In this exclusive extract from NBS Shortcuts externallink, written by noted industry figure Austin Williams, we discuss the control of the moisture content of timber.

Timber is one of the most ubiquitous products used in construction, but in order to perform adequately it needs to be carefully prepared, stored and maintained. To ensure satisfactory preservative treatment, accurate machining and efficient fabrication, and to avoid problems due to dimensional change and distortion in use, its moisture content must be controlled.

BS 6100-1 (also numbered BS ISO 6707-1), Building and civil engineering - Vocabulary, describes joinery (known as 'cabinetry' in America) as the assembly of worked components of timber and wood-based panels other than structural timber or cladding, together with associated mouldings used as finishing, such as architraves, skirting boards, weatherboards, etc. Carpentry, on the other hand, is defined as 'structural woodwork'. Woodwork is quite simple: it means working with wood.

The moisture content of wood is normally expressed as a percentage, and is calculated as: the difference between the weight of a sample of 'wet' (green) wood and the weight of the same sample after oven drying (to remove all moisture), divided by the oven-dry weight, all multiplied by 100. Thus, a piece of timber weighing 500 g, and containing 250 g of water will have a moisture content of 100 per cent or [(500-250) / 250] x 100.

The recommendations given in EN 942 Timber in joinery - general requirements for moisture content at the completion time of the product manufacture are useful where specific national product standards, etc. do not exist. It states that, at the time of manufacture, external joinery should have a moisture content in use of 12-19% and that the moisture content of 'internal joinery' should be:

  • 2-16% in unheated buildings
  • 9-13% in heated buildings between 12-21°C
  • 6-10% in heated buildings with temperatures in excess of 21°C.

These values vary slightly from UK national product standards, which typically recommend that at the time of installation the moisture content should be:

  • 18% in covered, generally unheated spaces
  • 15% in covered, generally heated spaces
  • 12% in internal conditions: in continuously heated buildings
  • 20% or more for external timber.

According to BS 5268-2 Structural use of timber. Code of practice for permissible stress design, materials and workmanship, if a moisture content lower than 18% is required at the time of erection, extra drying costs should be anticipated. Timber on site must be adequately protected, both before and after installation, to prevent the moisture content from rising. This is particularly important with material dried to below 20% moisture content since the full design load should not be applied if the moisture content rises above 20%.

The strength of timber is also affected by its moisture content - increasing as the moisture content reduces and vice versa. For example, the bending and the compression stresses for 'wet' timber, i.e. wood with a moisture content exceeding 20%, are, 80% and 60% respectively of those for 'dry' timber (wood with a moisture content less than 20%). Not only that, but the thickness and width of a piece of timber is predicted to increase by 0.25% for every 1 per cent increase in moisture content above 20%, and to shrink by the same amount for every 1% reduction in moisture content below 20%.

Therefore, it is essential that structural timber (not joinery timber) be strength graded at a moisture content appropriate for the exposure conditions of the timber in use.

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NBS Shortcuts is a new series of illustrated "how-to" articles and guides, covering a wide range of practice, regulatory and design guidance. The easy to follow text and detailed hand-drawn graphics will aid any building designer. They are available online as part of your subscription to NBS Building Regulations.