Technical Info About Plywood types

Kayak Jack

Well-Known Member
Aug 26, 2003
Okemos / East Lansing Michigan
Here is info from the Engineered Wood Association:

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. Epoxy would work better but it was not tested.
* 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.
* If you use epoxy ( you should), 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

The Engineered Wood Association


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 with 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.

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 manufacturing 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 C-C Plugged, PS-1, Group 1, EXTERIOR
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.


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 comes 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.


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.


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.


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


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.

The Engineered Wood Association
PO. Box 11700, Tacoma, WA 98411-0700
(253) 565-6600 Fax: (253) 565-7265
Internet Address: See our resin-fiberglass basic tutorial for more information about the use of materials.

BS1088 (British Standard 1088)
is defined by the following 4 basic requirements

1. Same species of wood throughout
2. WBP Phenolic or Advanced melamine resin glue line
3. No core gaps, some small pinhole gaps are accepted
4. Whole piece face and back, no spliced faces, allowed Grade is normally an A/B

BS6566 (British Standard 6566)

1. Wood can be mixed in species, normally the core will be different from the face.
2. WBP Phenolic or Type 1 Ext Melamine glue permitted
3. Some allowance of core gaps permitted, recommend filling any exposed edges before further encapsulation etc.
4. Faces can be made up of one or more pieces, spliced together. Grade of face is normally a B/C

The BS6566 is recommended where the wood is being encapsulated, or at least fully sealed.
It is not as structurally sound as BS1088, and does not pretend to be.
This also includes bending strength and modulus of elasticity.
Sometimes, the factory producing the 6566 will manufacture the plywood which will exceed the above conditions, (for example, they may put whole piece faces on just to complete production more quickly for an order that needs to be exported) Customers must be aware that at any time, grade must meet the above to be approved under BS6566 .

I also found this at another site:
Noah's Marine sent me this information.

"The only difference between the 6566 and 1088 is the face. The 1088 has no
voids in face, the 6566 has some. People use the 1088 if they are going to
do a clear finish, if you are going to paint the plywood i would use the
6566,it's cheaper. That's the only difference between the two."

The following is the criteria that must be met for a panel to be "legally" marked and marketed as BS1088. I havent heard of any counterfeit BS1088 being marketed. All the BS1088 panels I have ever bought (I dont build with luan) had a Lloyd's of London stamp along with the BS1088 marking.
As far as I know there is no standard that Luan has to met. Any core material is OK, any glue is ok, variation in thickness is ok, any face veneer is ok. That is why I say that Luan bought in Florida may be (act) different than luan bought in Texas, Australia, Canada, England, Scotland, Maine, Iowa, North Carolina or even where I live in Florida. While any BS1088 bought from any of those places would act the same as a piece bought from anywhere else.


From Wikipedia:

In materials, the BS 1088 specification is a marine plywood specification that applies to plywood produced with untreated tropical hardwood veneers that have a set level of resistance to fungal attack. The plies are bonded with WBP glue. Although the initials BS are for "British Standard", the finished product does not have to be "British made". The standard is associated with Lloyd's of London since it performs testing of products to this standard.

WBP Glue Line -- BS 1088 plywood must use an adhesive proven to be very resistant to weather, micro-organisms, cold and boiling water, and steam and dry heat. The product's bonding must pass a series of British Standard tests.

Face Veneers -- These must present a solid surface that is free from open defects. Face veneers must be free of knots other than "sound pin" knots, and there can be no more than an average of two such knots per square foot over the entire surface of the plywood sheet. The veneers must be reasonably free from irregular grain. The use of edge joints is limited, and end joints are not allowed.

Core Veneers -- Core veneers have the same basic requirements as face veneers, except that small splits are allowed, and there is no limit on the number of pin knots or edge joints. However, end joints are not permitted.

Limits of Manufacturing Defects -- Defective bonds, pleats and overlaps, and gaps in faces are not permitted. Occasional gaps may be repaired using veneer inserts bonded with the proper adhesive.

Moisture Content -- BS 1088 plywood must have a moisture content between 6% and 14% when it leaves the factory.

Finishing -- Boards will be sanded on both sides equally.

Length & Width -- The length or width of a board produced as a standard size shall not be less than the specified size nor more than 6.3 mm (0.25") greater than the specified size.

Squareness -- The lengths of the diagonals of a board shall not differ by more than 0.25% of the length of the diagonal.

Thickness Tolerances -- Tolerances vary as follows.

* 4 mm +.02/-0.6 6 mm +.04/-0.65 9 mm +.06/-0.75 12 mm +.09/-0.82
* 15 mm +.1/-0.9 18 mm +.12/-0.98 22 mm +.16/-1.08 25 mm +1.8/-1.16

From the above we can assume that 6 mm material will arrive at thickness' between 6.04 mm and 5.35 mm.

