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The Right Plywood and Why

Plywood is a versatile building material that has been used for construction projects for many years. It is made by bonding layers of wood veneer together to create a strong and durable sheet of ply-wood. Plywood comes in many different types and grades, each with its own unique properties and use cases. In this blog post, we will explore three types of common plywood: OSB, CDX, and ZIP Board. OSB (Oriented Strand Board) plywood is made by mechanically compressing wood chips together with a resin binder, while CDX plywood is made from multiple layers of mechanically pressed and bound wood veneer. ZIP Systems plywood, on the other hand, is a type of sheathing that combines a moisture-resistant barrier coating with the structural strength of OSB plywood. We will go over the pros and cons of each type of plywood, as well as their ideal use cases and the potential issues to be aware of. By the end of this blog post, you should have a better understanding of which type of plywood is best suited for your construction project and why.

Using a strong plywood when building is essential for ensuring the safety and longevity of the structure being built. Plywood is a widely used building material due to its strength, durability, and versatility. It is commonly used for sub-flooring, walls, roofs, and other structural elements of a building. When building a structure, the plywood used must be strong enough to withstand the weight, pressure and weather it will be subjected to over time. A weak or low-quality plywood can cause structural issues, leading to severe damage, racking, or even the collapse of the building structure. In addition to providing structural support, plywood also serves as a barrier against environmental factors such as wind, moisture and external temperatures. If the plywood used to build a structure is not strong enough, it can warp, crack or deteriorate over time, leading to further damage to the structure.

Using a strong plywood will ensure that the structure meets the safety standards and regulations set by local building codes as well as home insurance providers. Architectural engineers and building regulations often require the use of specific types, thicknesses and grades of plywood and specify the minimum strength and durability requirements for the builder to help ensure the safety of the building and its occupants under severe weather conditions. A strong and durable plywood can withstand the weight and pressure of the severe weather events that the structure will be subjected to over time, prevent racking of the structure in these events, serve as a barrier against environmental factors such as high wind and meet overall safety standards and regulations ensuring your investment isn’t just rotting away behind the stucco or siding. 

OSB, CDX, and ZIP are all types of plywood, but they have some key differences in terms of their composition, strength, intended use, lifespan and cost so please take all of these factors into consideration and weigh them equally. 

OSB (Oriented Strand Board) is made from strands of wood that are compressed and bonded together with a resin. It has a rough surface and is typically used for sub-flooring, wall and roof sheathing.

CDX plywood is made from thin layers of wood veneer that are glued together, with each layer oriented perpendicular to the previous one. The "C" in CDX stands for "grade C," which means that the plywood has some knots and other imperfections but is still structurally sound. The "D" in CDX stands for "grade D," which means that the plywood has some surface defects but is still suitable for shell or structural framing construction use. CDX plywood is also commonly used for roofing, walls, and subfloor sheathing in construction.

ZIP System is a type of sheathing that combines OSB with an integrated weather-resistant barrier. This product is designed to reduce the time and labor required for the installation of a Moisture Vapor Barrier (MVP) building envelope material by eliminating the need for a separate sheathing and house wrap system such as Tyvek. The advantages of ZIP Systems are that they provide superior moisture resistance through the exposure stages of construction, added air barrier performance, and added overall thermal protection compared to other types of standard plywood when properly installed. It is sometimes more durable and stronger than traditional sheathing and house wrap systems, due to its integrated design for weather and moisture resistance through construction. ZIP Systems are easier and faster to install due to a step being removed, which can save time and labor costs. ZIP System plywood is often used in residential and commercial construction as a more efficient alternative to traditional sheathing and house wrap systems.

Pros and Cons

One of the only advantages of OSB plywood is its affordability, making it a popular choice for builders on a budget and tract home builders. It is fairly strong and durable, with below average moisture resistance but has the ability to withstand fairly harsh weather conditions. Of all the plywood types it is of note that OSB plywood is more prone to water absorption, swelling and warping than other types of plywood when exposed to damp climates and heavy rains which can affect its structural integrity over time. Formaldehyde off-gassing can also be a concern with OSB. Formaldehyde is used in the manufacturing process of OSB plywood as a binder or adhesive. Formaldehyde off-gassing occurs when the formaldehyde in the plywood evaporates and is released into the atmosphere. This can be of concern because formaldehyde is a known carcinogen and can cause respiratory problems and irritation when inhaled. OSB plywood like particle board or MDF has been known to emit formaldehyde at levels that exceed recommended safety standards, especially in poorly ventilated spaces such as wall cavities. The off-gassing can also be higher in humid or warm conditions and should be a factor in your decision making.

