Plywood Problems
By James A. "Buck" Durham, P.E. Structural Engineer - Durham and Wehrman Engineering,
Phone 256-353-0504.
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. Structural Engineer - Durham and Wehrman Engineering,
Phone 256-353-0504.
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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