Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 49

A
Figure 2-7 Greatest shrinkage is in the direction of the annual
rings (A). Characteristic shrinkage and distortion of flat boards,
squares, and rounds as affected by the direction of the annual
rings. Tangential shrinkage (A) is about twice as great as radial
(B). There is little or no longitudinal (C) shrinkage. (Courtesy Forest
Product Lab)
This is an important consideration to be remembered when fram-
ing a building. For example, a stud in a wall will not shrink appre-
ciably in length, whereas it will shrink somewhat in both the 2-inch
and the 4-inch way. In like manner, a joist, if it is green when put in
place, will change in depth as it seasons in the building. These prin-
ciples of shrinkage also explain why an edge-grain or quarter-sawed
floor is less likely to open up than a flat-grain floor.
Density
The tree undergoes a considerable impetus early every spring and
grows very rapidly for a short time. Large amounts of water are
carried through the cells to the rapidly growing branches and leaves
at the top of the tree. This water passes upward mainly in the outer
layers of the tree. The result is that the cells next to the bark (which
are formed during the period of rapid growth) have thin walls and
large passages. Later on, during the summer, the rate of growth slows
and the demand for water is less. The cells that are formed during
the summer have much thicker walls and much smaller pores. Thus,