What is the difference between CDX plywood and OSB plywood?

You don't have a to an expert in the industry to tell the difference between the two products so we'll keep it simple. OSB, Oriented Strand Board, is composed of chips pressed together with resin. CDX is what many people consider “real plywood” and is 3-to-8 thin plies of wood layered to form one sheet.

croatancustomhomes.com - Structural Panels: OSB vs Plywood - Croatan Custom Homes

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Which is better, OSB or CDX? It’s a question that is often asked and hastily answered. The reference point of the one answering is as important as their answer. It most often comes down to background and personal preference, not facts or statistics. This post will identify the pros and cons, characteristics, and studies of OSB and CDX. You don’t have a to an expert in the industry to tell the difference between the two products so we’ll keep it simple. OSB, Oriented Strand Board, is composed of chips pressed together with resin. CDX is what many people consider “real plywood” and is 3-to-8 thin plies of wood layered to form one sheet. The composition of the product effects its performance.

Characteristics:

OSB – OSB is machine regulated and very consistent. More so than CDX. It doesn’t have the inconsistent layers, knots, or defects. It is denser. This is a main argument for its uses as decking material for your home. You don’t get the weak parts of a sheet which causes “give” or the squeaks that occur over time. -It typically has higher shear rating making it a good choice for walls. -Can be manufactured in 9’ or 10’ panels to better tie in the floor boxes.

-OSB also absorbs water at a lesser rate. Which has advantages.

Unfortunately, the flip side is it actually holds onto moisture longer. In the past there have been issues with the sides swelling due to this absorption (and OSB has issues with returning to original thickness once the moisture evaporates). This was once a larger concern than it is today. Massive complaints have driven manufactures to improve the process and change this. CDX – CDX dries out faster. Meaning it is less susceptible to rot. It can and should be used as underlayment for tile or flooring. Although OSB can still be used, it must be thicker material to acquire the same rating. Dampness may cause swelling and separation causing poor adhesion. Additionally, the flooring contractors I use do not recommend OSB as an underlayment. The durability of CDX has made it a favorite of many for roof applications. The idea is it doesn’t show the “ghost lines” or waviness that OSB sheets had once shown on roofs.

Similarities

When it comes to wall sheathing OSB and CDX are equal in regard to “nail holding and rack resistance” according to Kevin Chung an engineer for Western Wood Product Association. On average OSB is 7% less stiff than CDX, however, OSB is stronger than CDX in shear according to a study done by University of Massachusetts. Additionally, when the APA, The Engineered Wood Association, rate plywood, it rates them as equivalents. (As they rate spans, not whether it’s OSB or CDX.) With all that said, many users are beholden to their product. From my experience and research, you can find one study that gives OSB the advantage and then another that gives CDX the advantage. With data that is very close.

Price

When it comes to price, OSB is a clear winner. It is almost always more affordable than CDX. At time of this post OSB is approximately 25% less per sheet. As you can see these products have their advantages. People have their opinions, including myself, and love to voice them, but in my research, one is not far and away superior. Studies and tests prove, they each have their strengths and are generally equivalents.

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What grade of plywood is CDX?

CDX Plywood is used primarily by contractors to build exterior walls and roofs. CDX plywood has one side veneer grade “C” and one side veneer grade “D”. The two are bonded together with glue which can withstand a little moisture. The APA would recognize CDX Grade Plywood as C-D Exposure 1 plywood.

roof.atlasrwi.com - CDX Plywood 101 - Atlas Roofing Insulation

The letter “X” in CDX refers to the type of glue used in the factory to bond the plywood veneers. Some mistakenly think the “X” stands for “exterior”—which is not true. The letter “X” stands for exposure, which means the plywood is tough enough to withstand a little moisture—but for only a short time. Eventually, the panel must be covered by something more weather resistant—like bricks, siding or shingles. CDX PLYWOOD CDX Plywood is used primarily by contractors to build exterior walls and roofs. CDX plywood has one side veneer grade “C” and one side veneer grade “D”. The two are bonded together with glue which can withstand a little moisture. The APA would recognize CDX Grade Plywood as C-D Exposure 1 plywood. CDX DOES NOT AUTOMATICALLY MEAN PRESSURE-TREATED Be careful not to confuse the words “outdoor,” “exterior,” and “pressure treated” to all mean the same thing. Pressure-treated plywood, which is saturated with chemicals to ward off mold, mildew, and insects, can survive outside for decades without paint or a protective finish.