Moisture Control

The large size of timbers makes kiln drying impractical due to the drying stresses which would result from differential moisture contents between the interior and exterior of the timber. For this reason, timbers are usually dressed green (moisture content above 19 percent), and the moisture content of timber upon delivery will depend on the amount of air drying which has taken place.

Like dimension lumber, timber begins to shrink when its moisture content falls below about 28 percent. The degree of shrinkage depends on the climatic conditions of the environment. For example, timbers exposed to the outdoors usually shrink from 1.8 to 2.6 percent in width and thickness, depending on the species. Timbers used indoors, where the air is often drier, experience greater shrinkage, in the range of 2.4 to 3.0 percent in width and thickness. Length change in either case is negligible.

When constructing with Posts and Timbers or Beams and Stringers, allowance should be made for anticipated shrinkage based on the moisture content at the time of assembly. Where the building envelope relies on caulked seals between timbers and other building components, the selection of caulks should take into account the amount of movement which must be accommodated as shrinkage occurs.

Minor checks on the surface of a timber are common in most service conditions and therefore an allowance has been made for them in the assignment of working stresses. Checks in columns are not of structural importance unless the check develops into a through split that will divide the column.