Plywood Grading Rules :Part 11 MARINE PLYWOOD STANDARDS

Plywood Grading Rules :Part 11 MARINE PLYWOOD STANDARDS

Figuring out marine plywood grades in order to choose materials for stitch and glue and other methods of boatbuilding can be confusing.

Opinions range from the high and mighty: “Since you are going to spend a lot of time building, you should use the best material available no matter what the cost” to the economy version “Grab some Luan underlay, build a quick cheap boat and get out there and have some fun”

Most of us would prefer to understand the options a bit better.

Marine Plywood Grades

In order to be a Marine Grade the plywood has to conform to a certain number of criteria:

  1. The Glue MUST be waterproof
  2. Voids are to be avoided
  3. Face and inside veneers have to be good

Plywood Glue

Any glue used for marine plywood or for that matter also used in exterior use has to conform to the a stringent test. Plywood assembled from it needs to be able to withstand boiling in water for several hours without de-laminating. This is WBP glue and stands for “Weather and Boil Proof”. Glues vary in their tenacity while being boiled.

The capacity of a plywood to not de-laminate under boiling is not only dependent on the quality of the glue but also on the materials used and the care which was used in the manufacture.

Voids in Marine grade plywood spell disaster.

Voids not only weaken the plywood but create a ready path for water infiltration. For this reason Marine Grade Ply must not have voids.

The veneers used for the manufacture of the plywood must be of high quality.

Depending on the grade, knot size and frequency is controlled. If a repair needs to be made on an inside ply there are rules for this. If a ply needs to be joined inside it can not be butted. The thickness and number of plies is also controlled.

Marine Plywood Grades

BS 1088

Although a British Standard, it is used by marine plywood manufacturers world wide. The plywood does not have to be of British Manufacture. It is an extensive document which details all characteristics of the plywood.

  • The BS 1088 standard is a marine plywood specification that applies to plywood made from untreated tropical hardwood veneers that have a stated level of resistance to fungus growth.
  • BS 1088 plywood must use a glue, which has been tested and shown to be highly resistant to weather, micro-organisms, cold and boiling water, steam and dry heat. There are standard BS tests for the glue.
  • The Face Veneers must have a solid surface without open defects. Face veneers must be free of knots except “sound pin” knots, (no more than six in any one square foot,) the average of pin knots cannot exceed 2 per square foot over the whole surface of the plywood sheet. The grain in the veneers should be reasonably regular. Edge joints are limited, and end joints are not permitted.
  • Core veneers requirements are similar to face veneers except that slits are allowed as long as they are small. Pin knots and edge joints are also allowed. As in face veneer, end joints are forbidden.
  • Manufacturing Defects are strictly controlled. Poor bonds, overlaps and pleats, and gaps in faces are not allowed. Gaps may be filled using veneer inserts glued with appropriate adhesive.
  • The Moisture content of the plywood must fall between 6% and 14% when it leaves the factory.
  • Boards will be equally sanded on both sides.
  • Finished size The length or width of a standard sized sheet can not be less than the stated size nor more than 6.3 mm (0.25″) larger.
  • Squareness — The lengths of the diagonals of a sheet shall not vary by more than 0.25% of the length of the diagonal.
  • Thickness tolerances:
    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
  • Face Veneer thickness — For any three-ply construction, each veneer sheet will not be any thinner than 1/8 of the total thickness of veneers assembled dry. 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

Lloyd’s of London Certification

According to their website

“Lloyd’s Register Type Approval is an impartial certification service providing independent third-party Type Approval certificates attesting to a product’s conformity with specific standards or specifications, and verification of an appropriate production quality system.”

In other words Lloyd’s ensure that the standard in production is met. Lloyds do not continuously inspect production lines or individual plywood sheets.

Many plywood are made according to the BS 1088 marine plywood standard but they have not all gone to the trouble and expense of getting certified. On the other hand some manufacturers claim to be up to this standard but are not. As usual it boils down to you get what you pay for and it pays to go to a reputable dealer.

BS 6566

This standard is very similar to the BS 1088 with somewhat reduced quality.

