APA Plywood -- Plywood Specifications
All materials used in this tutorial can be found at boatbuildercentral.com
Boat Plans are available at bateau.com
Plywood for Boat Building
Below is a copy of Form Q220, an APA pamphlet about the use of plywood in the boat building industry. Some remarks are necessary:
- Resin in the document below means polyester. We use epoxy resin, which preforms much better.
- These are US standards but the information about plywood suitability for boat building is important for all builders. I will post some UK and French norms later.
- Redrying means drying the plywood before use, very important.
- Precoat all plywood parts with resin before any fiberglassing.
- When using epoxy, there is no need to buy pressure treated plywood!
Points of particular interest to the amateur boat builder are: Types of plywood, classifications. Recommended plywoods for boat building Fiberglassing plywood
APA
The Engineered Wood Association
PRESERVATIVE-TREATED PLYWOOD FOR BOAT MANUFACTURING APPLICATIONS.
The plywood industry consists of over a hundred mills
in North America utilizing dozens of wood species for manufacturing.
Since the major application for plywood has been for building construction,
the standards and plywood specifications have been primarily geared
to providing grades and layups that are optimized for construction
applications.
However, structural plywood also has a proven track record in boat
manufacturing. With the current grades, specifications and treatments
available, it is the best structural material to meet many of the
boat manufacturers needs. From the widespread use in PT boats during
World War II to today’s modern composite hulls, plywood has been
a preferred structural element due to its high strength-to-weight
ratio, machinability and excellent fastener holding capabilities.
Given the unique requirements of the boating industry, APA has developed
specific recommendations that best address the needs of the boat
manufacturer combined vith readily available preservative treatments,
plywood can provide long-term structural performance as boat components.
This guide provides specification details for best performance and
provides reference to a vast information base on plywood.
PLYWOOD STANDARDS AND SPECIFICATIONS
Voluntary Product Standard PS-1 for Plywood
Plywood grade and workmanship quality are defined in Voluntary Product
Standard PS-1. The standard defines the following panel attributes
that are important for the marine industry.
- Wood Species. Over 60 wood species may be used in the structural plywood industry. Coniferous species are the dominant and preferred species for boat manufacturing applications. The most popular of such species is Douglas-fir. Other species include western larch and western firs.
- Veneer Grades. Veneer grading is based on the size and frequency of natural growth characteristics such as knots, knot holes and sputs. The common veneer grades for plywood are A, B, C, C-plugged and D. The plywood panel itself is defined by the grade of the face and back veneers (e.g., A-B or C-D)
- Bond Requirements. virtually all structural plywood made today uses waterproof phenolic resins which maintain their bond during moisture exposure.
APA Industrial Specifiers Guide
Today, manufacturers of industrial grades of plywood have very
tight controls over how plywood is made. Current technology allows
for production of specialty plywood with fewer core voids and gaps,
which results in "tighter" panel construction. Such technology improves
upon the prescribed combinations in PS-1 by considering exactly
what attributes are needed by the industrial customer.
In order to fine tune the plywood grading system to more precisely
meet the needs of certain rnanufacturing industries, APA developed
an Industrial Panel Selection Guide. The grading system considers
the following panel attributes in a four-number ranking known as
the Industrial Category index, or ICI number. The four-digit ICI
number consists of the following:
- Face Veneer Quality Ranking. A numerical scale indicating the solidness and smoothness quality of the face veneer.
- Back Veneer Quality Ranking. Similar to the face veneer, the ranking addresses the back veneer quality required for the specific application.
- Inner ply veneer under the face. These veneers are often important for applications with heightened fastener holding demands, and where panels are going to be cut into smaller parts.
- Other inner plies. Similar to the veneer under the face, these are assessed for solidness.
A copy of APAs Industrial Panel Selection Guide can be ordered by calling or writing APA at the address listed on the last page.
APA RECOMMENDS THE FOLLOWING MINIMUM GRADE SPECIFICATIONS
FOR MOST BOAT CONSTRUCTION APPLICATIONS:
APA C-C Plugged, PS-1, Group 1, EXTERIOR
or
T the panel should meet an ICI Number of 7-3-3-3. Common Thicknesses
Plywood 1/4” to 1-1/8” thick is available, with the most common
thicknesses being 15/32”, 19/32" and 23/32".
*The best grade will vary depending on the application. Some boat
manufacturers use treated panels with an ICI number of 4-3-3-3 for
applications such as seats.
PRESERVATIVE TREATMENTS FOR PLYWOOD
Plywood has a long history of good service in the boating industry.
However as with all wood products, given extreme moisture conditions
for a long period of time, plywood may be susceptible to some degree
of fungal decay In many boat applications, the risk of elevated
moisture conditions is mitigated by coatings, laminates, encasernent
in fiberglass or other protective means that reduce the moisture
pickup or provide sufficient drying rate in order to reduce the
panel moisture content. For the ultimate assurance against the risk
of decay, commercial preservative treatments are available. Since
preservative treatments render the wood an unsuitable substrate
for decay fungi, treated plywood can be considered at the top level
of performance with respect to longevity.
