Posted on 04/13/2022  In: News PFTC  Rss feed  0 Comment  

The Plywood...

The Plywood...

Plywood is generally classified into two types: exterior and interior. Exterior plywood is made with waterproof glue only and should always be used for any exposed application. The interior plywood, made with very resistant glues, can indeed withstand a lot of humidity. There is interior plywood made with IMG (intermediate glue), which resists bacteria, mold and moisture, but no interior plywood is designed for exterior use.

When buying plywood, look for a back stamp or edge marking most plywood manufacturers represent, and they inspect and test all plywood to ensure the quality is high and the grading is accurate. The most critical plywood grading category for most home projects is the appearance quality of the panel faces (see plywood grades chart below).

To know more about all the abbreviations or letters accompanying plywood, learn to differentiate the different uses that can be made of it in the table that follows.

PLYWOOD GRADE

Interior Grade

 Face

 Back

Inner Plies

Common Uses

A-A

 A

 A

 D

Cabinet doors, built-ins, and furniture where both sides show.

A-B

 A

 B

 D

Alternate for A-A. Face is finish grade; back is solid and smooth.

A-D

 A

 D

 D

Finish grade face for paneling, built-ins, and backing.

B-D

 B

 D

 D

Utility grade. One paintable side. Used for backing, cabinet sides, etc.

C-D

 C

 D

 D

Sheathing and structural uses such as temporary enclosures, subfloor. Unsanded.

C-D

 C

 D

 C, D

For underlayment or combination subfloor-underlayment under tile and carpeting.

Exterior Grade

 Face

 Back

Inner Plies

Common Uses

A-A

 A

 A

 C

Outdoors, where appearance of both sides is important.

A-B

 A

 B

 C

Alternate for A-A, where appearance of one side is less important. Face is finish grade.

A-C

 A

 C

 C

Soffits, fences, base for coatings. 

B-C

 B

 C

 C

For utility uses such as farm buildings, some kinds of fences, base for coatings. 

C-

 C

 C

 C

Excellent base for tile, backing for wallcoverings, high-performance coatings. 

C-C

 C

 C

 C

Unsanded, for backing and rough construction exposed to weather. 

What is CDX?

CDX is veneer plywood, which is made by gluing and pressing together sheets of wood. The letters C and D represent the grades on each side of the plywood, while X represents the exposure.

What is OSB

OSB, on the other hand, stands for Oriented Strand Board which is a cheaper version of CDX. It is made of wood shavings, resin and glue, cast and fired together to produce a CDX type sheet.

Key Differences Between OSB and CDX

Water resistance

As mentioned above, the X in CDX stands for exposure. This means that the plywood can be exposed to moisture, but only for a limited period of time. Prolonged exposure will cause water damage. But, there is an exception to pressure treated CDX; they can withstand extreme humidity without being damaged by water.

OSB resists water quite well and is easily considered waterproof. Indeed, the glue and resin used in its processing prevent it from absorbing water even during extremely humid and rainy seasons.

Dimensional stability

Even though the CDX is not waterproof and easily absorbs water, it also loses this water quickly and returns to its original shape. This improves its dimensional stability

However, OSB does not absorb water easily, but when it does, it takes a while to lose the water. Even after losing the water, it will take time for it to return to its original state. CDX and OSB are used for almost similar things, but since they have different functionality, one is best applied to some tasks and the other to other tasks.

The letter X

The letter "X" in CDX refers to the type of glue used at the factory to bond the plywood veneers. Some people mistakenly think that the "X" stands for "outside" - which is not exactly true. The letter "X" stands for exposure, which means that the plywood is strong enough to withstand some moisture, but only for a short time.

For example, CDX plywood would be a good choice for laundry rooms, bathrooms, or any other place that might have occasional dampness or high humidity. Keep in mind that this type of plywood is water resistant, not waterproof, and can still be damaged by a leaky bathtub or toilet.

CDX plywood is often used under shingles and roofing felt, on walls (just behind sheathing and insulation) and as a subfloor just under the carpet or concrete of a tiled floor. Although it's easy to find at your local home improvement center, it's probably not the best choice for smaller projects, like a bookshelf or other furniture around the house. However, I've used it to create rough shelving in a basement or garage, and it works quite well for the types of projects where function is more important than looks.

However, be careful not to confuse the words "exterior", "exterior" and "pressure treated" to all mean the same thing. In many cases, they are not. Some types of exterior plywood can withstand years of direct exposure to rain and snow, while others cannot.

Pressure-treated plywood, which is saturated with chemicals to repel mold and insects, can survive outdoors for decades without paint or a protective finish.

Keep in mind that pressure treated plywood tends to corrode and destroy metal up to five times faster than untreated wood. This means that you will need to use hardware and fasteners specifically designed for pressure treated lumber. Look for fasteners labeled "outdoor," "galvanized," or "stainless steel."

ACX plywood

Among the many types of plywood available is ACX plywood. It is a type of very high quality softwood plywood. Although it's not a hardwood plywood, it looks like it's trying to compete. It is the highest grade of commonly manufactured softwood plywood, with a face surface that can be used for furniture or architectural applications where the wood is exposed and stained and varnished or painted. It is basically cabinet grade softwood plywood. There is also an AB softwood plywood rating, but it is rarely found.

