To Az Lumber
is a generic term that applies to various lengths of
wood used as construction materials. Pieces of lumber
are cut lengthwise from the trunks of trees and are
characterized by having generally rectangular or square
cross sections, as opposed to poles or pilings, which
have round cross sections. The use of wood as a construction
material predates written history. The earliest evidence
of wood construction comes from a site near Nice, France,
where a series of post holes seems to indicate that
a hut 20 ft (6m) wide by 50 ft (15 m) long was built
there 400,000 years ago using wood posts for support.
The oldest wood construction found intact is located
in northwest Germany, and was built about 7,300 years
ago. By 500 b.c. iron axes, saws, and chisels were commonly
used to cut and shape wood. The first reference to cutting
wood in a sawmill, rather than using hand tools, comes
from northern Europe and dates from about 375. The sawmill
was powered by the flow of water.
North America, European colonists found vast forests
of trees, and wood became the principal building material.
The circular saw, which had been developed in England,
was introduced in the United States in 1814 and was
widely used in sawmills. A large-scale bandsaw was developed
and patented by Jacob R. Hoffman in 1869 and replaced
the circular saw for many sawmill operations.
Lumber produced in early sawmills had varying dimensions
depending on the customer's specific order or the mill's
standard practice. Today, lumber pieces used in construction
have standard dimensions and are divided into three
categories, depending on the thickness of the piece.
Lumber with nominal thicknesses of less than 2 in (5
cm) are classified as boards. Those with nominal thicknesses
of 2 in (5 cm) but less than 5 in (13 cm) are classified
as dimension. Those with nominal thicknesses of 5 in
(12.5 cm) and greater are classified as timbers. The
nominal widths of these pieces vary from 2-16 in (5-40
cm) in 1 in (2.5 cm) increments. Most rough-cut lumber
pieces are dried and then finished, or surfaced, by
running them through a planer to smooth all four sides.
As a result, the actual dimensions are smaller than
the nominal dimensions. For example, a standard two-by-four
piece of dried, surfaced dimension lumber actually measures
1.5 in (3.8 cm) by 3.5 in (8.9 cm).
Pieces of lumber that are not only surfaced, but also
machined to produce a specific cross sectional shape
are classified as worked lumber or pattern lumber. Decorative
molding, tongue-and-groove flooring, and shiplap siding
are examples of pattern lumber.
Today, processing wood products is a billion-dollar,
worldwide industry. It not only produces construction
lumber, but also plywood, fiberboard, paper, cardboard,
turpentine, rosin, textiles, and a wide variety of industrial
The trees from which lumber is produced are classified
as hardwoods or softwoods. Although the woods of many
hardwoods are hard, and the woods of many softwoods
are soft, that is not the defining characteristic. Most
hardwood trees have leaves, which they shed in the winter.
Hardwood trees include oaks, maples, walnuts, cherries,
and birches, but they also include balsa, which has
one of the softest and lightest of all the woods. Softwood
trees, on the other hand, have needles instead of leaves.
They do not shed their needles in the winter, but remain
green throughout the year and are sometimes called evergreens.
Softwood trees include pines, firs, hemlocks, spruces,
Hardwoods are generally more expensive than softwoods
and are used for flooring, cabinetry, paneling, doors,
and trimwork. They are also extensively used to manufacture
furniture. Hardwoods are available in lengths from 4-16
ft (1.2-4.8 m). Softwoods are used for wall studs, joists,
planks, rafters, beams, stringers, posts, decking, sheathing,
subflooring, and concrete forms. They are available
in lengths from 4-24 ft (1.2-7.3 m).
Both hardwood and softwood lumber pieces are graded
according to the number and size of defects in the wood.
Defects include knots, holes, pitch pockets, splits,
and missing pieces on the edges or corners, called wanes.
These defects primarily affect the appearance, but may
also affect the strength of the piece. The higher grades
are called select grades. Hardwoods may also be graded
as firsts or seconds, which are even higher than select.