Treatments and Standards
Treated wood products are readily available and are often used in
construction where high decay hazards exist. Unfortunately, much
of the treated plywood found in retail lumber yards may not have
been redried to the degree required for boat construction. The following
recommendations are geared specifically for treated plywood for
boat construction. For best performance, care must be taken to specify
and purchase treated plywood in accordance with these recommendations.
First, make sure the plywood cornes from a mill that is a member
of APA - The Engineered Wood Association. That is your assurance
that the mill is subject to APA’s rigorous quality assurance program.
Treating is conducted as a secondary process following the commercial
treating standards written by the American Wood Preservers Association
(AWPA). The most common treatment and retention level for plywood
used in boat construction is CCA (Chromated Copper Arsenate) at
0.40 pcf retention. Other treatments for boat use are ACA, ACZA
and ACQ.
AWPA Standard C9, "Plywood - Preservative Treatment by Pressure
Process," specifies that the preservative-treated panel be redried
to a moisture content of 18% or less, unless waived by the specifier.
For use in boat manufacturing, the redrying of the treated plywood
is essential to good performance when laminating with fiberglass.
Treated plywood purchased from lumber yards is often used in construction
applications and is not necessarily re-dried after treatment. It
is essential for boat manufacturers to specify redrying.
Treated plywood is trademarked by a grading agency that monitors
treating quality. The trademark should specify the AWPA standard,
treatment and treating retention. Many suppliers of treated plywood
for the boat industry offer limited lifetime warranties. Check with
your panel supplier for warranty information.
For BEST performance of plywood in boat manufacturing, APA recommends
the use of treated plywood according to the following specification.
Treated in accordance to AWPA Standard C9 with < CCA, ACQ, ACZA
or ACA> to 0.40 pcf retention. Kiln dry after treating (~AJ) to
18% or less.
LAMINATING FIBERGLASS TO PLYWOOD
Many uses of plywood in boats involve laminating fiberglass over
a plywood boat component. APA recently contracted with a marine
testing laboratory to study the strength of fiberglass bond using
commercial resins applied to treated and untreated plywood. The
objective of the study was to assess the effect of preservative
treatments and panel moisture content on the strength of the laminate
bond.
The study assessed the laminating strength on treated and untreated
plywood. To study the effect of moisture content, half of the panels
were humidified to simulate the upper range of moisture content
of what may be expected from treated panels after redrying or from
panels stored at a boat manufacturer's facility.
The treated plywood developed bond strengths similar to the untreated
plywood.
As expected, the moisture content of the plywood influenced the
strength of the fiberglass bond. However, even at the highest moisture
condition, the ultimate test failure mode in the vast majority of
the cases was wood failure within the plywood itself, rather than
at the laminate bond interface. The influence of plywood moisture
content reinforces the need to specify drying after treating when
using treated plywood.
HANDLING AND STORING PLYWOOD
Like all materials used in manufacturing, plywood should be properly
stored and handled to assure proper performance. Protect the edges
and ends of panels during handling. Place panels to be moved by
forklift on pallets or bunks when received to avoid damage by fork
tines. Panels to be transported on open truck beds should be covered
with tarpaulins. For open rail transport, use "lumber wrap" to avoid
weather exposure. For best performance, store panels indoors away
from open doors to minimize moisture differentials along edges and
ends. Make sure the panels are not exposed to water, solvents or
other foreign matter that may interfere with establishing the fiberglass
bond. Covering and weighing down the top of the bundles assists
in keeping the panels flat. Stack panels on 4x4 stringers or other
blocking. To help assure continued panel flatness, use at least
three full-width stringers or bunks to avoid bending of the unit.
Covering and weighing down the top of the bundles assists in keeping
the panels flat.
RELATED PUBLICATIONS
APA maintains a vast library of literature regarding plywood. A
selection of titles follows:
- APA Technical Note: Fastener Loads for Plywood - Screws, Form E830
- APA Plywood Design Specification, Form Y510
- Consumer Information Sheet - Inorganic Arsenical Pressure-Treated Wood
- APA Product Guide Preservative-Treated Plywood, Form Q220
- EWS Technical Note; Controlling Decay in Wood Construction, Form R495
- Industrial Panel Selection Guide, Form T200
ABOUT APA
APA - The Engineered Wood Association is a nonprofit trade association
whose members produce 70 percent of the structural wood panels made
in North America.
APA has three main functions: 1) quality inspection and testing;
2) product and systems research; 3) promotion.
APA's Field Services Division is a national network of representatives
with regional offices in major market centers across North America.
APA Field Representatives help users, specifiers and distributors
market, design, and apply APA and APA EWS trademarked products for
countless end uses.
Most importantly, APA Field Representatives are available to help
you. If you have questions about structural wood panels or engineered
wood products please call the APA representative in your area.
APA
The Engineered Wood Association
PO. Box 11700, Tacoma, WA 98411-0700
(253) 565-6600 Fax: (253) 565-7265
Internet Address: http://www.apawood.org
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