Is OSB Waterproof? No, but You Can Make It More Water-Resistant

Curious about whether OSB (Oriented Strand Board) is waterproof? Let’s dive into the details!

How Waterproof Is OSB?

OSB is not waterproof. It’s made of wood strands and adhesives, which can soak up water and cause the board to swell, warp, or even fall apart when exposed to excessive moisture.

To enhance its water resistance, make sure to apply proper sealer or waterproofing coatings.

Is 18MM OSB Board Waterproof?

18MM OSB boards are not waterproof. These are types of OSB with a thickness of 18 millimeters. Since this is a wood-based material, it can absorb water and be at risk of damage when exposed to moisture, no matter the thickness.

Which Side of OSB Is Water Resistant?

Neither side of OSB is naturally water-resistant. Since OSB is made of wood, both sides have similar properties when it comes to soaking up moisture.

If you leave OSB out in the rain, both sides can absorb water, leading to swelling, warping, and potential structural problems.

Is OSB Subfloor Water Resistant?

OSB used as a subfloor isn’t water-resistant by default. Like other wood products, OSB can absorb water when it’s exposed to moisture.

If your OSB subfloor gets wet or stays damp for a while, it might swell, warp, and lose its strength.

To protect it from moisture-related issues:

Ensure proper installation without gaps for water seepage.
Install a moisture barrier (plastic or waterproof membrane) between the subfloor and the finished flooring.
Apply waterproofing sealer or coatings on the OSB surface for added protection.

What Is the Best Way to Waterproof OSB?

The best way to waterproof OSB is to seal all the edges with a waterproof sealer, and then apply a waterproof coating to the entire surface of the OSB.

You can also consider using a moisture barrier, like a plastic or waterproof membrane, for added protection in areas with water exposure.

Seal All Edges

Start by sealing all the edges of the OSB board with a waterproof sealer. This helps prevent water from penetrating into the board through exposed edges, where it is most vulnerable.

Apply Your Waterproofing Sealer

Coat the entire surface of the OSB with a high-quality waterproofing sealer, paint, or coating.

This creates a protective barrier that repels water and prevents it from being absorbed by the wood fibers.

Some of the best waterproofing products recommended by DIY aficionados for use on OSB are Flex Seal Liquid Rubber Sealant and Semco’s Liquid Membrane.

Here’s a quick video on how to apply Flex Seal Liquid:

Check out this video on how to use Semco’s Liquid Membrane to waterproof OSB.

⇒ Important Tips:

Painting liquid rubber on OSB flooring can make it slippery.

Add a Moisture Barrier

If the OSB will be exposed to moisture regularly, add a moisture barrier, such as a plastic or waterproof membrane, between the OSB and the finished material (e.g., flooring or siding) to prevent any water from reaching the OSB.

Keep an eye on your OSB by doing regular check-ups for any damage or signs of wear on the waterproofing.

If you spot any issues, make sure to fix them right away to keep the waterproofing doing its job.

Remember, these steps can help make your OSB more water-resistant, but it’s not totally waterproof.

Avoid using it in super wet or exposed outdoor areas. In those situations, you might wanna go for other materials specially made for the great outdoors.

Can You Paint OSB to Make It Waterproof?

You can boost OSB’s water resistance by painting it. However, keep in mind that painting OSB won’t make it completely waterproof.

While paint can enhance its water resistance, OSB might still be vulnerable to water damage if exposed to excessive moisture or prolonged contact with water.

Is OSB More Water Resistant Than Plywood?

Plywood is generally more water-resistant than OSB. Plywood is made by gluing together thin wood veneers with their grains alternating at right angles, giving it better moisture resistance compared to OSB.

However, neither plywood nor OSB is fully waterproof. Both materials need proper protection and shouldn’t be exposed to water for extended periods.

What Happens if OSB Gets Wet?

If OSB gets wet, it can swell, lose strength, and become a breeding ground for mold and mildew.

Additionally, delamination can occur, and the risk of rot and decay might compromise the board’s structural integrity over time.

Remember, keeping OSB dry is crucial to maintaining its performance and longevity.

Can You Leave OSB in the Rain?

Leaving OSB exposed to rain or significant moisture for a long time is not recommended. Since OSB is a wood-based material, it can absorb water, leading to swelling and warping.

If you need to use OSB outdoors or temporarily expose it to rain during construction, protect it with a waterproof covering or tarp to avoid direct water exposure.

For long-term outdoor use or consistently wet environments, consider alternative materials designed for exterior use.

Can I Use Flex Seal on OSB?

Yes, you can use Flex Seal on OSB to enhance its water resistance. Flex Seal is a popular rubberized sealant that forms a flexible, protective coating on various surfaces, including wood.

Flex Seal can create a protective barrier, helping to prevent water from penetrating the OSB, reducing the risk of water-related damage, and extending the board’s lifespan.

Before applying Flex Seal, make sure the OSB surface is clean, dry, and free from debris. Follow the manufacturer’s instructions on the product’s label for the best results.

Can You Seal OSB With Polyurethane?

You can seal OSB with polyurethane to improve its water resistance and protect it from moisture.

Polyurethane is a durable and versatile sealant that forms a clear protective coating when applied to wood surfaces, including OSB.

A polyurethane sealer is an ideal coating for an OSB floor. Just make sure to apply it correctly for the best results!