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Moisture content and timber floors - getting it right!

July 08, 2020

Timber flooring is unique in character and appearance as no two trees are exactly the same. Although there are similarities, each timber floor has a certain uniqueness.

The charm of timber floors is that they are totally natural and with this comes variability in colour and appearance.

Other considerations for timber floors are sunlight, heating and cooling that can affect the rate of expansion and contraction of the floor boards. Moisture from the air is absorbed into timber flooring when humidity is high and released when humidity is low. With increasing moisture, the floor boards go into expansion mode while with decreasing moisture, floor boards go into contraction or shrinkage mode.

The following information is a summary of more detailed information available from the Forest & Wood Product Australia which produces the Wood Solutions Timber Flooring Design guide for installation available from -

Moisture Content

One of the most important aspects of selecting the right timber floor boards is their moisture content.

Floor boards can be seriously damaged by improper exposure to wetting or drying, or both.  Reputable suppliers usually deliver timber building components to the site in the optimum condition suited for its end use.  Flooring is generally supplied within a moisture content range from 9% to 14%.  For larger jobs in specific environments, a different range may be specified.

Acclimatising is the process of allowing partial equalisation of the moisture content of the timber as supplied, to the moisture content of the surroundings in which the timber is to be installed.  Acclimatising relies on each board being exposed to the atmosphere in the room where they will be installed. Therefore opening packaging after delivery and restacking the floor boards in a way that allows airflow between each board can help in this process.

For a normal environment with an expected moisture content of 10% to 12.5%, no special consideration is required. If the flooring is to be laid in a moist environment (expected average moisture content of 12.5% to 15%) then allowance for future expansion should be made and acclimatisation is required.  If the flooring is to be laid in a dry environment (expected average moisture content of 8% to 10%), then allowance for shrinkage should be made and acclimatisation is required. 

Allowance for expansion

Timber floors need to take account of the likely expansion and contraction of the floor boards during their life.

Fitted floors generally require a minimum 10mm expansion gap between the floor boards and any internal or external wall structures.  However, where board ends abut doorways, the gap may be reduced to a neat fit with a small gap of approximately 1mm to prevent rubbing.

Floors up to 6 metres (measured at right angles to the run of the board) should not require intermediate expansion joints provided that normal atmospheric conditions exist.  For floor widths over 6 metres, or where extra allowance for expansion is required, e.g. moist locations, cramping pressure needs to be considered along with providing an intermediate expansion joint or a series of smaller expansion gaps every 800mm to 1000mm to provide equivalent spacing.

If cork expansion joints are used, the cork should be raised 2mm or so above the floor surface when installed.  This will be removed during the sanding process.  Cork at the perimeter should be installed level with the timber surface.

Installation of floor boards

When installing strip flooring over existing timber or sheet floors (plywood or particleboard), it is necessary to check that the existing floor moisture content is appropriate to accept the new floor.  The cause of any excess moisture (wetting during construction, leaks, inadequate sub-floor ventilation, etc.) needs to be addressed prior to installation.  Prior to laying the floor boards, the existing floor should be of similar moisture content, within a few percent, to the new floor.

Installation of flooring should not be done until other construction activities (particularly wet trades) are complete and after the building is roofed and enclosed, with temperature and humidity as close as possible to expected in-service condition.

Fixing flooring through sheet floors and into the floor joists provides a more robust fixing and is particularly appropriate where greater expansion is expected after installation.  If expansion after installation is expected to be small, then mechanical and adhesive fixing into the sub-floor (substrate) may be used.

Timber floors may be laid on battens or plywood over a concrete slab, or by direct fix.  Direct fix to the slab is a specialist field and appropriate professionals in this field should be consulted if considering this method. 

Care and maintenance

Sand and dirt 

The small particles of sand or dirt that can be brought into the house with footwear can act like sandpaper, resulting in scratches in the floor. A simple and effective way to reduce sand and grit from entering the house is the use of mats placed both outside and inside external doors.

Direct sunlight 

Direct intense sunlight can contribute to gaping and possible cupping of boards. It can also cause the colour of both boards and finish to change with time. By filtering the sunlight through the use of sheer curtains or blinds will slow the colour-change processes and is also effective in controlling gap size and possible cupping.

If the sunlight has not been controlled by window coverings, patio roofs or awnings, floors rugs can be used.

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