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OSB vs. CDX: Which Base Layer is Right for You?

“A three strand cord is not easily broken.” While this is speaking to the fact that there is strength in numbers, I am going to argue that those three strand need to be the highest quality they can to make the cord as strong as possible. The same is true when building home. While most people only see the finished product and do not give much thought to the steps it took to make that completed house, making sure each step is done properly and with the best quality materials possible is essential in ensuring a well built home. After your foundation is done, you frame your walls and put on your roof trusses. Next, you sheath the exterior of your home. This is a very important step and the quality and type of material you use is crucial in preventing future damage!

Sheathing gives your home rigidity and stability. It is the process of covering your studs to tie them together. This prevent them from twisting and bending, provides a nailing base for your exterior finish, and gives a layer of protection against exterior elements. While there are actually several materials you can use to sheath your home, the post popular are wood-based products, OSB And CDX plywood. While OSB is the top selling product by far, it does not mean it is the best choice. Let’s walk through the pro’s and con’s of each product and why we went with the one we did.

OSB- Oriented Strand Board are made from logs that are ground into thin wood strands to produce oriented strandboard. The dried strands are positioned as alternating layers that run perpendicular to each other, then combined with wax and resin, and formed and baked into plywood type sheets. Because OSB can be made from small, fast growing trees that can come from tree farms rather than forests, resource conscious builders prefer it. It can also be manufactured in lengths up to 16′, sometimes even longer whereas plywood can only done in 8-10′. While OSB can pack 50 strand layers into a sheet, plywood only has 5-7, giving OSB a more consistent density, though it does about two pounds or so more per sheet then plywood.

Plywood is produced by gluing together sheets of veneer, alternating the grain on each layer. Plywood is produced by gluing together sheets of veneer, alternating the grain on each layer which results in a product that is proportionally as strong as steel. So if both products are equally strong and OSB is cheaper and comes in more convenient sizes, it seems like an easy choice, right? Not so fast! There is one major element we have not covered yet, water! When building a home, water is one of the number one things you are combating. Not only do you have to think about the fact it may rain, snow or ice during construction, you also have to think about long term exposure to humidity or any flaws that may occur during the construction process which may allow moisture to reach your sheathing. Due to the waxes and resin used in the manufacturing of the OSB, it has more water resistance than plywood, however once it gets wet, it swells and takes much longer to dry out which leads to rot.

Plywood, on the other hand, swells consistently and dries quickly, returning to normal dimensions. OSB is also known for swelling along the edges when it gets wet which can cause an uneven subfloor and “ghost lines” on an asphalt roof shingles. Though manufacturer’s claim they now coat the edges in water-resistant seals to prevent this, because boards are commonly cut to fit the structure, there are still exposed edges. Because we are building in Florida, where we get an average of 59.21 inches of rain, we knew we could not risk using OSB during our build process. We had so many torrential downpours over the past 6 months, if we had used OSB anywhere, we would be dealing with some major swelling issues right now. However, it is not just during the building process you have to worry about. Long term constant humidity can affect the OSB and if there happened to any flashing or protective coverings installed incorrectly, OSB will not hold up as well as plywood. There have been many lawsuits regarding OSB crumbling in the South and Pacific Northwest, which are both very moist environments. While we plan on installing our protective coverings perfectly, in the case of human error, we want to ensure our base layer does not crumble away! So for us and our geographical location, plywood was an easy choice!

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