We have two classes of woods on the market which are used in different
parts of the structure, according to their special qualities. These are
commercially known as hard and soft woods, although this is not a very
scientific distinction, since some of the soft woods are harder than
some of the hard woods, and vice versa. Scientists have more accurate
names than these, but as the above are so well established, there is no
doubt as to what is meant.
In the market, lumber is not only classified according to the above,
but according to the species of tree it comes from, and also according
to certain standard grades of the same kind. These grades are
determined by the presence of certain defects. The recognized defects
are knots, shakes, checks, splits, streaks, pitch-pockets, stain, rot,
wane, warp, cupping, mineral streaks, pith on the face of the board,
and worm-holes.
Various large lumber associations issue rules governing standard sizes
and classifications for woods to be used in construction. The best and
the next best are the usual grades which are used for the interior
and exterior trim of houses. These grades have many designations, as
“clears” and “selects,” or “A” and “B,” or “No. 1” and “No. 2,” or
“firsts” and “seconds.”
The grades used for the rough framing, such as studs, joists, rafters,
subfloors, and sheathing, are not so good. They are designated as “No.
1 common” and “No. 2 common.” A poorer grade still, known as “No. 3
common,” is sometimes used for cheap temporary structures.
For the details of grading and standard sizes of lumber, one should
possess Circular 64 of the United States Department of Agriculture on
“How Lumber is Graded.”
Next to the grading of timber, the most important factor of quality is
the relative durability of the various woods, for upon this depends to
a large extent the choice of them for special places. The table on page
23 is taken from a government classification.
From this table it will be noticed that the soft woods as a class are
relatively more durable than the hard woods. This is true, because of
the fact that the structure of soft woods is simple, while that of the
hard woods is complex. When the former become wet and expand and then
dry out and shrink, the structure is not stressed internally as much as
is that of the hard woods, and they are therefore much more capable of
withstanding the action of the weather. Also certain of the soft woods
have natural properties of resisting dry or wet rot.
Certain species of woods are, therefore, selected for particular
parts of the house according to the needs of durability, strength,
appearance, and local supply.
Rough wooden framing requires a wood that is fairly abundant and
strong. The soft woods are generally used, and those which are
classified as _durable_ in the table are the most used.