It is a marine plywood standard using the same glues, but has somewhat more relaxed requirements as to the quality of the veneers. The face veneers are often of slightly lower grade and thinner. There are no voids and requirements for joins are similar.

It is generally considered an excellent material if not as pretty as the higher grade. Bright finishes are not necessarily as successful as with the BS 1088. I have used both grades and have not seen any defects in either.

Exterior Plywood

There are many exterior grades being manufactured. The glues are usually essentially the same as the marine grades. The outside veneers vary a great deal depending on the chosen grade. There is a possibility of voids. Boats have been successfully built using higher grade exterior plywood.

AS/NZS 2272

This is an Australia / New Zealand standard. Marine Plywood manufactured to AS/NZS 2272 standard, is a structural plywood intended for hulls of boats and other marine applications. It has a Type A (wbp) phenolic bond glue and is manufactured from woods based on density, bending strength, impact resistance and surface finishing characteristics.

None of the marine species are naturally durable and preservative treatment will be required. Hoop pine is often used.

The Engineered Wood Products Association of Australasia is a rich source of information and explains the standard in more detail.

The durability of the plywood wood is not always of paramount importance in modern boatbuilding

The marine plywood is rarely left unfinished. It very often has an epoxy coating, often with glass cloth as well. Okkume is a prime example of a widely used plywood wood which is not particularly resistant to rot and fungus.

You may not always need the absolute best grade of plywood. Often a high end exterior ply will do as well particularly in area that do not particularly stressed.

5 Things to Know About Marine Ply Before Your Next Project | Plyco
In the past, we have spoken about the differences between British Marine Ply and Australian Marine Ply standards when it comes to Marine Plywood, but that wasn’t enough for the team here at Plyco, so we’re back with another blog post. This time we’re focusing on some important facts everyone should know before starting their next project with Marine Grade Plywood.

At Plyco we offer a diverse range of Marine Ply products. We offer Pacific Maple Marine Plywood, which is manufactured to British Standard, as well as Gaboon Marine and Hoop Pine that are both perfect options if you need something to withstand use in wet areas while also having a beautiful architectural finish. Our Pacific Mable and Gaboon are available in 2440 x 1220mm sheet sizes with thicknesses varying from 3mm to 25mm, while our Hoop Pine comes in the standard 2400 x 1200mm sheet size and can be anywhere from 4mm to 25mm in thickness.

Plyco’s Pacific Maple Marine Plywood

But before you jump over to our online store to check out our entire range of Marine products, here are five facts you should know!

1. Marine Grade Plywood is manufactured using the best veneers
One of the key attributes of all great Marine Ply is that they use the highest quality grade of veneer possible. This is arguably the most important thing when it comes to this variety of plywood because without it you’re going to see whatever project you’re working on slowly deteriorate, which is less than ideal. The veneers used for Marine Plywood must have no defects and absolutely no core gaps present, while also lacking any knots, giving the board either an A or B face grade. This is done as a preventative measure to resist delaminating, fungal attacks and to stop water from getting inside the plywood to keep the glue bond stable. All of this proactive work combines to make our Marine Ply the only grade of plywood that is approved for constant submersion in water.

2. Despite the preventative measures, Marine Ply is still very similar to regular Plywood
Even with all the extra bells and whistles, at the end of the day, a sheet of Marine Plywood is still very similar to regular plywood. It’s still manufactured in mostly the same way, with numerous thin layers of wood being glued together and placed under extreme pressure to press it all together. Just like regular plywood, during this process, the grains are generally pressed at right angles to bolster the wood’s strength capabilities. While there are a number of extra add-ons used to ensure it’ll survive constant moisture, Marine Plywood is the same versatile timber product you would find in all of Plyco’s other plywood products and can be used for many of the same applications if you desire!

3. Hoop Pine is the highest quality Marine Plywood

If you’re looking for the highest quality Marine Plywood around that also has a stunning architectural finish to it, Hoop Pine Marine is the product you’re after. Hoop Pine is specifically manufactured from species that will maximise its density, core strength, impact resistance and high surface finishing capabilities. In addition to this, it has an A-grade face, back and core with an A-level glue bond and manufactured to Australian Standard. To top it all off, it’s manufactured in the most environmentally-friendly way possible to limit the number of emissions produced. There is no arguing that this is the defining product when it comes to Marine Plywood.

