Marine vs Pressure Treated Plywood


Perhaps the biggest controversy about needed repairs on boats is usually surrounding plywood or more specifically treated (big box store) plywood vs. harder to obtain and significantly more expensive marine plywood. My personal experiences with both varieties of plywood leaves me favoring Marine Plywood for my replacement / rebuilding needs. Our pontoon houseboat which we purchased quite used and in dire need of repairs actually contained both varieties courtesy of the previous owners. The boat as purchased was a 1972 model that had spent its life on a freshwater lake up till the time we got her and made extensive repairs or rather rebuilt her from the pontoons up. The front and back deck had been previously replaced at least once by one of the previous owners supposedly just a short time before we got her while the plywood under the cabin was still original vintage 1972 still in fairly sound condition (wish I could say that about the rest of the cabin though). Anyway the decks had been replaced with pressure treated plywood and common zinc plated nuts and bolts. The caustic effects of the treated lumber on the fasteners had caused severe rusting to the point that several of the fasteners were near failure and all of them had to be twisted off to remove. Also the treated plywood itself though not actually rotting it was failing because of delamination of the plies of the wood itself which in my observation is a common occurrence and a very undesirable real problem with boating projects. I’ve never observed delamination issues with marine plywood it seems to fail slowly by getting soft with gradual rotting you’ll generally notice before someone manages to put their foot through it.

I know many of you will ultimately decide to use the pressure treated lumber, just because of the cost and availability of the material (let’s face it we’d all like to save a buck here). But you really should follow the manufactures recommended fasteners you’ll find listed below and also try to isolate treated lumber from direct contact with dissimilar metals to reduce or hopefully eliminate galvanic corrosion and hopefully long term satisfaction of your project. The choice is ultimately yours to make and I’m hoping you choose wisely.

Marine Grade Plywood:

Marine-grade plywood generally is made entirely of Douglas-fir or Western Larch but some other woods are also available. The grade of all plies of veneer is B or better. B-grade veneer may have knots but no knotholes. A-grade veneer has no knots or knotholes. Both A and B grades may contain wood or synthetic patches. Panels are sanded on both faces or overlaid with a Medium Density Overlay (MDO) or a High Density Overlay (HDO). The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a “premium” panel grade for use in situations where these characteristics are required. It is available in 4x8-foot sheets of 1/4, 3/8, 1/2, 5/8 and 3/4-inch thickness. Sheets up to 5x12-feet are also available.

Available grades are A-A, A-B, B-B (face-back), MDO and HDO. Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard.

The detailed description of veneer grades and Marine-grade plywood is contained in Voluntary Product Standard PS 1-95 Construction and Industrial Plywood. Sample Specification for Marine Grade APA 3/8" B-B Marine Grade 4x8 10 pieces

Other Exterior Plywood Grades / Preservative Treated Plywood:

Read the information at this link see if it’s available anywhere close to you. Many others have purchased this plywood and have been very pleased with it. It costs much LESS then what Lowe's or Home Depot sells and is much better in quality too. The Inland games & Fisheries are also utilizing it everywhere to build docks and landings.

Environmentally Safe!

Borate Pressure Treated Wood with the added protection of EnviroSafe Plus®, a Colorless, Water-Repellant Polymer System for Interior & Exterior Applications

Features, Advantages & Benefits!

Lumber is pressure treated with Disodium Octaborate Tetrahydrate (DOT Borates), a Natural Preservative & EnviroSafe Plus®, a Colorless, Water-Repellant Polymer System

Contains No Metals, including Copper

Virtually Non-Corrosive – Standard Galvanized Fasteners can be used - Less Corrosive than Untreated Wood

Fire Retardant Properties

Low Toxicity – low risk to humans & other non-target animals

Paintable Stainable upon completion of installation

Odorless Non-Volatile – no negative effects on indoor air quality

Resists attacks by Subterranean Termites, Dampwood Termites, Drywood Termites, Powder Post Beetles, Carpenter Ants and other Wood-Destroying Insects Decay Fungi

Wood retains natural color characteristics after pressure treatment

No adverse effects on strength or elasticity of lumber

40 Year Transferable Limited Warranty

If you can't find it anywhere close to you then get the self sticking rubber tape self drilling deck screws from pontoonstuff.com. I used both of these together along with the BORATE treated plywood & it worked great for me the deck turned out fine. Hope this helps. Never ever lay treated plywood directly on the cross members as a galvanic reaction WILL take place a lot sooner then you could expect.

Plywood panels rated as EXTERIOR but not Marine, such as A-A, A-B or C-C EXTERIOR, may contain any other permissible species and contain C-grade veneer. Unless specially improved, C-grade veneer is permitted to contain knots and knotholes up to approximately 1-1/2 inches across, and the inner plies may have core gaps up to 1 inch wide. Most construction applications don’t require preservative treated wood. But in some uses and in some climates treated wood is both recommended and desirable. Treatment can reduce and protect against decay, fungi, termites, and other wood destroying insects, and against marine borers in sea-water exposure.

Preservative treated plywood is impregnated with preservative by a pressure process and the resulting penetration of the chemicals provides the protection against decay and insect attack. Non-pressure preservative treatments are available, but superficial applications provide limited protection against decay. Preservative treated plywood maintains its stiffness and strength, thermal properties, workability, light weight and economy. Wood treated with preservatives needs to be handled appropriately by following the recommendations of the manufacture.

