Plywood Problems
By James A. "Buck" Durham, P.E.
Plywood is made by peeling thin strips of wood (plies) from logs and gluing them together. The
log is placed on a rotary saw and the wood is peeled from it, coming off in much the same way
wrapping paper comes off a roll. The thin sheets are then glued together so that the grain in
each sheet is at a 90-degree angle to the grain in the sheet above and below it. Because of this
cross-grain construction, plywood is much less likely to warp than solid wood. Plywood was first
made in France in the 1860's. It did not live up to its promise of being an all purpose
construction material because it could not be exposed to the elements.
The fledgling automobile industry first used plywood for the manufacture of cars. Early
floorboards, running boards and side panels were made of plywood. No matter how many coats
of protective finish were applied, however, the plywood deteriorated quickly when exposed to the
outdoors. The glue used to join the different layers of wood was not waterproof or even
water-resistant; when plywood was exposed to dampness, the glueline melted and the sheets of
wood bulged and flaked away.
Late in 1934, an Irish chemist, Dr. James V. Nevin, announced that he had developed a
completely reliable waterproof glue. Extensive tests were conducted and by January 1935, the
Harbor Plywood Corporation produced what it first called Super plywood. All kinds of exterior
uses for plywood were now feasible.
The manufacture of plywood represents effective wood conservation, because little waste is
tolerated in extracting the finished product from the tree. Logs used for plywood are peeled
against a clean cutting edge into continuous sheets of wood, or veneers. When the log has
been unpeeled as far as the first lathe will take it, it is cut in half and the two logs are placed on
smaller lathes and peeled further to produce the inner plies of plywood panels. Ultimately, two
core spindles, just big enough to make two sections of 2 X 4, are left.
As the long sheets of veneer come from the lathe, a clipper, operated by a skilled technician,
cuts them to desired widths; the sheets are then sorted and dried. Next, the sheets of veneer
are fed through a glue spreader, where they are coated uniformly with adhesive, and assembled
to form three-, five- and seven-ply panels. A hydraulic press bonds the adhesive under
temperatures of 260 to 285 degrees Fahrenheit and a pressure of about 200 pounds per
square inch. The rough panels are then trimmed to standard sizes, sanded, and stamped to
certify their type and grade.
Douglas Fir is the most common softwood plywood. Widths range from 24" to 60" with 48" being
most common. Plywood can be manufactured from more than 70 species of wood, divided into
five groups on the basis of bending strength and stiffness, with the stiffest species in group 1.
This group includes Douglas Fir, Western Larch and American Birch. Groups 5, the least stiff,
include Basswood and Poplar.
Usually a panel is made from an odd number of plies, each approximately of equal thickness.
Panels are available as thin as 1/8 inch, but run in regular stock thickness ranging from ¼ inch
(3-ply) to 1 and 1/8 of an inch (seven ply). Two types of plywood, exterior and interior, are
available. Interior plywood is marked AD; and exterior is marked AC (whether it's interior or
exterior is usually printed on the board surface as well).
Exterior type has a 100 percent waterproof glueline, and interior type has a highly moisture
resistant glueline. Most interior type plywood is manufactured with exterior glue. Interior type
panels with exterior glue are suitable for applications where ability to resist moisture during long
construction delays is required. Because the lower grade of veneer permitted for backs and
inner plies of interior type panels may affect glueline performance, only exterior type plywood
should be used for permanent exposure to the weather or moisture. Manufacture guidelines are
issued by the American Plywood Association, and APA trademarks appear only on products
made by APA-member mills These mills produce about 80 percent of the structural wood panel
products in the United States. This trademark indicates the plywood manufacturer is subject to
an APA audit for quality.
The veneer, or exposed front and back surfaces of the plywood slab, is usually used to
determine its grade. Veneer grades define veneer appearance in terms of natural unrepaired
growth characteristics and allowable number and size of repairs that may be made during
manufacture. The highest quality veneer grades are N and A. N is a smooth surface "natural
finish" veneer, made of all heartwood or all sapwood, free of open defects, with no more than six
repairs allowed per 4x8 panel, with each made parallel to grain and well-matched for grain and
color.
The lowest veneer grade is D. Knots and knotholes up to 2-1/2 width across grain are allowed.
Panels with B-grade or better veneer faces are always sanded smooth in manufacture to fulfill
requirements of their intended end-use (applications such as cabinets, shelving, furniture and
built-ins). Plywood panels are sometimes marked based on their intended end-use, such as APA
RATED SHEATHING, or APA RATED STURD-I-FLOOR.
A conscientious effort has been made by the authors to provide accurate information; however,
neither the authors nor Alabama Residential Inspection Services, LLC will assume any liability
for its use. Readers are advised to perform additional research, seek other professional advice,
and to act on the information provided, herein, very carefully.
