, also be metal/glass, plaster, brick, mastic asphalt, chipboard / strandboard (OSB), hardboard, anhydrite/gypsum, magnesite, as well as other, often unsuitable, previous floors. If unsuitable, they must be removed.
Approved document E of the Building Regulations (1991) sets out sound transmission values which the floor/sub-floor must pass in relation to airborne and impact sound. If requirements are not met, further insulation is necessary.
The sub-floor must be sound
‘Sound’ means that it is appropriate to the occupancy of the building, taking structural considerations and design into account. The structure and design should be based on stress figures published by the Institute of Structural Engineers. Floors subjected to heavy loading or abnormal concentrated loads should be designed by a corporate structural engineer or be tested in accordance with the procedure for prototype testing as per BS 5268.
If the sub-floor is sound it must also be dry
Even though a damp-proof membrane (DPM) is normally applied either between the screed and the concrete or, alternatively, below the concrete, a vapour barrier may still be required.
If the humidity is too high, it can be due to lack of drying and/or due to the lack of suitable DPM. A 6” concrete floor slab will take about 6 months to dry. Remember: site conditions at time of installation are to be the same as those which will generally prevail.
BS 8201 (1987) – British Standard recommends that the moisture in concrete slabs and screeds does not exceed 75% RH or 5% MC measured with a sealed humidity box. However this corresponds to a wood equivalent of 16%.
As per below simplified scale, 75% RH is too much and not recommended, even if the installation is taking place in a non-heated, covered environment. To install wooden floors in an environment with an MC >16% is also not recommended. The cause for the high MC or damp condition is to be found and rectified, likely by letting the concrete and/or screed dry properly!
The importance of maintaining a uniform temperature and humidity throughout the year is highlighted by the fact that just a small change in MC of the wooden floor, (2-3% - from using under-floor heating to intermittent heating, or simply the natural difference between winter and summer), can produce an approx. 1.0 – 1.5 mm expansion / contraction per linear meter across the grains.
There must therefore be an expansion gap all around the room, including at radiator pipes etc., of minimum 12 mm (1/2”). If installed above under floor heating, then please see below. You must ensure that the MC of your floor, at time of installation, is a maximum of 1-2% within the expected EMC when in use.
