When enjoying a piece of out luxury oak outdoor furniture, we may look at the detail of the joints or appreciate the fine smooth finish, without giving much thought to how the wood was sawn from the original log and how that affects the quality of the finished piece.
The quartersawn boards are like taking the prime cut of fillet steak, ie the best part of the timber. When the timber is quartersawn, the board will expand and contract in a more stable way than if it had been Plain sawn. This is vitally important when being made into furniture for outdoor use which will need to withstand the swings in moisture that life outdoors brings.
All timbers expand and contract with moisture differently in the three directions of the original tree. The length (originally the height of the tree) never changes, whereas there is a ratio of expansion and contraction between the radial direction (from the centre of the tree to the outside) and the tangential direction (lines running at a tangent to the annual rings). This is normally between 1:1.5 and 1:2.2 for different types of wood. For oak this ratio is generally 1:1.8. This means that when oak dries, it shrinks about 80% more in the tangential direction than in the radial direction and equally expands around 80% more when it is exposed to humidity.
This expansion and contraction is commonly called movement in the wood and is why it is important that wood is well seasoned (dried) before it is used. Whereas for interior use the ratio is no longer significant as it is in a stable moisture environment, for outdoors this ratio is critical as it determines the way the wood behaves between summer and winter each year. It equally applies to wood used in old Tudor buildings which did not have the benefits of central heating or air conditioning, which is why you always see the beautiful medullary rays on old oak panelling in many great halls.