Is Cat Rated Sheathing Treated

What is OSB Sheathing?

OSB (Oriented Strand Board) sheathing is a common structural panel made of compressed wooden strands. According to Inspectapedia, “OSB panels are made of compressed rectangular shaped wooden strips compressed and bonded together with resin, wax and heat into sheet goods in layers that are cross-laminated for strength.”

The wood strands are compressed and bonded together in specific orientations. The outer layers orient the strands parallel to the panel’s strength axis, while the inner layers orient them perpendicularly. This cross-oriented construction gives OSB panels their characteristic strength and rigidity. (

OSB sheathing is widely used during construction to provide structural integrity to exterior walls, floors, and roofs. It adds shear strength and diaphragm stiffness to the frame and allows the structure to resist lateral loads from wind or earthquakes. OSB panels are a cheaper alternative to plywood sheathing and can be used in a similar manner. They are easy to work with using standard tools and fasteners.

OSB Sheathing Types

There are two main types of OSB sheathing: standard OSB and treated/weather-resistant OSB. They differ in their composition and intended applications.

Standard OSB is composed of wood strands bonded together with adhesives under high heat and pressure. It has good structural strength but is not meant for exposure to moisture and weather. Standard OSB is used for sheathing walls, floors, and roofs in dry areas protected from the elements by exterior siding or roofing.1

Treated or weather-resistant OSB has additional chemicals mixed into the wood strands or applied to the surface to improve its water resistance. Examples include water-repellent wax, asphalt emulsion coating, zinc borate treatment, and dip-treating strands in phenolic resin. Treated OSB can withstand humidity, rain, and other weather exposure before final exterior finishing is installed. It’s used for structural sheathing directly behind vinyl, stucco, brick, and other siding.2

Treated vs Untreated OSB

OSB (oriented strand board) sheathing comes in both treated and untreated varieties. The key differences between the two are:

  • Treated OSB contains chemicals that help protect it from rot, fungi, termites, and other pests. Untreated OSB has no added protection.
  • Treated OSB costs 20-30% more than untreated on average (Source:
  • treated osb costs more than untreated

  • Treated OSB has an extended lifespan compared to untreated when exposed to moisture and other harsh outdoor conditions.

The main benefits of using treated OSB sheathing are:

  • Increased weather, water, and pest resistance.
  • Longer lasting and more durable where moisture exposure is a concern.
  • Meets code requirements for sheathing in direct contact with concrete and masonry.

It’s recommended to use treated OSB when:

  • The sheathing will have direct contact with concrete, masonry, or soil.
  • There is a risk of termites or other wood destroying insects.
  • use treated osb to resist pests and decay

  • The structure will be exposed to excessive moisture or damp conditions.
  • Maximum durability and lifespan are desired.

Untreated OSB is suitable for dry, protected areas like interior walls and roof sheathing with proper moisture barriers. It provides adequate performance at a lower cost in applications where moisture exposure is limited.

CAT Rated Sheathing

CAT (Category) ratings indicate the level of transient overvoltage resistance for electrical equipment like multimeters. CAT ratings are designated as CAT I, CAT II, CAT III, and CAT IV, with higher numbers indicating higher energy surges the equipment can safely withstand (CAT Ratings Explained).

CAT I equipment is designed for low energy electronic circuits. CAT II equipment can handle local power distribution levels like wall outlets and appliances. CAT III equipment is safe for distribution panels and main feeders. Finally, CAT IV is for very high power distribution near building service entrances (What do CAT ratings mean?).

Using properly rated test equipment reduces risk of injury or damage from transient overvoltages. Higher CAT ratings provide more protection, but the equipment is often larger, heavier, and more expensive. CAT III or IV ratings are recommended for testing power circuits, but CAT II may suffice for basic residential wiring (What do CAT Ratings Mean?).

Weather Resistance

One downside of using OSB as sheathing is that it does not have natural weather resistance to moisture like some other sheathing materials. Untreated OSB will absorb water quickly, resulting in swelling, warping, and potentially structural damage if exposed to rain or high humidity for too long (Source). The strands in OSB can expand up to 3% in size when absorbing moisture (Source).

To improve the weather resistance of OSB, manufacturers offer treated panels or panels with a sealed/painted surface. Treated OSB is infused with wax and/or resin during the pressing process to make it more hydrophobic. This helps repel water and prevents excessive swelling and warping. Special coatings or paint can also be applied as a protective barrier. Properly finished and installed, treated OSB generally provides adequate weather protection for most regions (Source).

In very humid climates like the Southeastern US, untreated OSB will need to be covered quickly after installation to prevent damage. Treated or sealed OSB is recommended for these regions. In dry climates, untreated OSB may provide sufficient weather resistance if protected by finishes. Consult local building codes for requirements based on climate. With proper protection, OSB can perform well as a structural sheathing even in wet environments.

Structural Strength

OSB sheathing provides excellent structural strength for load-bearing walls and meets building code requirements for shear strength. The cross-laminated structure of OSB gives it enhanced stiffness and strength compared to traditional plywood sheathing.