These grades have very few defects and are used for
trim, molding, and finish woodwork where appearance
is important. The higher the grade, the fewer the number
of defects. The lower grades are called common grades
and are used for general construction where the wood
will be covered or where defects will not be objectionable.
Common grades are designated in descending order of
quality by a number such as #1 common, #2 common, and
so on. Pieces of softwood common grade lumber may also
be designated by an equivalent name, such as select
merchantable, construction, and so on. Lumber intended
for uses other than construction, such as boxes or ladders,
are given other grading designations.
The Manufacturing Process
In the United States, most trees destined to be cut
into lumber are grown in managed forests either owned
by the lumber company or leased from the government.
After the trees have reached an appropriate size, they
are cut down and transported to a lumber mill where
they are cut into various sizes of lumber.
Here is a typical sequence of operations for processing
trees into lumber.
Selected trees in an area are visually inspected and
marked as being ready to be cut down, or felled. If
a road does not already exist in the area, one is
cut and graded using bulldozers. If operations are
expected to extend into the rainy season, the road
may be graveled, and culverts may be installed across
streams to prevent washouts.
Most tree felling is done with gasoline-powered chain
saws. Two cuts are made near the base, one on each
side, to control the direction the tree will fall.
Once the tree is down, the limbs are trimmed off with
chain saws, and the tree is cut into convenient lengths
•If the terrain is relatively level, diesel-powered
tractors, called skidders, are used to drag the fallen
tree sections to a cleared area for loading. If the
terrain is steep, a self-propelled yarder is used.
The yarder has a telescoping hydraulic tower that
can be raised to a height of 110 ft (33.5 m). Guy
wires support the tower, and cables are run from the
top of the tower down the steep slopes to retrieve
the felled trees. The tree sections, or logs, are
then loaded on trucks using wheeled log loaders.
trucks make their way down the graded road and onto
public highways on their way to the lumber mill. Once
at the mill, giant mobile unloaders grab the entire
truck load in one bite and stack it in long piles,
known as log decks. The decks are periodically sprayed
with water to prevent the wood from drying out and
Debarking and bucking
Logs are picked up from the log deck with rubber-tired
loaders and are placed on a chain conveyor that brings
them into the mill. In some cases, the outer bark
of the log is removed, either with sharp-toothed grinding
wheels or with a jet of high-pressure water, while
the log is slowly rotated about its long axis. The
removed bark is pulverized and may be used as a fuel
for the mill's furnaces or may be sold as a decorative
The logs are carried into the mill on the chain conveyor,
where they stop momentarily as a huge circular saw
cuts them into predetermined lengths. This process
is called bucking, and the saw is called a bucking
Headrig sawing large logs
If the log has a diameter larger than 2-3 ft (0.6-0.9
m), it is tipped off the conveyor and clamped onto
a moveable carriage that slides lengthwise on a set
of rails. The carriage can position the log transversely
relative to the rails and can also rotate the log
90 or 180 degrees about its length. Optical sensors
scan the log and determine its diameter at each end,
its length, and any visible defects. Based on this
information, a computer then calculates a suggested
cutting pattern to maximize the number of pieces of
lumber obtainable from the log.
The headrig sawyer sits in an enclosed booth next
to a large vertical bandsaw called the headrig saw.
He reviews the suggested cutting pattern displayed
on a television monitor, but relies more on his experience
to make the series of cuts. The log is fed lengthwise
through the vertical bandsaw. The first cut is made
along the side closest to the operator and removes
a piece of wood called a slab. The outer surface of
the slab has the curvature of the original tree trunk,
and this piece is usually discarded and ground to
chips for use in paper pulp.
The carriage is returned to its original position,
and the log is shifted sideways or rotated to make
subsequent cuts. The headrig sawyer must constantly
review the log for internal defects and modify the
cutting pattern accordingly as each successive cut
opens the log further. In general, thinner pieces
destined to be made into boards are cut from the outer
portion of the log where there are fewer knots. Thicker
pieces for dimension lumber are cut next, while the
center of the log yields stock for heavy timber pieces.
