Building Maintenance Problems and Their Solution—IV

Abstract

Good-quality joinery should be free from cracks, large or loose knots and rough or raised grain. The softer, lighter spring grain has a tendency to greater shrinkage with the possibility of the darker, harder autumn grain being raised on the surface. Careful selection and proper conversion of the timber will assist in minimising this defect and a good standard of rubbing down with glasspaper by the painter will also help. External joinery which contains too much sapwood may deteriorate and shrink and cause fracture of the paint film. In extreme cases, rotting may occur beneath the paint film. It is now well established that external window joinery made from present-day supplies of redwood (Pinus sylvestris) is liable to decay within a few years, because of the presence of a large proportion of sapwood in the timber. Sapwood offers little resistance to wood-destroying fungi, and soon decays if it remains wet. This has resulted in the widespread adoption of preservative treatment of redwood joinery timber since the mid 1970s. On the other hand, Western red cedar and Douglas fir with no sapwood have sufficient natural resistance to decay without preservative treatment.¹