According to LP Corporation, Structural 1 rated OSB sheathing provides greater strength and stiffness than regular OSB or plywood for demanding portions of a build like shear walls. The panels have improved racking strength to meet corner bracing requirements with a low coefficient of variation (

OSB Structural 1 panels exceed the racking strength of plywood by up to 20% and provide up to twice the stiffness of standard plywood sheathing of equal thickness. OSB has about 2 to 3 times the transverse stiffness of plywood. The high strength and stiffness of OSB Structural 1 makes it ideal for shear walls and meeting lateral load resistance requirements (

Overall, OSB Structural 1 provides superior load-bearing abilities over standard plywood while meeting building code requirements for shear strength in wall systems.

Cost Comparison

OSB tends to be less expensive than plywood. According to, installing OSB sheathing over a 500 square foot area would cost around $100 in materials, while the same amount of plywood would cost about $160 in materials. The installation costs would be comparable for both OSB and plywood.

osb cheaper than plywood but doesn't last as long

When looking at treated versus untreated OSB, the treated OSB does come at a premium. According to Home Depot, a 4×8 sheet of 7/16″ untreated OSB ranges from $15-22, while the comparable treated sheet is $30-40. This is a significant difference in cost, but the treated OSB offers more weather protection and may be worth the extra investment for exterior applications or areas with high moisture.

While OSB is generally cheaper upfront than plywood, it’s important to consider the lifespan and performance differences. Plywood is more resistant to delamination and deterioration when exposed to moisture over time. The higher grades of plywood may lead to lower maintenance costs and better ROI in the long run for applications like roof sheathing, despite the higher initial price tag. Properly weighing the cost differences against the expected performance and lifespan is important when deciding between OSB and plywood.

Installation Tips

Proper installation is crucial for getting the full benefits of CAT rated OSB sheathing. Here are some tips for installing OSB sheathing correctly:

Use the right fasteners – Use galvanized nails or screws specifically designed for fastening OSB. Ring shank nails provide maximum holding power. Follow the manufacturer’s recommendations for fastener type, length, and spacing.

Seal the panels – Be sure to seal all edges with a high quality caulk or foam sealant. This prevents moisture intrusion and air leaks. Pay special attention to sealing around openings like windows and doors.

Stagger panel joints – Staggering the vertical seams between OSB panels adds strength and rigidity. Avoid aligning seams on adjacent rows.

Fill gaps – Inspect for any gaps between panels or around openings. Use lumber blocking and/or sealant to fill any gaps larger than 1/8 inch.

Allow for expansion – Leave a 1/8 inch gap between panels and any abutting surfaces like floors, ceilings, or framing. This allows room for expansion and contraction.

Start at the bottom – Begin installation from the bottom up. This prevents moisture from getting trapped between panels.

Following the manufacturer’s recommended installation techniques is key to maximizing the weather-resistance and structural performance of CAT rated OSB sheathing. Taking steps to properly seal and fasten the panels will help avoid issues down the road.


OSB sheathing requires some basic maintenance to keep it in good condition and extend its lifespan. It’s important to periodically inspect the OSB boards for any signs of damage or deterioration.

Look for cracks, holes, delamination, bowing, warping, and excess moisture. Small holes and cracks can be repaired with exterior wood filler. For larger holes, damaged boards may need to be replaced. Take care of minor damage right away to prevent further deterioration.

It’s also critical to ensure OSB stays protected from moisture. Apply exterior paint or sealant to create a water-resistant barrier. Check areas around windows, doors, joints, and seams where water could penetrate behind the siding. Keep gutters clean and make sure water drains properly away from the structure.

Storing OSB boards properly before installation is also key – keep them off the ground on a flat surface and covered with a breathable waterproof cover. With periodic inspections and proper care, OSB sheathing can last for decades.


OSB is a common building material, but there are still some common questions and myths surrounding it. Here are some of the most frequently asked questions about OSB:

What is the difference between OSB and plywood?

The main difference is that OSB is made from wood strands, while plywood is made from thin sheets of wood veneer. OSB tends to be cheaper than plywood, and has comparable strength and performance for many applications like sheathing and roofing. However, OSB does not have the same smooth surface as plywood.

Is OSB waterproof?

Untreated OSB is not waterproof, and will swell and deteriorate when exposed to moisture over time. However, some types of OSB are treated to be water-resistant or waterproof, such as OSB sheathing rated for exposure to the elements. Look for products advertised as weather or moisture resistant.

untreated osb not waterproof, can deteriorate

Can OSB be used for load-bearing walls?

Yes, OSB has sufficient structural strength to be used as sheathing on load-bearing walls when properly installed. Consult local building codes for exact specifications.

Does OSB contain formaldehyde?

Most OSB does contain some formaldehyde resin, which is used as a binder during manufacturing. However, modern OSB products meet stringent formaldehyde emission standards for indoor use.

For more information, check out these additional resources:

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