Face Veneer thickness -- For any three-ply construction, which applies to 3 and 4 mm material, each face veneer shall be not thinner than 1/8 of the total thickness of veneers assembled dry. Since the dry thicknesses of the boards are 3.6 and 4.6 respectively, we can assume that for these thicknesses only the face veneers will be as follows:

* 3.6 mm dry x 12.5% (1/8 ) = 0.45 mm 4.6 mm dry x 12.5% (1/8 ) = 0.575 mm

Multi-Ply Construction-- This applies to boards thicker than 4.8 mm (3/16")

* Each face veneer shall be a minimum of 1.3 mm and not thicker than 3.8 mm.
* Each core veneer shall be no thicker than 4.8 mm


Well-Known Member
Feb 22, 2011
Not sure what they mean by "precoat all plywood parts before fiberglassing."

Do they mean precoat, let the epoxy set hard, prepare the surface, and then glass? That's going to risk unnecessary epoxy thickness between wood and glass.

Do they mean paint the epoxy on first, and while it is wet, drop the glass on it and fill as needed with more epoxy? That's OK as long as one makes sure the FG gets down tight against the wood.

I've glassed various spruce or luan marine plywood paddles, and after thoroughly cleaning and smoothing the wood surface, I lay the FG over the wood dry, and then wet it out with epoxy. Have had absolutely no problems with that procedure, and these are whitewater paddles that take high stress.

Maybe they're concerned that some plywoods will have voids or will be so spongy that they will drink epoxy and cause voids in the glass. The plywood I've used is high grade, very uniform, and doesn't drink much resin at all.

Kayak Jack

Well-Known Member
Aug 26, 2003
Okemos / East Lansing Michigan
Some like to pre-saturate the plywood with epoxy, let it dry (less than 72 hours to get a chemical bond next, rather than a physical bond), sand off any "hairs", and then lay the glass on a dry, smooth surface and wet it out. Others just sand the wood, lay the glass, and then wet it out. Pre-saturating the wood adds a bit of weight - and strength.


Well-Known Member
Feb 22, 2011
I haven't seen technical info on the "chemical" bond versus "physical" bond issue. West does say that there may be an advantage for subsequent epoxy coats to go on before the previous coat has "set" so that amine blush does not interfere. I've done that, and it works.

I don't see any reason why laying the glass and epoxy on clean, prepared plywood wouldn't result in just as strong a result as doing it in two steps. The epoxy should go right through the glass cloth and into the wood grain. For a slightly better result, one might warm the plywood before applying the glass and epoxy. As the wood cools, it should tend to draw the resin in, and there should be no outgassing.

I realise that I am talking to folks who have extensive experience glassing plywood. Mine is limited to plywood paddle blanks and paddle shafts. But to me, the best way to get an optimum bond between glass and plywood is to do it in one step.

Kayak Jack

Well-Known Member
Aug 26, 2003
Okemos / East Lansing Michigan
The 72 hour bit comes from System 3, and it may not apply to other epoxies? They state that within the first 72 hours, a subsequent coat will bond chemically instead of just physically. I suppose that epoxy sloshing through fiberglass will soak in as much as epoxy without glass.
Each of we builders vary in many ways, and then report results, or at least, perceived results. Sometimes we learn something new and better. Sometimes we make a mess, clean it up, and don't say much about it. Evidently your methodologies have met with success; maintain your headway. If you experiment with something, and it seems to work, let us all know? (News about good food is just as interesting as news about good epoxy, BTW. :D )

tx river rat

Well-Known Member
Feb 23, 2007
Waco Tx
A precoat of epoxy is what I use also then the cloth and a second coat. I have done it both ways and seem to have less problems with out gassing and dry spot doing it that way. I live in Texas where you have a heck of a change in heat and humidity in a 24 hour period.


Well-Known Member
Aug 25, 2003
Central , Florida
I epoxy saturate the wood before glassing it. The epoxy saturation at times ( that I do ) is with a diluted epoxy of epoxy and acetone mix , nice and runny. I believe that lets the epoxy saturate the wood more then the standard , thick , coat of epoxy.

If I used the diluted mix then I follow it the next day with a normal mix ( which each boat gets ) and the day after that a light sanding and then the glass with epoxy.

The way I see it is that the epoxy saturation will add strength to the wood , one report I had was up to 4 times the strength. Take some scrap wood , a plain piece and one that has been epoxy saturated , the saturated piece is a lot harder to break or has more tinsel strength then the untreated piece. Not sure about that 4 times the strength but it is a lot stronger.

I do know that when the epoxy saturates the wood it sure gives the epoxy with the glass some tooth to bond to , more so then just bonding to plain wood.
One other benefit to the saturation.... A epoxy saturated piece has the epoxy deeper in the wood then just a piece that has been glassed and epoxied. This offers a 2nd safety measure in case the boat runs up on something causing a cut in the glass and even into the wood. Better to have a scratch in the saturated wood then just plain wood , epoxy does waterproof the wood it saturates.



Well-Known Member
Aug 25, 2003
Central , Florida
LesForgue said:
During the building stages, would the deep epoxy saturation make plywood panels harder to bend and twist without breaking?
You mention tensile strength, so I'm guessing the opposite, that the epoxy stauration might make the panels more able to bend and twist without breaking. Would that be true?
I have never saturated the wood before having it all together. The saturation is after the boat is made and before glassing it.
The saturation adds epoxy to the wood (soaks into the fibers) so the epoxy and glass have something to bond with besides just the wood. The boat is glassed within a 24 to 48 hour period after the saturation or last saturation , usually 24 hours.