Bandsawing small logs
Smaller diameter logs are fed through a series of
bandsaws that cut them into nominal 1 in (2.5 cm),
2 in (5 cm), or 4 in (10 cm) thick pieces in one pass.
The large cut pieces from the headrig saw, called
cants, are laid flat and moved by chain conveyor to
multiple-blade bandsaws, where they are cut into the
required widths and the outside edges are trimmed
square. The pieces that were cut from smaller logs
may also pass through multiple-blade bandsaws to cut
them to width. If the pieces are small enough that
they do not need further cutting, they may pass through
a chipper, which grinds the uneven edges square.
Drying or seasoning
The cut and trimmed pieces of lumber are then moved
to an area to be dried, or "seasoned." This
is necessary to prevent decay and to permit the wood
to shrink as it dries out. Timbers, because of their
large dimensions, are difficult to thoroughly dry
and are generally sold wet, or "green."
Other lumber may be air dried or kiln dried, depending
on the required moisture content of the finished piece.
Air-dried lumber is stacked in a covered area with
spacers between each piece to allow air to circulate.
Air-dried woods generally contain about 20% moisture.
Kiln-dried lumber is stacked in an enclosed area,
while 110-180°F (44-82°C) heated air is circulated
through the stack. Kiln-dried woods generally contain
less than 15% moisture and are often specified for
interior floors, molding, and doors where minimal
shrinkage is required.
dried pieces of lumber are passed through planers,
where rotating cutting heads trim the pieces to
their final dimensions, smooth all four surfaces,
and round the edges.
Grade stamping and banding
piece of lumber is visually or mechanically inspected
and graded according to the amount of defects present.
The grade is stamped on each piece, along with information
about the moisture content, and a mill identification
number. The lumber is then bundled according to the
type of wood, grade, and moisture content, and the
bundle is secured with steel bands. The bundle is
loaded on a truck or train and shipped to a lumber
yard for resale to customers.
There are very few pieces of perfect lumber. Even though
great care is taken to avoid or minimize defects when
sawing the wood to the required sizes, there are almost
always some defects present. The number and location
of these defects determines the grade of the lumber,
and the purchaser must choose the grade that is appropriate
for each specific application.
As the number of older trees available for logging diminishes,
so does the lumber industry's ability to selectively
cut pieces of lumber to the sizes needed for construction.
Many of the trees being logged today are second-generation
or third-generation trees that are younger and smaller
in diameter than the original old-growth trees. These
younger trees also contain a higher percentage of juvenile
wood, which is less dimensionally stable than older
To counter this trend, the lumber industry is literally
taking trees apart and putting them back together again
to manufacture the sizes, strengths, and stability required
for construction. Actually, they have been doing this
for decades in the form of plywood and glue-laminated
beams, and some of the new products use similar technology.
One of the new manufactured lumber products is called
parallel strand lumber. It begins much like plywood
with a thin veneer of wood being peeled off a log. The
veneer passes under a fiber-optic scanner that spots
defects and cuts them out, sort of like an automated
cookie cutter. The veneer is then dried and cut into
0.5 in (1.3 cm) wide strips. The strips are fed into
one end of a machine, which coats them with a phenolic
resin glue and stacks them side-to-side and end-to-end
to form a solid 12 in by 17 in (30 cm by 43 cm) beam
of wood. The beam is zapped with 400,000 watts of microwave
energy, which hardens the glue almost instantly. As
the beam emerges from the other end of the machine,
it is cut into 60 ft (18.3 m) lengths. It is then further
cut into various sizes of lumber, and sanded smooth.
The resulting pieces are significantly stronger and
more dimensionally stable than natural wood, while being
attractive enough to be used for exposed beams and other