4. Marine Ply is the king of the high seas
When a boat travels through water it comes under impact from plate action as well as any floating debris. To combat this Marine Plywood is specifically designed to be able to withstand and absorb these forces without losing any of its quality. Our Marine Ply is able to easily handle these conditions thanks to it being the closest plywood to solid wood, and actually goes one better as solid wood has defects that need to be graded out, while Marine Ply doesn’t.

Sitting on the dock of the bay

5. Not just a sea lover
While the name “Marine” Plywood does immediately send your mind to the waterfront, it’s not just boats, docks and other projects by the water that are perfect for this range. Marine Plywood can be used for limitless applications wherever the danger of moisture is present. Whether that is something being constructed and in danger of being rained on or construction in kitchen or laundry areas where water from taps and household appliances are present, you’re only held back by your imagination when it comes to utilising Marine Ply.

Now that you know just a little bit more about the world of Marine Ply, why not head over to Plyco’s online store where you can pick up Marine and a whole bonanza of other plywood varieties? You can explore our Marine Ply range and while you there you might discover something you never knew you needed!

BS 1088 (Hydrotek)

4’ x 8’

Strong – Medium Density – Easy to Work – Hull Components – Floors – Bulkhead

BS 1088 Hydrotek is a multi-ply marine plywood that is superior to all standard Meranti plywood currently produced.  Both face and inner plies are constructed from whole piece, color matched, Meranti veneers.  Hydrotek is used in boat building applications where only the best will do.  Hydrotek’s durable rating will ensure that you are using a marine plywood that will last a lifetime.

BS 1088 presents a solid surface that is free from open defects and knots.  The veneers are free from irregular grain.  Boards can be sanded on both sides.  Hydrotek has excellent machining properties as it can be easily sawn and bored leaving smooth edges.  It also has good nailing and screw-holding properties and is easy to stain and glue.

Hydrotek Meranti has superior durability and insulation properties compared to other timbers commonly used in marine applications.  Due to the multi-ply construction, its strength properties are superior to other marine plywood available.

Face Veneers:  No structural defects allowed/Appearance grade face and back, color matched.

Core Veneers:  All core veneers are comprised of whole piece veneers ensuring absolutely no core voids.

Glue:  WBP (Phenolic)

Bending Strength:  92    N/mm2

Modules of Elasticity:  11400   N/mm2

Compression Parallel to Grain:  52.9   N/mm2

Impact Strength:  0.74

Durability:  Durable

Movement:  Stable

Working Properties:  Good


3mm   (1/8”) – 3 ply

4mm   (5/32”) – 3 ply

6mm   (1/4”) – 5 ply

9mm   (3/8”) – 5 ply

12mm   (1/2”) – 7 ply

15mm   (5/8”) – 9 ply

18mm   (3/4” – 11 ply

21mm   (7/8”) – 13 ply

25mm   (1”) – 15 ply

BS 6566?

There have been inquiries on the Boatbuilder Connection regarding the difference between BS 1088 and BS 6566. I knew there was a difference in the outer veneer thickness, but not whether there were other differences. The following are my notes. I hope to add a more complete description to the Wood & Plywood Information section of the web site when I feel I have all the information. I am including these notes for those “who want to know…”. I welcome any additions.

My only “in person” contact so far has been Dick Garwood of Shelman USA. As far as I know, Shelman is the largest manufacturer of okoume marine plywood. They are very concerned about the confusion over plywood standards and will be one of the manufacturers that Woodenboat magazine will talk to for the series on marine plywood.

Summary of a phone conversation with Dick Garwood of Shelman USA, 3-13-03. The question being, “what is the difference between BS 1088 and BS 6566”.

BS 1088 (This is the only plywood Shelman refers to by number.)

  • Face and back: 1.3mm (regardless of total plywood thickness). There may be some very minor differences, because the 1.3mm is before final sanding.
    WBP glue (glue is rated on a scale of 1-10. Shelman uses between 5 and 10)
  • No voids or patches
  • Outer veneer in one piece.
  • Rotary cut, all the same species
  • The panel is completely balanced

okoume is rated as a non-durable wood. Shelman adds a fungicide to the glue which changes the okoume Lloyd’s rating to “moderately durable”. At the present time, Shelman is the only one to do this. This material is sold under the trade name “Shelmarine® plywood”. They also make a non-fungicide okoume that is sold as okoume Marine.