One word of warning ........ When the wood is epoxy saturated the wood fibers soak it up and some of them become what I call whiskers. They rise up to a small degree and need a light sanding before the glass is placed on them for epoxying. The whiskers will cause some runs in the glass if they are not lightly sanded. I stress a light sanding , not a hard sanding since all you need to do is to smooth off the the few ends that are sticking up.

I have taken some scrap pieces and saturated one piece and left one raw wood , the raw wood piece was a lot easier to bend and break then the one that was saturated with the epoxy.


Well-Known Member
Sep 10, 2003
Oscoda, michigan
I am in the process of making another cedar strip canoe and want to reduce the weight as much as possible over previous strippers I have built. One of the new things I am trying is to use lighter woods where possible. I have always used ash or white or red oak for the gunnels and keel, when I added a keel. A few years ago Joe Fennell bought several 1 x 12 x 20' cypress boards for me and I've only used them for the occasional paddle. But, I thought I'd cut strips and use them as gunnels for this next canoe. After cutting them and sanding, shaping, etc. I'm having second thoughts. It seems to be too prone to split, too delicate to stand up under stress, dent too easily with rough handling, etc. Has anyone out there used cypress for gunnels? Has it worked O K for you? Thought I'd get some advice before I take that final step and mount them, then be sorry and have to go back and replace 'em. Sure would appreciate any ideas you can share on this subject. Thanks.

Dapper Al---Keep Moving.


Well-Known Member
Jul 22, 2006
Denham Springs, LA
we use cypress for nearly every boat we build. The difference is that we use old growth cypress that is typically 800 to 1500 years old. The newer cypress, called grow back around here, is usually lighter in color, stiffer and more brittle than the old stuff. If your cypress has less than 30 or 40 growth rings per inch, it's likely grow back.

the grain orientation can have a big effect on what you're trying to do. Chances are your strips vary from plain sawn (growth rings parallel to the face) to quarter sawn (growth rings perpendicular to the face). Sorting them out will improve their suitability for your application.


Well-Known Member
Sep 10, 2003
Oscoda, michigan
Thanks for that information Seedtick. I'll go out in the workshop and count the rings. The boards Joe got for me have a beautiful grain and make very pretty paddles, but the way I build the inwhales causes me to be hesitant about using it. On the inwhales I make a mark every two inches , and this particular inwhale is 154 inches long where it goes into the stem ends. I make a mark every 2 inches on the inwhale then use a router on my router table to make an indentation 1/4 inch deep on every other two inches. It usually takes two, sometimes three passes to remove the 1/4 inch material. I put the finishing touches on these goughed out areas with files and sandpaper.
This further reduces the weight, but also diminishes the strength of this strip of wood. Why do I do that? The finished look gives an appearance of having ribs like the old model Old Town cedar ,canvas covered canoes, plus each indentation allows for a tie down for your gear when tripping. I always tie an old shoestring, or 1/8" piece of nylon to each bundle placed into the canoe when I head into fast water or even when travelling over a long distance on lakes or flat water. But, with this much material removed I am getting edgy about the cypresses ability to hold up the seats, and the center thwart. Oak or ash is so tough that I never had second thoughts about it. But this cypress has me second guessing. I'll count the rings and make my decision based on the count.

Thanks again. Wish I had your knowledge of cypress, 'cause it it a nice wood to work with. Dapper Al.


Well-Known Member
Jun 20, 2007
Spring Hill, FL
I'm thinking that maybe you are making something that looks a lot like the "ladder" inner rail that many use on their pirogues. I'm also thinking I might be wrong.

But if you are, rather than remove wood from the rail, why not just attach spacers to the side first, then put the rail on top of those? The spacers can be whatever thickness you want and the ends shaped to whatever look you want. Round them off, make the ends concave, or cut them at angles. You can also play with the distance between the spacers to suit your self. Lots of ways to go.

Look at some of the boats in the Boat Bragging section of the Form to see what I mean. I think you could probable get the look (and function) that you want this way and it should be easier than what you are doing. The end result is something that would add strength, rather than reduce it.

Other than that, pretty much what seedtick said. (I just got "wordier," as usual.)

Mike S.
Spring Hill, FL


Well-Known Member
Sep 10, 2003
Oscoda, michigan
Thanks guys. I appreciate the ideas but adding spacers doesn't help reduce the weight. I considered that technique but rejected it because it makes the inside gunnel wider than the outside gunnel and doesn't look right, It would be stronger than the "laddered" effect but with oak or ash I didn't have to be concerned. At this stage I plan on going ahead with the cypress. I counted the rings as Seedtick suggested and there 48 rings, on average, per inch. I guess I've got regrowth wood, but if I put screws thru the gunnels just in front of and behind each bolt used for hanging the seats it should add strength and reduce chances of splitting (I'M hoping). It's a gamble,but what the heck, experimentation is half the fun of creating something,or so I've been told.

Dapper All--------Keep Moving.