| Sub-floors solid or wood need to be dry. British Standards state a
screed should be tested using Hygrometry as described in annex A in the
standards. The maximum level of relative humidity in the screed is 75%.
There are many manufacturers of moisture testing equipment such as
Tramex and Protimeter whose instruments can be used to identify areas
for further testing with a hygrometer. These instruments can also be
used to check the relative humidity to British Standards. The duration
of the test will depend on the sub-straight. Sand and cement will
normally require 2 to 3 days against power floated which will require at
least 7 days. Never test floors with underfloor heating artificial
drying aids (de-humidifiers) switched on. Switch off for at least 4 days
prior to setting the hygrometer and they should remain off during the
As a guide a new sand and cement screed will dry at a rate of 1mm per
day for the first 75mm and 0.5mm per day up to 100mm. Thickness greater
than 100mm can take considerably longer (150mm up to and over 1 year)
given ideal drying conditions. Anhydrite screeds dry at a similar rate
providing the surface laitance has been sanded off to allow evaporation
or treat as power floated.
Some types of sub-floors can be coated with a liquid damp proof
membrane to prevent excess moisture affecting the floorcovering. Always
consult the DPM manufacturer for suitability.
Rooms below ground level are particularly vulnerable to high moisture levels see section 9 below.
Wood sub-floor moisture also needs to be checked. This can be done
using the equipment described above with spike attachment. These work by
pressing the spikes into the wood with the spikes (2) in line with the
grain. The maximum moisture level is 15% although ideally 13% should be
considered. Moisture levels above 17% need to be investigated. High
levels could be caused by poor ventilation under the suspended
Taken from http://www.mflor.com