Re. Lloyd’s certificate. There are lots of Lloyd’s certificates, the question is “what is it certified for?” A Lloyd’s certification by itself means only that it is certified for what the certificate says it is certified for.

BS 6566 (Shelman lists this as “BBX”)

  • Face and back: 1mm
  • WBP glue
  • Patches acceptable on interior cores. Not as select veneers for the outer plies as BS 1088, but in one piece.
  • Does not have to be the same species, but Shelman’s is.
  • The panel is completely balanced

“There are no BS 1088 police.” No one polices the standards. There have been cases of distributors re-marking BS 6566 panels as marine. Some of the cheaper panels that are marked BS 1088 do not meet the standards. It is best to get your plywood from a reputable manufacturer if you want BS 1088 that meets the standards.

Woodenboat plans a series of articles on BS 1088. We look forward to having the difference in the standards clarified.

Questions for Dick Garwood:

  1. BS 6566: Does not have to be the same species, but Shelman’s is. Is this correct?
  2. A Lloyd’s certificate… the question is “what is it certified for?” How does one know what it is certified for? Is there a copy of the certificate on each sheet?
  3. Are okoume and exterior plywoods below NOT Lloyd’s registered? Or just rated differently?
  4. Shelmarine: Glue is “melamine” reinforced. This is not the case with the other two grades?
  5. Shelmarine “is approved for application of pleasure craft and small craft construction”. So, what is okoume marine rated for, if not pleasure craft or small boat construction?
  6. In the paragraph under Lloyd’s Registered: “In addition, the front and back panels have a minimum thickness of 1.3mm.” I changed panel to plies.


There are a number of okoume exterior plywoods on the market. It is important to understand the difference in these okoume plywoods so that you select the correct panel for its intended use. Shelman produces three okoume exterior plywoods, two for marine use and one for exterior use. The higher quality of the face, back and interior cores insure a longer life for the plywood.

Shelmarine® okoume Plywood

okoume as a species is rated as a non-durable wood. In order to obtain a rating by Lloyd’s as equivalent to a minimum of moderately durable, Shelman okoume plywood is treated with a preservative. Because of this treatment Lloyd’s Register gives us an approval for application of pleasure craft and small craft construction. No other okoume plywood has this rating or this marine application. Shelmarine® okoume meets or exceeds all BS 1089 and BS 6566 standards, The core has no voids, patches or gaps. The panel itself is all rotary cut okoume, balanced throughout. In addition, the front and back plies have a minimum thickness of 1.3mm. Glue is melamine reinforced. The surface of the panels is carefully selected to ensure the highest quality. The careful selection of all layers, thickness, face and back panels and special preservative insures you of a panel that is superior to any other okoume plywood on the market.

There are a number of Lloyd’s registered plywoods on the market. Being registered by Lloyd’s is a registration only. What is important to note on the registration is the “application” and rating”. This spells out where the plywood be used. Again, Shelmarine® okoume is the only Lloyd’s rated okoume plywood as “equivalent to a minimum of moderately durable”. All other okoume plywood is rated as non-durable. For application, our plywood can be used for “marine, offshore, industrial use for construction of pleasure craft and small craft”. No other okoume plywood is rated for pleasure craft and small craft.

Shelman also offers Shelmarine® Sapelli with the same high standards registered by Lloyd’s.


This plywood meets or exceeds the BS 1088 and BS 6566 standard. The glue line is phenolic. Face and back are a minimum of 1.3mm with no voids or patches in the surface or core and is rotary okoume throughout. Panel is completely balanced.


This plywood is an all rotary cut veneer okoume plywood for exterior use according to BS 6566 standard. Face and back are one piece with a thickness of 1.0mm. Glue is phenolic. Interior core is joined pieces of veneer. Panel is completely balanced.

Shelman offers you a complete line of okoume plywood panels depending on use. Please contact Harbor Sales at (800) 345-1712 or other Shelmarine® distributors.

