Moisture Content

  • The moisture content of a piece of wood can affect its performance and properties including workability and strength, which will generally start to increase once the m/c falls below 25-30%.

  • Moisture content is associated with shrinkage of timber seasoned beams as they will start to shrink as it falls below the 30% fibre saturation point. Pre-drying can avoid problems with this or the m/c can be measured to estimate the shrinkage that will occur.  

  • The susceptibility of timber to degrade by fungi or insect attack also depends on the moisture in the timber, as these both require the moisture content to be above 20%.

  • As timber dries it becomes lighter and therefore easier to transport.

Recommendations as in BS 5268 Part 2

 End Use of Timber

 Av. m/c in Service (%) 

 Max. m/c at Time of Erection (%)

 External uses, fully exposed 

 20 or more

–

 Covered and generally unheated 

18

24

 Covered and generally heated 

15

20

 Internal uses, continuously heated building 

12

20

 

 

 

Measuring moisture content. Measuring moisture content.


Recording moisture content and shrinkage against time for ash. The shrinkage of exposed timber was compared to timber left under cover.