Whether you’re building a new home or a shed in your backyard, you’ll likely be working with Oriented Strand Board (OSB) as one of your construction materials. OSB panels are created by using a thermal press to compress strands of young wood, which are layered in a specific pattern and bonded with a resin compound, making OSB water-resistant. However, like any wood product, OSB will warp and swell if too much water is absorbed, like after heavy rain.
OSB needs to be protected from prolonged exposure to rain and moisture. There are several ways to seal and protect the OSB board. These include using a weather-resistant wrap or painting the surface with a sealant.
So, how much rain is too much? How long can OSB be exposed to water before it becomes compromised? What are the different ways to protect OSB from the rain? And what are some tips to protect your construction as you’re building? This article answers these questions to help you make the best use of this strong and resilient material.
Do You Have to Protect OSB from Rain?
Unless you live in an arid climate, it will likely rain on whatever you are building before you’re finished it. And it’s also likely that your building site will sit for more than a day or two until the rain (or snow) passes. In other words, houses under construction routinely get wet before the frame and roofing are completed.
Fortunately, OSB for exterior use may already have some degree of waterproofing, as indicated by a stamp on the board.
However, if OSB gets too wet, such as with prolonged rain, the board will swell and then shrink with drying, especially around the edges. Additionally, like with any wood or wood product, mould can begin to grow on OSB within 48 hours with enough exposure to moisture. Wood rot starts when the wood reaches a 20% moisture level.
Of course, you can avoid these issues completely if you take steps to protect OSB from the rain, to begin with.
How to Protect OSB from Rain with Sealants
Sealants are one way to protect OSB from moisture before damage has the chance to occur. To apply sealant, follow these steps:
- Plan and cut the pieces of OSB you’ll be working with. Remember that every time you saw a piece of OSB board, you have to waterproof the edge(s) you cut as well as the board itself. Lay the pieces out on a tarp or other clean surface as you cut them. (Or lay them on sawhorses, so you’re not bending over during the waterproofing process.)
- Sand the OSB panel to create a good adhesion surface. You can use a palm sander with 100-grit sandpaper. Finish with 220 grit for the best surface effect.
- Paint before sealing. If you are painting the board to add colour or additional protection, use paint appropriate to where you’re using the board: interior or exterior. If interior, an acrylic latex primer is a great base coat. You can also use stain on OSB if desired. Be sure the paint has dried for the time noted on the paint label.
- Apply the sealant. Waterproof the OSB using a sealant appropriate for wood (you can use a polyurethane sealer.) Using a paintbrush to apply the sealant, coat the board surfaces and all edges (not just where cuts were made.) Most manufacturers recommend that the coated pieces dry or cure for 12 to 14 hours.
- Once dry, turn the pieces over, and if needed, coat the other side. Repeat if additional waterproofing is needed.
How to Protect OSB from Rain with a House Wrap
Another way to protect exterior OSB panels during the building process is by using a waterproof barrier such as Tyvek House Wrap. If you are planning on using vinyl siding, it’s especially important to use house wrap first to keep any water from coming through the siding onto the OSB.
House wrap ensures that the OSB remains protected from external environmental factors like rain and snow. Here are the steps to install it:
- Calculate the amount of house wrap you need by measuring the OSB panels used and adding 10%.
- Cut wrap to the width of the area you are covering and add 12 inches to either side to wrap around the corners.
- Start with the house wrap across the bottom of a wall, press flat, and staple to hold it in place.
- Continue up and out, holding the house wrap flat against the OSB and stapling every 8 inches in both horizontal and vertical directions.
- Continue until all the exposed OSB is covered.
How to Protect OSB from Rain While Building
If you plan on using OSB for an ongoing project, you’ll want to ensure the boards stay protected from rain over time, especially during the part of the building process where they will remain exposed until some form of roof is built.
Here are some steps you can take to protect OSB from water damage during the building process:
- After a rainstorm, remove the water as soon as possible (sweep away standing water, etc.).
- Keep the site as clean as possible: remove scraps of wood so that nothing is covering (i.e., trapping moisture) on OSB flooring.
- Let things dry out as much as possible.
- Stack OSB and other lumber four or more inches off the ground. Tent, but don’t tie a tarp over it (so the lumber can breathe).
- If you’re in a particularly rain-prone area, consider using OSB panels with built-in grooves, so the rain runs off of them.
Additional Considerations for OSB and Moisture
Given that OSB is used in flooring, siding, and roofing in a home (in other words, almost everywhere in framing), there are some things to keep in mind before you even consider waterproofing.
- Check your OSB’s grade for water resistance. This will help you determine which protection method is best. OSB grades range from OSB/1 for interior, non-loadbearing work to OSB/4, where loadbearing and water-resistant sheathing is needed.
- Orient the OSB correctly before waterproofing. If used for an interior wall, then the board’s smooth side faces out. If you’re using OSB for roofing, then the textured side faces out to make it easier to walk on. Also, because you are more likely to paint an interior wall, the smooth side is easier to paint (and it also absorbs less paint).
- Give the OSB a chance to dry after a rainstorm. This may mean waiting longer than you’d like to finish the roofing or flooring process. Waiting is better than re-doing if the OSB is compromised by sealing in dampness, leading to decay or mould.
- Leave 1/8 space between OSB panels as you install them. This lets the OSB slightly expand and adjust as needed to the environment.
- Consider using OSB combo sheathing that is created with a moisture-resistant barrier in place. Most all building and home improvement stores carry this version of OSB.
Just knowing that OSB panels are resistant to rain means less worry when that summer downpour happens on the shed you just started framing in. But knowing how to protect OSB panels from the rain in the first place is an important step in the building process.
Although OSB panels can withstand some rain, they should be waterproofed using a house wrap or sealant as part of the building process. The proper use and waterproofing of OSB can ensure its integrity and preserve your building projects’ value through many seasons of weather to come.