Water Boil Proof Plywood
When one is looking for the ultimate in weather resistant plywood, it is important to look for WBP. This abbreviation stands for “Water Boil Proof” or “Weather and Boil Proof;” a standard used to determine the water resistance of the plywood product. In actuality, this is more a rating of the glue used in the manufacture of the plywood, than it is in the wood itself.
Please note that WBP is not a separate type of plywood from others; rather, it is a rating that some plywood will have, if the manufacturer chooses to have it tested to determine that it qualifies. This is most often done on marine grade plywood, but it could also be done for other types of plywood that are intended to be used in wet or humid environments, such as phenolic plywood.
WBP plywood is not waterproof plywood. It is made out of the same sort of wood veneer that other types of plywood are. This rating doesn’t mean that it is pressure treated or impregnated with resin as construction lumber and plywood that is intended to be truly weatherproof and waterproof, although it could be. All the rating applies to is the adhesive resin that is used to hold the veneer layers of wood together in the manufacture of the plywood.
Nor is WBP plywood the same as marine grade plywood, although marine grade plywood can also be rated MBP. What makes marine plywood unique, is that it is guaranteed to be 100% void free, without any knotholes or cracks. WBP plywood may still contain these, although WBP rated marine grade plywood will not.
Many are saying that the WBP rating is no longer in existence. Yet you can still find plywood that carries this rating. Those who say it is obsolete, point to the European Standard EN314, Class 3. This refers to plywood which is suitable for exterior use, or a similarly wet/moist/humid environment. Please note that the European Standard provides the same testing specifications as found for WBP plywood, so the only difference is which standard is being applied; the plywood and adhesive are the same.
WBP testing consists of immersing a sample piece of the plywood in boiling water, and leaving it there for an extended time. If the plywood doesn’t delaminate from the heated water, then it is considered WBP.
In this testing, the rationale behind using boiling water, rather than room temperature water or cold water is that many adhesives are softened by heat. By using hot water, the adhesives being tested are subject to both the solvent action of the water itself and the softening potential of the heat. On top of that, the moving air bubbles from boiling agitate the plywood, making it more likely to delaminate. Delamination is always more likely to happen when adhesives are subject to the stress of movement, due to the constant stress on the glue joint, than it is like to happen when sitting idle.
Using boiling water provides for consistency in testing, as well, as water boils at 100°C or 212°F at sea level. There is no other temperature which is as consistent, other than freezing; but freezing water is not going to do much to stress the adhesive quality of the rosins used in manufacturing the plywood.
Never assume that plywood is WDP, if it is not so marked. While the plywood might have the ability to pass the WDP test, it hasn’t. The specific characteristics of WDP plywood are not repeated in other types of plywood, even exterior plywood that is chemically treated to withstand the rigors of being exposed to the weather.
There is no one glue that is considered to be WBF glue. Rather, WBP is a property that is determined by testing. A number of different types of glues can be WBF. However, different types of glues provide for different levels of protection against water. Therefore, the WBP rating is only a minimum. Often, the specific plywood carries an additional rating of how long it can withstand the WBP test.
The most common WBP glues used in plywood are melamine and phenolic. Ordinary melamine plywood can withstand the WBP test for 4 to 8 hours, without delamination. There is also a better grade of WBP melamine plywood, which can withstand the test for over 10 hours, in some cases up to 20 hours. However, phenolic WBP plywood outperforms melamine, surviving WBP testing for 24 to 72 hours.
Using the right adhesives isn’t the only thing that makes a particular plywood type WBP, how the adhesive is applied makes a difference as well. Even the best adhesives, when applied poorly, will not provide a secure bond. Likewise, the quality of the substrate will make a difference, as in the case of many adhesives, which are often stronger than the materials they are bonding together.
Please note: When we refer to WBP phenolic adhesive it isn’t the same as Phenolic plywood. Standard phenolic plywood is not WBP, as the phenolic resin is only applied to the face laminations. The interior laminations use a different adhesive, which is not WBP. However, WBP phenolic plywood would use phenolic as the adhesive on all laminating layers, in addition to having the face layers impregnated with it, making it highly water resistant.
The WBP rating originated in Great Britain, as part of the British Standards Institute standard 1203:1963, which is titled, “Specification for synthetic resin adhesives (phenolic and aminoplastic) for plywood.” It identifies the four commonly understood grades of glue used to bond the layers of plywood together, based upon their durability. WBP is the highest of these layers.
- WBP – Weather and boil proof
- BR – Boil resistant
- MR – Moisture resistant
- INT – Interior
You may find any of these markings on plywood products you buy at the local lumberyard or home improvement center.