The name "ACX Plywood" does not refer to any specific type of tree or wood, although all softwood plywood is made from some variety of pine, fir or spruce. The name refers to the quality of wood and glue used to make the plywood product.

BCX plywood

BCX plywood has a nice well sanded face and a less finished back face. It usually contains C-grade inner layers and is bonded with outer glue. BCX is available in a variety of thicknesses. The thickness of this plywood is normally measured in 32nds of an inch; standard sizes being 1/32" less than nominal thickness. So 3/8" plywood is actually 11/32", ½" plywood is actually 15/32", plywood 5/8" plywood is actually 19/32" and ¾" plywood is actually 23/32".

These utility panels are perfect for work buildings, farm construction projects, truck and boxcar liners, and similar projects. You can make painted projects out of them, especially in cases where only one side needs to be shown. They are an excellent base for the application of exterior coatings. You may find that these panels have been pressure treated; but if so, they will be specifically marked as such.

CCX plywood

CCX plywood is unusual and hard to find. Nevertheless, it is a standard grade plywood, often made with pressure treated softwood. This type of plywood has more defects than BCX plywood, and the face and back have the same characteristics. Just like BCX, CCX plywood comes in a wide variety of thicknesses, normally contains grade C inner plies and is glued with outer glue.

PTS plywood

In many construction projects, ACX or CDX plywood can be used as a sheathing or underlayment. While this works well, voids in the center layers of plywood can cause premature sponginess or delamination of the wood, especially where high floor loads are expected or where some floor flexibility is required.

The solution is to use a plywood product with no voids. One option for this is to use marine grade plywood. Another option is PTS plywood, which stands for plugged, touched and sanded. This refers to the way the plywood was made. During this process, all knot holes are cut out, plugged and then finished to match the surface area and thickness of the rest of the veneer layer. This extra effort provides a superior plywood product for underlayment, where you don't have to worry about voids or unevenness in the sheets.

GC plywood

If you need a plywood that is more moisture resistant than normal exterior grade plywood, you may want to consider GC plywood. This rating refers to plywood intended for actual "ground contact", hence the name. GC plywood is always pressure treated, protecting it from decay and decay, making it truly impervious to the normal rot that occurs in wood in contact with the ground.

The protection applied to GC plywood also protects it from termites, making this plywood and able to have direct contact with the ground. While other wood left in contact with the ground will be attacked by termites, GC plywood will not.

Typically, GC plywood will be stained, due to the rosin used to pressure treat it.

WBP plywood

Water Boil Resistant Plywood

When looking for the ultimate in weather resistant plywood, it is important to seek out 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 reality, this is more of an assessment of the glue used in the manufacture of the plywood than of the wood itself.

Please note that WBP is not a separate type of plywood; rather, it is a rating that some plywood will have, if the manufacturer chooses to have it tested to determine if it qualifies. This is most often done on marine grade plywood, but it can also be done for other types of plywood intended for use in damp or wet environments, such as phenolic plywood.

WBP plywood is not a waterproof plywood. It is made from the same type of wood veneer as other types of plywood. This rating does not mean that it is pressure treated or resin impregnated like lumber and plywood which is meant to be truly weather and water resistant, although it could be. The rating applies only to the adhesive resin used to hold the wood veneer layers together during the manufacture of the plywood.

WBP plywood is also not the same as marine grade plywood, although marine grade plywood can also be graded MBP. What makes marine plywood unique is that it is guaranteed to be 100% void free, free of knots and cracks. WBP plywood may still contain some, although WBP marine grade plywood does not.

RTD Plywood

RTD plywood is the next generation of plywood specifically designed for the construction industry. As with WBP plywood, the letters R, T and D do not refer to the quality of the plywood, but rather to an important part of the manufacturing process. Specifically, it refers to plywood that has been fabricated using a resistance temperature detector. This is a device used during the manufacturing process that provides a very accurate reading of the temperatures reached during the bonding process.

How is plywood made?

So know that we know how plywood is graded, how is it made? Plywood is made by gluing together thin layers of wood veneer. In cases other than hardwood plywood used for furniture and cabinetry, this veneer is peeled from a log into a single continuous sheet, rather than sawn as boards are. However, hardwood plywood often uses sawn veneer for facing layers.

Before the layers of wood veneer are glued together, they are soaked for up to 40 hours. This is done in order to soften the wood which makes the pressing process much smoother. Various sheets of veneer often need to be laid together, butted against each other to eliminate or minimize gaps. To increase strength and minimize shrinkage, each layer is laid perpendicular to the previous layer.

All plywood has a minimum of three veneers, although more is preferable, especially as the plywood gets thicker. The more veneers in a piece of plywood, the stronger it will be. These layers, along with the glue, are heated and pressed together to form a tight bond. The type of glue used is what really dictates the use of the plywood.

Plywood R-Value

Is plywood a good insulator? The short answer is “no”, at least not in relation to products designed to insulate against cold and heat. Even so, plywood products offer minimal R-value, as well as structural advantages. Most modern structures contain plywood or OSB, so knowing these R-values can help you calculate the R-value of your home's walls, floors, and attic.

In general, one inch of solid wood is considered to have an R-value of 1.

The general R-value of plywood per inch is 1.25.

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