Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 41

Chapter 2
Wood as a Building Material
Wood is the most versatile, most useful building material, and a
general knowledge of the physical characteristics of various woods
used in building is important for carpenters and builders.
Growth and Structure of Wood
Wood, like all plant material, is made up of cells (or fibers ) that,
when magnified, have an appearance similar to (though less regular
than) that of the common honeycomb. The walls of the honeycomb
correspond to the walls of the fibers, and the cavities in the honey-
comb correspond to the hollow or open spaces of the fibers.
Softwoods and Hardwoods
All lumber is divided as a matter of convenience into two great
groups: softwoods and hardwoods . The softwoods in general are
the coniferous or cone-bearing trees (such as the various pines,
spruces, hemlocks, firs, and cedar). The hardwoods are the non-
cone-bearing trees (such as the maple, oak, and poplar). These
terms are used as a matter of custom, for not all so-called soft-
woods are soft, nor are all so-called hardwoods necessarily hard. In
fact, such softwoods as long-leaf southern pine and Douglas fir are
much harder than poplar, basswood, and so on, which are called
hardwoods.
Other (and perhaps more accurate) terms often used for these
two groups are the needle-bearing trees (softwoods) and the broad-
leaved trees (hardwoods). In general, the softwoods are more com-
monly used for structural purposes such as for joists, studs, girders,
and posts, whereas the hardwoods are more likely to be used for
interior finish, flooring, and furniture. The softwoods are also used
for interior finish and, in many cases, for floors, but are not often
used for furniture.
A tree consists of the following:
Outer bark —The bark is living and growing only at the cam-
bium layer. In most trees, the bark continually sloughs away.
Inner bark —In some trees (notably hickories and basswood)
there are long tough fibers (called bast fibers )inthe inner bark.
In other trees (such as the beech), these bast fibers are absent.
Cambium layer —This can be only one cell thick. Only these
cells are living and growing.
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