Although more commonly used in England, you can find WBP plywood in the United States. That’s the reason why the prices are usually expressed in British pounds (£). Since WPB is a rating for water resistance, this term can be applied to a number of different types of plywood products. So, an exterior grade plywood may or may not be “Water Boil Proof”, depending upon the manufacturing process.
The difference between WBP plywood and exterior grade plywood is that the WBP rating refers to the adhesive resin used to bond the veneer layers of the plywood together. On the other hand, the “exterior” rating on plywood refers to chemical treating of the wood veneer, not the resin. One does not imply the other; you would need to look for both ratings, if you need them. Exterior WBP plywood is obviously going to withstand the weather better than either exterior grade plywood or WBP rated plywood.
WBP plywood can be used as exterior grade plywood for construction. There are those who say that WBP plywood is the only plywood which should be used for exterior sheathing, due to its weather resistance. It might be specified in cases where the exterior sheathing or subflooring is going to be exposed to the elements for more time than normal. Compared to normal exterior grade plywood, WBP can withstand the effects of weather for considerably more time. Specifying it in these cases would help ensure that the sheathing or subfloor was not damaged by the elements during construction.
It is common to use WBP plywood for subflooring, especially in areas where the flooring is likely to be subject to spills and other moisture. Even though the flooring itself is usually waterproof in kitchens and bathrooms, having a subfloor which is moisture resistant is an added benefit, in case the flooring becomes damaged.
However, it is more commonly used in place of Marine plywood, especially for interior applications. The difference between WBP and standard marine plywood is that marine plywood is guaranteed 100% void free, without any knotholes and cracks, while other WBP plywood may not be.
Keep in mind that WBP rated plywood is not marine grade plywood and should not be expected to be or to provide the same benefits. This is especially true in actual marine applications, where water might seep into and build up in the voids contained in the core veneers of the product. While WBP plywood may be better than marine grade in certain ways, it cannot just replace marine grade plywood in all applications.
Sign board, a specialized form of plywood, is often rated WBP, because a highly weather resistant wood product is needed for signage, which will be left out in the weather. A number of types of interior plywood are also rated WBP, such as many types of cabinet plywood, even though they are not used in an application where they are exposed to the weather.
Rating cabinet grade hardwood plywood as a WDP plywood is more about showing the quality of the manufacturing process, rather than any expectation that the finished cabinets would be subject to being soaked in water. Nevertheless, having your cabinets built out of WDP rated cabinet grade plywood can prove useful if you have water leaks and flooding in your kitchen or bathroom. Whereas other plywood products might delaminate, they would survive the water.
Of course, the WDB rating in the adhesive used in making the plywood doesn’t protect the wood veneers in the plywood from rotting, if they are exposed to continual humidity or flooding. That would require plywood that has been treated, ensuring that the plywood can’t absorb water.
Working with WDP plywood
WDP Plywood is no different to cut, sand, mill or connect than any other plywood. In most cases, the rosins used for making WDP plywood are the same as those used in the manufacture of plywood that does not carry the WDP rating. The only real difference may be that the manufacturer hasn’t bothered to have the plywood tested.
Keep in mind that any of the rosins used in the manufacture of plywood, when bonded together with wood fiber, create an abrasive mixture, which will work to dull saw blades. It is important to change your saw blades periodically, so as to prevent this dulling from causing more splintering in the wood.
Today’s carbide saw blades withstand the abrasive action of plywood much better than high speed steel blades do. This allows you to use the blades longer, before resharpening. It also makes resharpening a better financial decision, as the difference in cost between resharpening a saw blade and buying a new carbide tipped blade is greater than the difference between sharpening and replacing a high speed steel blade.
The WDP rating on the plywood does not affect the use of glue for attaching plywood sheets or parts together. Nor does it affect the use of any particular fasteners. However, if there is enough concern about moisture to specify WDP plywood, it would be advisable to use some sort of fasteners which are coated to protect them from rusting. Many types of fasteners are offered now in a coated version, such as various styles of drywall screws, which have become the standard all-purpose fastener for woodworking and construction.
WBP plywood is often sold in metric thickness, rather than fractional inch. It is available in both 18mm and 19mm thicknesses, with 18mm being slightly thicker than 5/8” and used in place of it and 19mm being slightly less thick than 3/4” and used in its place.
It is also possible to find 6mm WDP plywood, which is a five ply board, approximately 1/4” thick. It is useful as an underlayment to smooth out the existing subfloor. Being more water resistant, it is ideal in wet or high moisture applications. One such situation would be the installation of ceramic tile. The water in the mastic used to attach the tile and the grout could adversely affect some types of plywood, weakening the floor. By using WDP plywood, instead of interior grade, there is no risk of the plywood delaminating.