Wood Works No. 4 – Wood flooring movement

A beautiful wood floor can enhance any room, but be aware of these factors to avoid excessive movement in your wood flooring …

Make sure your wood flooring is supplied and installed at the correct moisture content to avoid movement in service (See Wood Works No.1)

As we covered in Wood Works No.1, the issue of moisture content should be borne in mind when ordering, receiving, and installing your new wood flooring.

It is important to be aware of potential issues with flooring movement, which in turn is associated with changes in moisture content.  It is therefore important to consider the relationship between these two factors.

Timber contains a large amount of water when it is freshly felled, and it needs to be dried so that it can be used without excessive shrinkage and distortion.  The amount of water in a piece of wood is referred to as its moisture content, which is expressed as a percentage of its dry weight. Below around 30% moisture content (known as “fibre saturation point”), timber will begin to shrink as moisture is lost from the wood fibre of the cells. It can also take on moisture below this level, and subsequently expand. This is because timber and wood-based materials are hygroscopic; in other words, the amount of water they contain rises and falls in response to changes in temperature and humidity. 

Although it is not possible to prevent flooring from expanding and contracting in use, this movement can be minimised by drying the boards to a level that they are likely to achieve in service, referred to as their equilibrium moisture content (“EMC”). 

In this respect, the relevant British Standard (BS 8201) provides general guidance on the moisture content that timber flooring will attain in different environments, as follows:

Table 1:

EnvironmentRecommended MC for wood flooring
Unheated conditions15% to 19%
Intermittent heating (substantial drop in temperature between periods of heating)10% to 14%
Continuous heating (temp maintained at reasonably constant level throughout year)9% to 11%
Under floor heating6% to 8%
Recommended moisture content when laying a wood floor in different service environments

Consider the environmental conditions that the wood flooring will be subject to in service

Shrinkage in oak flooring
Uneven shrinkage in oak floorboards installed at too high a moisture content

Although the Standard references temperature, it is the amount of water in the air, or relative humidity (“RH”), which has the greater influence over the EMC that the flooring will attain in service. The RH of a normal domestic environment is likely to range between 35% – 65% over a yearly cycle, and wood flooring will shrink and expand in response to these conditions.

Table 2 provides a guide to the expected moisture content that timber is likely to attain under different environmental conditions. This shows, in theory at least, that during the year timber might experience a change in moisture content of anywhere between 8.5% – 14% at a temperature of 20°C. In practice, and depending on the heating regime of the household, an EMC of around 10% – 11% might be expected to prevail, except when laid over underfloor heating where it is subject to direct heat from below.

Table 2:

MC in flooring at different conditions of Temp and RH

Expect some flooring movement in service

The amount of timber movement that occurs in response to changes in moisture content varies depending on the species and nature of the flooring. Movement in solid timber boards only occurs across the grain (movement along the grain is negligible) and can be expected to be greater than that in engineered flooring.

For example, a solid 150mm wide oak floorboard could move by 0.4mm in response to a 1% change in moisture content. Not much. But when considered across a 5m wide floor, this equates to 12mm. Were the floor to experience seasonal movement of 3%, then there needs to be provision to accommodate movement of 36mm.

Plywood, by contrast, is inherently restrained and dimensional movement is quite small compared to solid timber.  The same change in moisture content would result in around a 0.015% change in plywood, or a 0.75mm change across a 5m floor. This is one of the benefits of bonding a solid timber wear layer to a plywood core in engineered flooring as the plywood restrains the movement of the solid timber.

In all cases, it is necessary to leave gaps at the floor edges as well as all other abutments, such as door thresholds and linings, floor sockets, hearths, radiator pipes etc. to allow for possible expansion.  For large floor areas where it is not possible to accommodate the expansion in a perimeter gap alone, particularly with solid flooring, arrangements should be made to accommodate the movement between individual boards or at predefined regular intervals.

It is also worth bearing in mind that if your property is to be left unoccupied for a prolonged period, then it would be worth arranging for a key holder to ventilate the premises regularly, or otherwise consider installing a humidifier. The reason for this is that when buildings are not occupied and the heating remains operational, even at a low level, there is a danger that heating levels can build up. As warm air holds more moisture, the RH can fall resulting in a dry environment which can lower the EMC of the flooring, and this can result in the development of gaps and fissures.

Be prepared for some minor floor distortion and seasonal gaps

BS 8201 provides guidance on how to assess potential defect issues when inspected from a normal viewing position, either standing or seated.

A common form of distortion in flooring is concave or convex cupping, where the centre of the boards appears raised or lower than the edges. The Standard considers that some cupping across the face of the floorboards can be expected. The permissible limits are deemed to be 0.3% of the face width for solid wood flooring and 0.2% for engineered flooring, and up to 0.5mm should be regarded as acceptable.

Movement in wood flooring resulting in distortion in form of crowing and cupping
‘Crowning’ and ‘cupping’ (right) in wood flooring in adjacent boards

BS 8201 also recognises that some movement across the width of flooring is likely in response to seasonal changes in conditions which will give rise to longitudinal gaps.  Although no precise limit is stated, it details that longitudinal gaps at joints will vary throughout the year and could be up to 2mm wide at times, whilst header joints should not exceed 0.6mm. It also points out that gaps should be relatively even across the floor and that uneven or inconsistent gaps which stand out in an otherwise closely fitted floor should be investigated.

Look out for our forthcoming Wood Works Blog on top tips when laying and maintaining your wood flooring.

For further information:

BS EN 8201: 2011 Code of practice for installation of flooring of wood and wood-based panels (“BS 8201”). 
The Wood Shop Consultancy – www.timberconsultancy.co.uk

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