The properties of green, air dried or kiln dried oak
The posts and beams we use are milled from the log so that the centre of the tree is in the centre of the beam this is called boxed heart.
There are three areas affected by shrinkage, most shrinkage will occur around the circumference (tangentially). Secondly less shrinkage will occur along the radius and the least amount will be Longitudinal.
If a large log is sawn to produce a number of beams, they will be a combination of halved and quartered beams and will become cupped or diamond in section when affected by shrinkage. A boxed heart sawn beam will remain quite square in section with most deformation occurring, as dishing on all four faces. Twisting of timbers whilst drying is not common unless the tree was twisted during its’ growth.
Fresh sawn oak
Fresh sawn Oak can contain up to 75% moisture. Shrinkage starts at 30% moisture and will shrink approximately 7.5% in total. Oak dries naturally at approximately 25mm per year and a board sawn at 50mm will air dry to about 14%. Drying time will be accelerated if fresh sawn or air dried timber is brought into a heated environment and as a consequence the timber may exhibit more superficial shakes or cracks.
Air dried oak
Air dried beams have normally spent a minimum of two years drying outdoors. Air drying will make a beam relatively dry on the surface (about 20%) but leave the centre at 30% or more.
Air drying is the only practical way to season large section timber because kiln drying Oak is not practically possible over 100mm in thickness. Kiln drying will reduce moisture to about 10%.
The percentage of continued shrinkage of an air dried beam will be less than a freshly sawn beam but further movement must be anticipated.
Avoiding problems with shrinkage
To avoid gaps appearing in plaster work around timbers such as the shrinkage around rafters, the roof can be clad on top of the rafters with tongue and grove boards called sarking boards, If painted they should be painted before they are laid.