Is OSB Plywood Water Resistant?

When building residential wood structures, a good home builder will invest a lot of time in choosing the materials that will best serve future homebuyers. Sometimes, the cost of materials will be a primary concern, forcing the homebuilder to consider the cheapest alternatives. Other times, the focus might fall on materials that are more practical to a given building scenario.

For the longest time, there were few alternatives to good plywood. In recent years, the builder use of oriented strand board, or OSB has gained in popularity. Depending on where the actual building process is taking place, there might be a need for the homebuilder and future homeowner to be concerned about the waterproofing of building materials. That would especially be true if the development was taking place near the ocean, in a high humidity region, or where rainfall is a legitimate concern.

The question at hand is OSB Plywood water-resistant? In the following sections, the focus of the discussion will be to answer that question.

What is Oriented Strand Board, or OSB?

Oriented strand board (OSB) is a type of engineered wood that constitutes a cross between particleboard and plywood. OBS is formed by adding strong adhesives (wax and synthetic resin adhesives) and then compressing multiple layers of wood strands (flakes) into specific sizes of material.

From a structural standpoint, OSB is at least as strong and durable as plywood, if not stronger and more durable. On the downside, OSB is a bit more expensive than both particleboard and plywood simply because of the cost of materials needed for the manufacturing process. Also, the manufacturing process itself takes more time, which adds to the cost.

Is OSB Plywood Water Resistant?

Keeping in mind that the finest OSB is created by compressing and gluing wood flakes together to form a strong and durable surface, OSB panels have no structural internal gaps or voids. At a very basic level, it makes OSB naturally water-resistant. At the same time, it is impermeable because of the need to also create membranes for additional structural strength. What does that mean to a builder?

Because OSB has impermeable properties, it is water-resistant to a point. From a practical standpoint, it would be water-resistant enough for an interior building project where the only real exposure to moisture would come from humidity in the air. That being said, OSB is not recommended for exterior building projects without the addition of other waterproofing materials.

How to Waterproof Your OSB Plywood for Exterior Use

If you heed the warnings, you will want to provide extra waterproofing to any OSB materials you plan to use on the exterior of your building project. It is not difficult to do your own waterproofing, but you do need to follow the right procedures. To aid in your waterproofing of OSB, you can follow these step-by-step instructions.

Step 1 - Verify the Need for Waterproofing

Sometimes, OSB manufacturers will go ahead and apply extra waterproofing in anticipation of the materials being used in an exterior building project. If the manufacture decides to do this, they will stamp the materials as "waterproofed." If you purchase OSB without an official stamp, you should move forward with your own waterproofing.

Step 2 - Cut Your Pieces to Scale

Before applying any extra treatments to wood, you should go ahead and size up and cut your pieces. This will prevent any potential damage to the treatment process. By cutting OSB, you will be opening some of the aforementioned membranes, which will reduce the water-resistance of the OSB. After cutting, place your material in a safe, clean, and dry area.

Step 3 - Paint or Stain the OSB

If you plan to paint or stain the OSB, you would certainly want to do that before applying your waterproofing materials. Both oil-based and latex-based paints will be fine with OSB. The most important part of the painting process is to make sure the paint or stain has fully dried before starting the waterproofing process.

Step 4 - Apply Waterproofing

When it's time to apply your waterproofing materials, you will want to pay particular attention to the materials you purchase and use. Many sealers are multi-surface sealers, which work well for porous building materials like bricks and concrete. However, they don't work so well with OSB. You want to find a waterproofing product that the manufacturer has developed specifically for hardwood surfaces.

Each piece should be methodically painted with a waterproofing material on all sides. Most waterproofing products take at least 12 hours to dry. For optimum protection against moisture, you should apply at least two coats.

Step 5 - Testing

To test for success, take a piece of leftover scrap and apply the waterproofing product. After letting it dry, pour some water on it and let it stand for at least two hours. The results you want would include zero absorption of water. If successful, you are good to go. If not, additional coats of waterproofing would be necessary.

Precaution when building is always the best approach. Regardless of what an OSB manufacturer might say about the product being water-resistant, the time and money you invest in additional waterproofing of materials for exterior building should give you the peace of mind you want.