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The Design Process (Carpentry Drawings)

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.


Should I treat Oak with preservatives ?

Oak is a very durable timber used in boat and house building for hundreds of years. Oak has its own natural preservative, The tanins in Oak are unattractive to pest and diseases. The structure of Oak is so dense that moisture will only penetrate the first 2mm.

What will happen to the Oak if I let it ages naturally?

When the Oak ages, the sun and rain combine to remove tanin from the surface of the Oak. Tanin is brown and as this is lost and the sun bleaches the surface the Oak turns silvery grey.

What should I do if I want to keep the golden colour of the Oak.

There are many micro porous surface treatments in the market, Greenwood Oak would recommend Osmo 410 UV Protect for external Oak as this retains the Oaks natural colour.

Why are there cracks in the Oak?

There are two main types of cracking in Oak, sun checking, this is small cracks caused by the rapid drying of the surface when fresh sawn and exposed to the sun.

The larger cracks are caused by differential drying within the Oak causing stresses which pull the surface apart. Unlike in some species of timber such as softwood,

Oak cracks do not normally join together to cause a weakness. Cracks are a normal and feature in naturally dried Oak – Oak that has been dried quickly is likely to have larger cracks.

Will the Oak move and cause problems with glazing?

Greenwood Oak always use Oak that is sawn to create the most stable section i.e. boxed heart. The timber used for glazing is also air dried for two to three years.

Very little movement is expected in glazed frames, although minimal expansion and contraction will occur seasonally which is common in all timbers.