Notes (These come from a variety of sources.):

  • BS = British Standard. These people formulate standards for all sorts of things, not just plywood.
  • BS 1088 veneers may be rotary or sliced cut. The method of cutting is at the option of the manufacturer.
  • Although we have referred to BS6566 because this is a used terminology, this standard is officially withdrawn and superseded by BS EN 315.
  • WBP glue: Weather & Boil Proof
  • All plywood is graded based on the quality of the veneers and the size and number of permissible voids and patches. Grades A-C are the most common. To be suitable for marine construction, panels must be made with waterproof glue and marked as exterior plywood (EXT) or marine plywood.
  • As a general rule for boatbuilding, use marine-grade plywood for planking because the wood used to make it is of higher quality with smaller voids and fewer patches. Inner veneers are also of higher quality, grade B or better. Marine plywood is therefore more stable, has less potential for checking, and the overall strength and stiffness are better than exterior plywood. Considering the overall cost of a project, the price for marine grade plywood is a good insurance. For applications where the plywood is not bent (transom, stems, breasthook, knees, gussets, etc.) we would generally use ABX (X = exterior). If planking is applied in multiple cold-molded layers, ABX can also be used.
  • Domestic marine plywood is made to standards formulated and monitored by the American Plywood Association (APA). It is not a US government standard.
  • Domestic marine plywood: Species: Douglas-fir No 1 or western larch. Glue: waterproof. Face and back veneers A-A, A-B, B-B smooth cut and free of knots, pitch pockets and open splits and other open defects. Inner plies, B or better. Repairs: no more than 9 repairs allowed on A faces in a 4×8 sheet.
  • There are some plywoods available that are not called “marine plywood”, but use marine in the description. These plywoods are made from wood that is other than those listed in the marine standard.
  • Pressure treated plywood: Pressure treated plywood is often heavily warped. This is primarily the result of the high temperature and rapid moisture take up during processing. Some plywood delaminates during the process. In addition, some pressure treating methods incorporate water repellent materials. These materials can easily jeopardize bonding so you won’t be able to glue or paint the plywood successfully. See # Preservative-Treated Plywood for Boat Manufacturing Applications – Best Practice Guide in the Wood & Plywood Information section of our web site for treatment techniques that are boatbuilding compatible.
  • BS 1088 (Marine Plywood) is currently undergoing a revision (the latest available version is 1966, updated in 1988) and may not be available for at least another four months. BS 6566 (Plywood) has been withdrawn from circulation for some time now and has been replaced by various European standards, including BS EN 636-1, BS EN 636-2, BS EN 636-3 and DD ENV 1099.
  • Bruynzeel used to “own” the US market, they are now out of business. There may be a company producing limited amounts of plywood under the Bruynzeel name, but it would be very limited.
  • It is important that any plywood you use for boat construction be “balanced”. This means that there is a center ply with with an equal number of veneers on either side and the corresponding veneer the same thickness.
  • Plywood contains end grain on all its edges, which makes it susceptible to moisture migration and damage. Whether exterior or marine, any exposed edges need to be sealed.


BS 1088 uses more select material and is defintly a superior panel. Having said that, BS 6566, like Douglas-fir exterior is suitable for some boatbuilding projects. In a perfect world, where we would only use the best, marine would be the only plywood used for plywood boatbuilding.

Personal opinion: On hulls that do not have to be the lightest weight, I would choose Douglas-fir over okoume. It is stronger and more rot resistant than okoume. Shelmarine® may be equal on the rot part… don’t know for sure. Okoume is possibly more attractive than DF, but only just. For a natural appearance, I wouldn’t choose either. I would consider using veneer on larger hulls. Some web sites that I have visited on my search have suggested that using interior plywood is acceptable for boatbuilding as long as it is encapsulated in epoxy. Not my boat. There is normally a lot of other stuff that goes into a boat, not the least of which is epoxy. If you drill a hole in the boat and do not properly seal it, your boat can turn to poop. If the glue bond fails, the plywood turns to rubber and there is not much you can do about it. In defense of those other guys, IF no water gets to the glue, it will probably be fine.

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