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This page contains information for customers about some of the more technical aspects of fitting and caring for your wooden floor.

When laying any type of timber floor, it is very important for the subfloor to meet the required parameters for moisture and levels. The final installation is only as good as the subfloor you are installing over, so care should be taken to carefully inspect the subfloor.

Moisture Content

Timber floors laid onto a subfloor with high moisture content can produce numerous problems. The flooring elements can expand on the underside producing a wave like effect over the floor surface, this is known as cupping. The floor can over expand, lifting the entire floor from the centre (bridging). Engineered boards can delaminate; the top hardwood layer becomes separated from the bottom layer. The flooring elements can separate from the sub floor due to failure of the adhesive.

The recognised drying times for a new screed is 1mm per day up to 40mm and 0.5mm per day thereafter. This means that a 75mm thick traditional screed will take 110 days to dry to an acceptable level for applying sensitive floor finishes. The drying time is also very reliant on a good airflow around the screeded area plus a controlled temperature of 20 °C & 50% RH.

Once the required dry time has elapsed the screed would then be tested .Testing methods for the moisture content of floor screed vary in both time taken to achieve results and their reliability.

An air hygrometer is the testing method recommended by The British Standard to record the Relative Humidity (RH) in screed. When using a hood to get results from above the surface of the screed the hood has to be left in place for 72 hours to allow the chamber under it to reach equilibrium with the humidity and the temperature of the floor slab. This is a timely testing method requiring a return visit to site and it is often impractical to ensure the hood is not disturbed by traffic on a busy site.

An RH Probe can be used to test the relative humidity in the screed below the surface in situ and requires a number of holes to be drilled in the screed. A plastic (sleeve) hole liner is placed into the drilled holes and these are then sealed left in place for 72 hours to allow the humidity in the sleeve to reach equilibrium with the humidity in the screed before making the humidity measurement with the RH Probe. The probe is inserted into the sleeve to record the RH and for each recording it needs to be left in place in the sleeve for approximately 40 minutes to reach the temperature and humidity equilibrium of that within the concrete floor slab. Readings should then be taken over a period of 20 minutes to allow for any drift in the meter readings.

For an accurate speedy result, the Moisture Content (MC) test is recommended. This is the testing method preferred by most product manufacturers particularly when testing modified screeds which are not currently covered by the latest revision of The British Standard. The Moisture Content test can be carried out either by a destructive method, a calcium carbide test, or non-destructive method using a Tramex Meter.

Figure 1
figure 1.

The British standard for the moisture content within a concrete screed, prior to laying a timber floor, is 75% RH (Relative Humidity) for unheated screed and 65% RH for screeds incorporating an under floor heating system (see figure 1). Moisture tests should be Figure 1 carried out prior to any installation regardless as to how dry the surface looks. If the readings taken are in excess of the figures stated then either a longer drying time or a moisture barrier will be required. On engineered floating floors, it may be possible to use an underlay incorporating a moisture barrier; however, if the moisture content is very high then even this may not be enough. On floors that are to be stuck down, a liquid DPM (Damp Proof Membrane) will be needed (see figure 2). Various products are available depending on the sub floor composition (concrete, sand and cement, calcium sulphate screeds etc.) and as to whether the screed is heated.

Figure 2
figure 2.

Timber subfloors such as joists or existing floorboards should also be tested. This test is carried out using a timber moisture meter such as a Tramex or Protimeter. The difference between the timber floor and the subfloor should be no more than 3%; so, if an acclimatised floor has moisture content of 9%, prior to laying, then the timber sub floor should not exceed 12% (see figure 3). If higher readings are taken, then a moisture barrier such as a Bitumen flooring paper should be used.

Figure 3
figure 3.

Subfloor levels

It is extremely important that the subfloor levels are within the recognised industry standards of +/- 3mm over a 3 meter straight edge in all directions. Floors laid on unlevelled subfloors will produce excessive vertical movement on floating floors, this can damage click systems, pull joints apart, create squeaks and de-laminate engineered boards. Floors laid onto an unlevelled sub floors incorporating under floor heating will produce an uneven heat distribution over the floor surface. All manufactures require their floors to be laid in accordance to these levels; failure to meet them can affect the manufactures guarantee.

Levelling to concrete subfloors should be carried out using a self-levelling compound. If high moisture readings have been found then the application of the DPM should be carried out prior to levelling. It is recommended that the levelling compound and the DPM used come from the same manufacture. If no moisture barrier is required then the sub floor should be primed in accordance to the manufactures recommendation (do not use PVA a mix). Failure to prime the floor will prevent the levelling compound from adhering to the subfloor. On new screeds, it may be necessary to grind the surface using a copper plate to remove any contamination created during the drying process. The type of levelling compound will depend on the thickness required and the subfloor composition (new screed, tiled surface, timber sub floor etc.). Levelling should be carried out in one application using a levelling compound rake and a spiked roller (see figure 4).

Figure 4
figure 4.