Wood as a Building Material - Carpenter's and Builder's Math, Plans, and Specifications - page 65

Biological attack of a wood surface by microorganisms is rec-
ognized as a contributing factor to color changes. When weathered
wood has an unsightly dark gray and blotchy appearance, it is caused
by dark-colored fungal spores and mycelium on the wood surface.
The formation of a clean, light gray, silvery sheen on weathered
wood occurs most frequently where microorganism growth is in-
hibited by a hot, arid climate or where there is a salt atmosphere,
such as in coastal regions.
The contact of fasteners and other metallic products with the
weathering wood surface is a source of color, often undesirable if a
natural color is desired.
Insect Damage
Insect damage may occur in standing trees, logs, and unseasoned or
seasoned lumber. Damage in the standing tree is difficult to control,
but otherwise insect damage can be largely eliminated by proper
control methods.
Insect holes are generally classified as pinholes , grub holes , and
powder postholes . The powder post larvae, by their irregular bur-
rows, may destroy most of the interior of a piece, although the
surface shows only small holes, and the strength of the piece may
be reduced virtually to zero.
No method is known for estimating the amount of reduction in
strength from the appearance of insect-damaged wood, and, when
strength is an important consideration, the safe procedure is to elim-
inate pieces containing insect holes.
Summary
Wood is the most versatile building material. Softwoods and hard-
woods grow as trees with a fibrous cellular structure. Logs are con-
verted into lumber and seasoned by drying. The physical charac-
teristics of wood affect its performance and specific uses. Defects
in lumber relate to defects in the logs it was cut from and how
the lumber was handled during drying and storage. After wood is
installed in a building, it can deteriorate by weathering, decay, or
insects unless protected.
Review Questions
1. Describe the physical structure of wood with a drawing. Label
the parts.
2. What are the characteristics of softwoods and hardwoods?
How is each type of wood used in building?