Tim Burrell

Tim is Carpenter Oak & Woodland’s managing director. Tim has extensive experience of managing companies of all sizes, in many business sectors, including construction and building. Tim’s no-nonsense hands-on approach, coupled with his outstanding management experience and commercial acumen, make him the ideal complement to the craft-based skills of the rest of the team.

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Previous posts

• Timber Supply for Oak Frame Houses
• Timber frame for supermarkets
• The quandary of externally exposed oak frames
• Oak frame and building budget
• Infill panels and building warranty

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By Tim Burrell

I’ve just been asked what ‘green’ oak means.  For such a fundamental material in the timber industry, you’d think that there would be a universal readily understood definition, but surprisingly this doesn’t seem to be the case!  I’ve often heard green oak referred to as ‘freshly felled’, but this isn’t really the issue – what is more important is the moisture content.  Thinking I’d like to get an ‘official view’, I called TRADA for their definition, but it seems even they do not yet have one!  So, here’s my take.

Timber can contain two types of water: ‘free’ water that sits in the cell cavities, and ‘bound’ water that is contained within the cell walls.  Freshly felled oak is high in both, but will lose water from the time it is felled.  At the point where it has lost all its ‘free’ water, but still retains its ‘bound’ water, it is at its ‘fibre saturation point’ – in the case of oak this is at a moisture content of 30%.  Below the fibre saturation point, the timber is losing that bound water, and to that extent could be said to be (to some extent) dry.  This is when the timber will shrink, and possibly distort.  Oak will naturally air dry down to 18%, but below this will need to be kiln dried.  Of course, the moisture in the wood will always gravitate to the moisture content around it: drier when the central heating is on full blast and variable and wetter where exposed to the elements.  But I digress…

So, my definition of green is “timber that has a moisture content greater than its fibre saturation point”.  Often, but not necessarily freshly felled, and usually having rested a while ‘in the round’.

A last thought.  We work with green oak because larger section oak is either extremely expensive or quite possibly unavailable – plus it’s easier to work.  How that timber will behave when drying is a question for skilled and experienced graders.  The shrinkage from green timber need not be a problem with good selection, good detailing and the skill of the craftsmen.  Buyers beware, and make sure that your suppliers really know their material!