The ZIP Systems main feature is being a type of plywood sheathing that is designed to provide a moisture-resistant barrier for buildings. However, if the ZIP System is not installed properly (which it rarely is), it can lead to even more leaks and more water damage especially when used under lathe and stucco or rainscreen systems as it offers a false sense of moisture vapor security and protection.

One of the ways that the ZIP System can and often does fail and rot out is if the nails used to secure the sheathing to the framing members are set too deep into the plywood. This happens as soon as the nail head breaks the very thin paint like waterproofing membrane surface of the Zip system which happens almost every single nail when the framers nail gun pressure is set too high. This is very common and typical in production framing. When the nails are set too deep, they can puncture the waterproofing barrier of the ZIP System and create exposed pockets that then allow water to enter the building structure ultimately causing swelling and rot of the (OSB) ZIP system and other structural elements. This is why it is important to use the correct type and length of nails and to follow the manufacturer's very detailed guidelines for installation. If the manufacturers guidelines are not followed the products waterproofing warranty is void and your structure is at severe risk due to the lack of a proper moisture barrier system. 

Another way that the ZIP System can leak is if the seams between the sheathing panels or structural strapping over the Zip panels are not properly taped and rolled out. This Zip Tape System is "pressure activated" so just putting it on isn't enough. If the Zip Tape is not properly rolled out and compressed you will most certainly have leaks and water intrusion as the adhesive membrane on the tape was never "activated." The seams between the panels are all a potential entry point for water into the structure, and if they are not sealed correctly, water can then roll into the structure through all these gaps. This is why it is important to use the correct type of ZIP Tape and to ensure that the tape is properly adhered to every seam and over every strap and rolled out to create a tight seal with no bubbles or wrinkles in the ZIP Tape. Every Zip Tape roller has small Z's on the roller which imprint on to the tape so that you can see if your Zip Tape was or was not rolled out properly. When the ZIP System leaks, it can result in water damage to the structure and potentially lead to the growth of mold and mildew along with the potential for structural rot. This can be costly to repair and can cause health problems for occupants of the building so please take your time installing if you choose a Zip System. 

When it comes to CDX the main advantages of CDX plywood is its structural strength and longevity. The multiple layers of wood veneer make it stronger than OSB plywood, and it can withstand heavy loads and severe storm pressures. CDX plywood is also more durable than OSB plywood and is less prone to swelling and warping. This durability makes it an ideal choice for roofing, walls, and floors in construction projects. Another advantage of CDX plywood is its resistance to moisture with a simple backpack spray on sealer application. It becomes highly water-resistant and can withstand humid environments without losing any structural integrity through swelling and deterioration like OSB. This makes it suitable for construction projects in areas with high humidity or rainfall and long rainy seasons. CDX plywood is also stronger than OSB plywood due to its multiple layers of wood veneer. In terms of cost, CDX plywood is more expensive than OSB plywood but often less expensive than ZIP System plywood on a per sheet basis. CDX plywood will however require a house wrap and type system such as Tyvek over the plywood for a water tight envelope. The superior strength and durability of CDX however make it a worthwhile investment for construction projects that require a strong and reliable building material.

It is difficult to definitively say which type of plywood is the best as it largely depends on the specific needs, budget and requirements of the specific construction project. However, CDX plywood is often considered the most reliable while OSB is the most cost-effective option for construction projects while Zip is a new system that must be installed per manufacture specs which can integrate a risk and warranty factor. When selecting a plywood for a construction project, it is important to consider factors such as the environmental conditions of the construction site, the weight and pressure that the plywood will be subjected too, and the budget available for the project. By taking these factors into consideration, builders can select the best type of plywood for their specific needs.

If you want to know what we use here at Shell Basix for shell construction it’s simple. We don’t want the cheapest, we want the strongest. We always push our clients towards APA rated CDX on all roofs and walls and on all subfloors we use AdvanTech. We then look to wrap our projects with a Commercial Grade Tyvek or VP100 for a superior envelope. Investing a few thousand dollars more in a construction project can significantly impact its longevity and durability. By choosing to prioritize quality and integrity over the bottom line, you can ensure that your structure will stand strong for a century or more, unlike many of the homes being built today. Unfortunately, we have already observed that many of these homes built for the bottom line will not even endure the duration of their mortgages ultimately burdening future generations with a legacy of deterioration and decay they will not be able to maintain or sustainably rebuild. By selecting higher-quality materials and construction methods, you can contribute to a more sustainable future and protect your investment in the long run.

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