Moisture Content

Whatever product our diverse businesses are involved with, BWF manufacturing members all use wood as our basic and key material. Accordingly we all also have to consider on an on-going basis, an unavoidable fundamental feature of this favoured commodity – one which brings both benefits and challenges – it is of course ‘Moisture Content’ (MC).

It can be considered that although MC is critical to our products performance its importance is regularly dismissed or even disregarded, particularly by those who should be best placed to manage it.

Why is moisture content so important?

The MC level is expressed as a percentage of the oven dry weight of the timber – growing trees will typically be stated as having a MC of more than 100% and even at a MC of around 25% water will be partially filling the structural wood fibres. Initially as the timber dries this moisture is lost, lowering the weight although not affecting the timber dimensionally. However if the bound moisture in the empty wood cell walls also dries, without some control, the fibres contract and shrinkage occurs. Conversely if dried wood is allowed to absorb moisture then the reverse of this process occurs, creating dimensional increase and distortion.

Against this background, unless the component required is going to be used in and designed for a wet environment, it is a fundamental that wood must have been dried to a target MC level before use. This will ensure minimal and restrained movement.  The correct MC level will relate to the intended product use and this is defined in various guidance and standards such as BS EN 942. As we know joinery grades of timber are readily available at these appropriate MC levels for joinery manufacturing.

What can manufacturers do to avoid costly product issues and claims as a result of moisture?

It is easy to simply assume that the wood provided has an MC suitable for the intended use. It is important to check MC as part of your quality control process. If we start from the wrong base the chance of product failure later is obviously increased. We all acknowledge that  internal joinery needs to be drier than external if movement risk is to be minimised, but do we assume our timber stock will be universally suitable?

The biggest danger however is the often unsatisfactory conditions that are found on site with wet trades prevalent even as internal joinery is being installed, or exterior components being fixed days or even weeks before full decorative protection is applied. In both cases we are often pressurised into complying with time and contract deadlines but if the product fails we all know where the finger of blame will point.

It is not easy but it is in all of our interests for us to challenge bad site practice and to push back at those customers and our own staff, who fail to acknowledge the unavoidable link between movement in timber and the maintaining of optimum correct moisture content.

Fundamentally products and components, however well made and with an accurately controlled MC will move or distort if they are either dried out or wetted via the effect of ambient storage or installed conditions.

When supplying internal doors to a newly constructed building, it is advisable to instruct your customer in good practice that will ensure the good performance of the doors. BWF members can access further advice here such as ‘What Advice Can I Give My Customers Concerning The Correct Care And Treatment Of Timber Doors On Site?’, and download content such as our BWF Members' Day presentation on 'Right moisture: Right Timber, Coatings and Processes'.

Engineered timber components for window and door production

Work undertaken by TREP, in conjunction with the Wood Window Alliance, the British Woodworking Federation, and the Timber Trade Federation has defined a simplified range of engineered timber components which will be available from timber and builders merchants on an ex-stock basis.