Technical Data Preservatives:

Pressure preservative treatments listed in AWPA Standard U1 include organics such as creosote, pentachlorophenol, copper naphthenate, oxine copper and waterborne inorganic formulations. Proprietary preservative treatments that meet AWPA requirements and local building codes and approvals are permitted for use.

Organic Preservatives:

Organic preservatives listed in AWPA Standard U1 for treating plywood include creosote, pentachlorophenol, copper napthenate and oxine copper. The first two are “restricted use” chemicals and are not available for residential construction use so I’m going to skip them and proceed to the actual plywood you may encounter.

Copper naphthenate (CuN) is suitable for above ground uses when treated to the proper retention level. CuN is often dissolved in heavy oil solvents or light hydrocarbon solvents for use as preservative carriers. CuN treated wood mau have a light green coloration that fades during weathering. After through drying it can be stained of painted but a stain blocking primer is recommended to minimize finish discoloration caused by CuN treatment.

Plywood treated with oxine copper (Cu-8-Q) preservative can be used in applications where food is harvested, transported, or stored, and is suitable for above ground uses. The preservative is dissolved in liquid petroleum gas or light hydrocarbon solvents so that the treated plywood has a clean surface and id free of solvent odors. Oxine copper solutions may leave a greenish-brown coloration.

Inorganic Preservatives:

The most common inorganic preservative used for treating plywood is the leach resistant, waterborne preservative alkaline copper quat type C and type D (ACQ-D), which are highly effective in preventing decay and attack by insects and marine organisms. Other waterborne preservatives that are listed in AWPA Standard U1 for treatment of plywood are ammoniacal copper zinc arsenate (AZCA), ammoniacal copper quat type B (ACQ), acid copper chromate (ACC), chromate arsenate type C (CCA), copper azole type A (CBA-A), copper azole B, copper HDO type A (CX-A), inorganic boron (SBX), and copper bis dimethyldithiocarbonate (CDDC). CCA may be only used in applications where frequent human contact will not occur. Consult local and state building codes prier to specifying the use of CCA treated wood. These preservatives are forced into the plywood under extreme pressure and become insoluble or fixed into the wood cells after impregnation and drying. Waterborne preservatives are recommended when clean, odorless and paintable plywood products are required.

Field Cuts:

It is strongly recommended that all fabrication, trimming and boring of plywood be performed prier to pressure treating. Should there be any field fabrication or surface damage to the plywood, all cuts, holes or damaged areas must be field treated to protect the exposed wood material. Copper napthenate can be used to reseal areas of plywood treated with creosote or pentachlorophenol. It may leave a greenish coloration and a through treatment should be applied to the point of rejection to insure total saturation of any field modifications.

Fasteners:

All fasteners used to connect preservative treated plywood should be corrosion resistant to withstand the effects of the high moisture environment which treated plywood is typically exposed. Corrosion of fasteners is heavy influenced by the amount of moisture present, temperature, wood ph, extractives, chemicals in the treatment and environmental factors such as chlorine, salt and pollutants.

Hot dipped galvanized steel or Type 304 or 316 stainless steel fasteners and connectors are recommended for treated plywood used in above ground applications. Hot dipped galvanized fasteners should meet ASTM A 153 and connectors should meet ASTM A 653, Class G185 sheet with minimum 1.85 ounces of zinc coating per square foot. Fasteners and connectors used together should be the same type. Do not use standard carbon steel, aluminum, copper or copper based fasteners or connections in direct contact with pressure preservative treated plywood or lumber. Only stainless steel fasteners are recommended for attaching treated plywood to lumber below grade in wood foundations. Follow preservative treatment recommendations of the manufacture for best recommended fasteners for their product.

Glue-ability:

Plywood can be glued when treated with most of the waterborne preservatives, and some of the penetration treatments using light petroleum oil or solvent, liquid petroleum gas, or petroleum solvent/water as a carrier. Consult supplier or their trade associations for specific recommendations for the particular treatment and application.

Use and handling precautions:

The chemical formulations used for preservative treatments of wood and plywood products are registered with the U.S. Environmental Protection Agency (EPA), which has approved guidelines for the use of pressure treated wood to ensure safe handling and avoiding environmental and health hazards.

Treated wood in some forms is considered to be a hazardous waste and as such can only be disposed of in an approved hazardous waste disposal site.

Treated wood from commercial or industrial use (construction sites) may only be burned in special incinerators in accordance with state and federal regulations.

Avoid frequent or prolonged inhalation of sawdust from treated wood. When sawing or machining treated wood always wear a dust mask.

Whenever possible all cutting operations should be done outdoors to avoid indoor accumulations of airborne sawdust from treated wood.

When sawing or machining always protect your eyes and eyesight by wearing safety goggles.

Avoid frequent or prolonged skin contact with any form of treated wood. When handling wood treated with chemicals, wear long sleeved shirts and long pants and use gloves.

After working with treated wood, and before eating, drinking, or using tobacco products, wash hands and exposed skin areas thoroughly.

If preservatives or sawdust accumulate on clothing, launder before reuse. Wash clothing separately from other clothing.

All Specifications sourced from the: Technical Services Division © 2006 APA – The Engineered Wood Association

You can also look here for a bit more about plywood and other choices of materials for your project.

Updated 12/19/10






Posted by blog.houseboatodyssey1.com at 1/27/2010 3:18 PM
Categories: Buying Tips, Maintenance, Various 101's:101, Supplies, Pontoon Boat Maintenance
Tags: plywood pressure marine treated Borate cca marine plywood galvanic corrosion
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