By James A. "Buck" Durham, P.E.
Plywood is made by peeling thin strips of wood (plies) from logs and gluing them together. The
log is placed on a rotary saw and the wood is peeled from it, coming off in much the same way
wrapping paper comes off a roll. The thin sheets are then glued together so that the grain in
each sheet is at a 90-degree angle to the grain in the sheet above and below it. Because of this
cross-grain construction, plywood is much less likely to warp than solid wood. Plywood was first
made in France in the 1860's. It did not live up to its promise of being an all purpose
construction material because it could not be exposed to the elements.
The fledgling automobile industry first used plywood for the manufacture of cars. Early
floorboards, running boards and side panels were made of plywood. No matter how many coats
of protective finish were applied, however, the plywood deteriorated quickly when exposed to the
outdoors. The glue used to join the different layers of wood was not waterproof or even
water-resistant; when plywood was exposed to dampness, the glueline melted and the sheets of
wood bulged and flaked away.
Late in 1934, an Irish chemist, Dr. James V. Nevin, announced that he had developed a
completely reliable waterproof glue. Extensive tests were conducted and by January 1935, the
Harbor Plywood Corporation produced what it first called Super plywood. All kinds of exterior
uses for plywood were now feasible.
The manufacture of plywood represents effective wood conservation, because little waste is
tolerated in extracting the finished product from the tree. Logs used for plywood are peeled
against a clean cutting edge into continuous sheets of wood, or veneers. When the log has
been unpeeled as far as the first lathe will take it, it is cut in half and the two logs are placed on
smaller lathes and peeled further to produce the inner plies of plywood panels. Ultimately, two
core spindles, just big enough to make two sections of 2 X 4, are left.
As the long sheets of veneer come from the lathe, a clipper, operated by a skilled technician,
cuts them to desired widths; the sheets are then sorted and dried. Next, the sheets of veneer
are fed through a glue spreader, where they are coated uniformly with adhesive, and assembled
to form three-, five- and seven-ply panels. A hydraulic press bonds the adhesive under
temperatures of 260 to 285 degrees Fahrenheit and a pressure of about 200 pounds per
square inch. The rough panels are then trimmed to standard sizes, sanded, and stamped to
certify their type and grade.
Douglas Fir is the most common softwood plywood. Widths range from 24" to 60" with 48" being
most common. Plywood can be manufactured from more than 70 species of wood, divided into
five groups on the basis of bending strength and stiffness, with the stiffest species in group 1.
This group includes Douglas Fir, Western Larch and American Birch. Groups 5, the least stiff,
include Basswood and Poplar.
Usually a panel is made from an odd number of plies, each approximately of equal thickness.
Panels are available as thin as 1/8 inch, but run in regular stock thickness ranging from ¼ inch
(3-ply) to 1 and 1/8 of an inch (seven ply). Two types of plywood, exterior and interior, are
available. Interior plywood is marked AD; and exterior is marked AC (whether it's interior or
exterior is usually printed on the board surface as well).
Exterior type has a 100 percent waterproof glueline, and interior type has a highly moisture
resistant glueline. Most interior type plywood is manufactured with exterior glue. Interior type
panels with exterior glue are suitable for applications where ability to resist moisture during long
construction delays is required. Because the lower grade of veneer permitted for backs and
inner plies of interior type panels may affect glueline performance, only exterior type plywood
should be used for permanent exposure to the weather or moisture. Manufacture guidelines are
issued by the American Plywood Association, and APA trademarks appear only on products
made by APA-member mills These mills produce about 80 percent of the structural wood panel
products in the United States. This trademark indicates the plywood manufacturer is subject to
an APA audit for quality.
The veneer, or exposed front and back surfaces of the plywood slab, is usually used to
determine its grade. Veneer grades define veneer appearance in terms of natural unrepaired
growth characteristics and allowable number and size of repairs that may be made during
manufacture. The highest quality veneer grades are N and A. N is a smooth surface "natural
finish" veneer, made of all heartwood or all sapwood, free of open defects, with no more than six
repairs allowed per 4x8 panel, with each made parallel to grain and well-matched for grain and
color.
The lowest veneer grade is D. Knots and knotholes up to 2-1/2 width across grain are allowed.
Panels with B-grade or better veneer faces are always sanded smooth in manufacture to fulfill
requirements of their intended end-use (applications such as cabinets, shelving, furniture and
built-ins). Plywood panels are sometimes marked based on their intended end-use, such as APA
RATED SHEATHING, or APA RATED STURD-I-FLOOR.
A conscientious effort has been made by the authors to provide accurate information; however,
neither the authors nor Alabama Residential Inspection Services, LLC will assume any liability
for its use. Readers are advised to perform additional research, seek other professional advice,
and to act on the information provided, herein, very carefully.
EFIS / SYNTHETIC STUCCO
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