Why are timbers seasoned?

Answer 1

Timber is seasoned to bring it to a condition ready for its final use. It needs to be at the same moisture content as it will be in its final use so it will not move or bend once in place, and to have sugars in the sap reduced to deter fungal attack.

When first cut from a tree, timber is very moist, containing sap sugars and water. It also has areas of tension and compression as a result of stresses during growth, and the difference between the heartwood (centre of the trunk) and sapwood (outer part of the trunk).

Sap sugars can be driven out by using moisture such as steam before the final drying, but this can reduce the strength and elasticity of the timber.

As the timber dries it will shrink and this shrinking can occur differentially across the piece causing further stresses. The amount of shrinkage depends on the species of tree, the amount of drying and whether the timber is heartwood or sapwood.

Movement through the piece of timber to release these stresses causes a change in the shape of the piece, known as warping. There are four main types of warping:

* cupping, where the flat faces of a board become concave on one side and convex on the opposite side across the short dimension of the face forming a cup shape that if face up on floorboards would hold water on the board, * bowing, where the wide faces of a board become concave on one side and convex on the opposite side along the length of the board, forming a shape like an Archery bow

* crooking or springing, which is similar to bowing, but the wide faces stay flat and the thin edges are bowed,so if laid on its wide face it looks a little like a C shape

* twisting, where the board has no flat face or edge except the ends and looks like one end has been rotated while the other held still. Ways to reduce the warping during drying include:

* restraining the timber in the desired shape * drying the timber evenly and slowly or in a very controlled way * cutting in directions that will reduce the stresses * doing some cuts after final drying

Timber being seasoned is usually stacked with small pieces of timber (fillets) separating each piece to allow even air flow around them.

It can be done slowly and naturally outside, or in a large airy shed, or quickly in a controlled kiln. Some timber is treated with preservatives prior to seasoning.

Timber with a shape critical use, such as doors and flooring, may be brought into the building in which it will be used, and left to sit for some weeks to come to equilibrium with the moisture levels in that building, to allow any final movement before fixing in place.