Concrete sub-floors need particular care and attention when being prepared for the installation of a wood or laminate floor. All concrete will contain some level of moisture and this is especially true of newly laid concrete. Therefore, a new concrete sub-floor should be left for at least 2 months to cure and dry out before any further work is carried out. Even after this period and indeed, this applies to any concrete sub-floor, it is strongly recommended to carry out tests to determine the moisture content. Issues arising from damp getting into wood floors can include bowing and cupping (this is where moisture penetrates the bottom layer of the wood, causing it to expand more than the top layer, thus creating a convex bending across the width of the board). Problems like these can be troublesome and costly to resolve at a later time so, taking all precautionary measures in the preparation stages is essential.
There are various methods to test for moisture in concrete, two of which have been standardised by the American Society for Testing and Materials (ASTM). Standard test method for indicating moisture in concrete by the plastic sheet method (ASTM D 4263) This test involves taping a plastic sheet down to an area of the concrete and leaving for 72 hours after which the relative humidity (RH) is measured using a dew point hygrometer. A maximum RH of 80% is recommended for wood floors and similar floor coverings. Further information on recommended relative humidity of concrete sub-floors can be found here.
If this test is to actually be carried out according to the standard then 3 tests for the first 1000 square feet (93m² approx) plus tests for any additional areas should be performed. The test uses anhydrous calcium chloride which, being hygroscopic, readily absorbs moisture from the atmosphere. You may have noticed small sachets of this substance in the packaging of products ranging from shoes to electrical goods being used to keep things dry and protected from humidity. A small amount, that has been kept sealed, and thus is free from moisture is weighed and placed in a container under a sealed enclosure or taped plastic sheet as in the first test. Again, this is left for 72 hours after which it is again weighed. Calculations that take into account the weight change and the size of the sealed area where the test was carried out are then performed and the result is expressed as the number of pounds of water emitted per 1000 square feet (93m² approx) per 24 hour period. For solid wood floors, engineered wood floors and laminate flooring it is recommended that this value does not exceed 3 pounds (1.36 Kg).
Whilst it is prudent to try and adhere to the above recommendations, it might not always be possible. For example, concrete is a porous material which means it can absorb, as well as emit, moisture. It may be that, in some situations, moisture is absorbed into the concrete from beneath where a damp course or vapor retarder has not been used. If this is the case then either a damp proof membrane sheet or a 2 part liquid damp proof membrane should be used over the concrete sub-floor. Ideally, and for total peace of mind, you should try and achieve the recommended moisture levels and use a damp proof membrane or vapor barrier.
For a perfect wood floor installation, you should aim to have as level a sub-floor as possible that is also free from any other defects, paint, grease, dirt or adhesive left from any previous flooring installation. To test how level your floor is you need to take a plank or straight edge that is 3 meters (10 foot) long on its edge and lay it at various positions and in all directions around the room. If you detect any gap between the floor and the board that is greater than about 6 mm (1/4 inch) then you will need to use a 2 Part Latex Leveling Compound in order to level the floor. This is a liquid compound that will flow across the sub-floor creating a level surface on which to lay the floor. This should be used before and underneath any damp proof membrane that may also be employed and is suitable for use with under floor heating systems.