An alternative—and some say the replacement—for plywood is oriented strand board, or OSB. While both are made of particles of wood that have been pressed together, OSB is created when strands of young wood are mixed with resin to form a very hard and pliable board. This material is also often waterproofed before being sold in stores, as indicated by a stamp on the back side. However, if you're planning on using this wood for an exterior project, you will need to add additional moisture protection. Follow the steps below to find out how you can do this before starting your next building project.
Step 1 - Make Sure Your OSB Is Waterproofed
As stated above, you will want to check that your OSB is stamped with "waterproofed" when you buy it. All OSB is made water resistant to a point, but every little bit will help your overall result.
Step 2 - Plan and Cut Your Pieces
Every time you make a cut, any sealer at the edges will disappear and open up a new place where moisture and water can be absorbed. Therefore, you will need to make sure your OSB is cut into the pieces you will need before you add your waterproofing so these edges can be protected. Measure, mark, and cut all the pieces you need, and set them aside on the tarp. You don't want grass or dirt to get in your waterproofing either.
Save a scrap piece of OSB at this time as well. You'll want to treat it like the rest of your wood so you can use it to test the success of your waterproofing later.
Step 3 - Paint if Desired
A good exterior paint can actually add to your OSB's water resistance, as well as add a splash of color to your project. If you intend to paint your final project at all, make sure it is added before any waterproofing so the sealer can also protect your top coat. Oil-based and latex-based paints will both work in this case, but be sure you purchase a high-quality exterior variety to use.
Start by sanding the surface lightly, just enough to rough up the existing sealer for better adhesion of the paint. Then, brush your paint on the surface of your pieces. Allow each area to dry for the time provided on your product's label before you flip the pieces or add additional coats, and let the last coat dry completely before moving to the next step.
Tip: You can also stain OSB for a more natural look. Choose a water-resistant type to get an added layer of protection similar to paint.
Step 4 - Apply Waterproofing
A high-quality, exterior waterproofing solution can be purchased from your local hardware or home improvement store. The product you use is up to your own personal preference; however, be sure that what you choose is made for use on wood. Many sealers are multi-surface, but others are specifically designed for other porous surfaces like concrete or brick.
Soak the tip of your paintbrush in your sealer and begin brushing it over the surface and on the ends of the OSB pieces. Make sure you repeat this process on each side, not just where your cuts were made.
Coat the pieces as they lay on the tarp, and then let the waterproofing sit to dry for 12 to 14 hours. Carefully turn the wood over, and coat any remaining surfaces. You will likely want more than one layer of waterproofing to ensure the wood stays protected; just let the solution dry for another 12 to 14 hours between each coat. If you live in a particularly humid climate, your dry times may be longer.
Step 5 - Test
To make sure you have put enough sealer on your OSB, pour some water on your scrap piece from earlier and let it sit for a couple of hours. If it has not soaked in after that time, chances are you have finished your job successfully. If not, you will need to apply two to three more coats and let them dry before you test again.
OSB is said to be more water resistant than plywood by some and not so by others. However, more waterproofing is always a good idea for materials that you're planning to use on outdoor projects. After following these steps, you will never risk moisture ruining your work down the road.
OSB Waterproofing FAQ
What is the best waterproofing for OSB?
Waterproof sealer made for plywood works very well to coat OSB with a protective layer. Simply paint this sealer onto OSB and give it a waterproof finish.
Can you paint OSB to make it weatherproof?
Oil- and latex-based paint can be used to add color and design to OSB. If you use exterior paint, it can add another layer of weather resistance.
However, paint will not make OSB weatherproof. To truly waterproof and weatherproof OSB, you will want to cover it with a sealer once you're done painting it and have it looking the way you want it to look.
What happens to OSB if it gets wet?
OSB tends to swell around moisture. Usually, however, the boards only swell around the edges and the swelling does not really cause damage.
It does take OSB a long time to dry once it is wet, usually hours and sometimes days depending on how wet it gets and the environment it's in.
Can you put polyurethane on OSB?
Polyurethane works well as a sealer for OSB and gives the material a glossy finish. The polyurethane coating adds a layer of waterproofing protection to the OSB.
Is there waterproof OSB board?
OSB board is treated with a water-resistant wax coating and then the edges of the panels are sealed to provide protection from the elements. This material stands up well to water and outdoor conditions as-is, but you can finish OSB with a sealer to make it even more waterproof.