How to Waterproof OSB

How to Waterproof OSB

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If you are planning to build your own house, shed, or deck and the type of engineered wood that you are using is OSB or oriented strand board, then one thing that you need to consider is its water resistance. Now, the fact of the matter is that oriented strand board is a fantastic material to use for a variety of building purposes as it is quite strong and comparatively cheap, especially when compared to plywood.

With that being said, whether or not the material is 100% waterproof is questionable,  something that we will discuss below. Plus, we are going to teach you how to waterproof your OSB.

Is OSB Waterproof by Default?

One of the important things to note about OSB is the fact that it is much more resistant to water than plywood is. Now, this really has nothing to do with the type of wood that was used to create these two types of engineered wood, but rather it has to do with the materials that were used to bond them together. When oriented strand board is made, a series of wax and resins are used to hold those various pieces together, and yes, these resins and waxes are actually waterproof. Of course, this does make quite a big difference.

There is also the fact that the edges of oriented strand board are also sealed with these same materials and are therefore much more water-resistant at the edges than plywood is. At the same time, OSB tends to be denser than plywood, and it also has no gaps or voids in it. Two more factors that make it water-resistant.

However, you might have noticed that we keep saying that oriented strand board is water-resistant, but we have not said that it is waterproof, and this is simply due to the fact that while oriented strand board is very resistant to moisture, it is not 100% waterproof. With prolonged exposure to high levels of moisture, it will begin to work and degrade. The strands can swell and change shape over time.

The other important thing to know about OSB is that there are different grades of it, most of which are either grades one and two, or three and four. One and two are rated for dry conditions, whereas grades three and four are rated for humid conditions.

Now, you definitely don’t want to use interior OSB for exterior purposes because it’s just not going to last very long, but the exterior OSB, or in other words, grades three and four, are ideal for outdoor use in humid conditions. However, with that being said, exterior oriented strand board is still not 100% waterproof, and if it sits in the rain for weeks or months on end, it’s still not going to turn out very well.

How to Waterproof OSB

Alright, so now that we have established that oriented strand board is in fact not 100% waterproof, let’s figure out how to make it waterproof, or at least as waterproof as humanly possible. Keep in mind that the only real way to waterproof a piece of OSB is to paint it or seal it with some kind of waterproof sealant. Depending on your exact needs, this could be waterproof paint, lacquer, stain, or even a special waterproof sealer designed specifically for this kind of purpose.

Therefore, the first thing that you need to do is to figure out what the differences between paint, stain, lacquer, and normal sealant are, so you can figure out which one is the best for your purposes. On a side note, do keep in mind that the number one recommended paint to use for this purpose is waterproof latex paint, although the choice is yours.

Once you have chosen the specific product and type of sealant that you want to use, you can then move on to the instructions as listed below.

Step 1: Gather Your Materials

There are a few different materials and pieces of equipment that you will need for this task, so first, you will need to gather them. Refer to the list below for everything that you will need to accomplish this task.

  • Workbenches or sawhorses
  • Tarps
  • Paintbrush
  • Paint roller
  • Paint tray
  • Paint, sealant, stain, or lacquer of choice

Step 2: Prepare the Area

In case you are painting indoors, you may want to lay down some tarps just to protect the general area from getting paint on it. Moreover, if you want to make your life a bit easier, you should prop up the various pieces of oriented strand board on either workbenches or sawhorses so you don’t have to bend down to the ground to perform the painting.

Step 3: Prepare the OSB

Before you can start painting, you do want to prepare the OSB, by which we mean that you should just wipe it down with a slightly damp rag in order to remove any debris. Just make sure that the rag is not too wet or else you will have to wait until it totally dries before you can paint. Either way, you need to make sure that there is absolutely no debris present on the OSB when you go to paint it because that debris will get ingrained into the paint.

Step 4: Paint the Faces

All you have to do now is to take your paint roller, dip it in the paint of choice, and apply an even layer to one of the faces. Once the layer is dry enough to touch, you can then flip it over and apply a layer of paint to the other side. For the maximum amount of waterproofing power, you should apply two layers.

Step 4: Paint the Edges

Once you have painted both of the faces, you can then use your paintbrush to apply two layers of paint to all four of the edges. You don’t have to wait for the faces to try to do this, as long as you have the OSB propped up on sawhorses or workbenches. All you have to do now is to wait for the paint to dry and then the board is ready for use.

How Long Can OSB Be Exposed to Rain?

Generally speaking, depending on the exact type of OSB, it can be exposed to rain and high levels of moisture for anywhere from 20 to 40 days without being damaged or absorbing water. It may even last for a few weeks longer if the OSB is standing up or sloped, as most of the water will run off before it can be absorbed. With that being said, there are also some types of OSB, the highest quality types, that can last for up to 200 days in the rain before being damaged.

However, once OSB does absorb water, the damage is irreversible.

Mistakes to Avoid, Tips & Tricks

Lastly, here are some additional tips regarding the topic:

  • Do make sure that the paint you use for this purpose is indeed rated for outdoor purposes and is also waterproof.
  • When painting OSB, make sure to do so in decent weather conditions or in other words, don’t go out and paint in the rain because it’s not going to work very well.
  • Keep in mind that for this purpose you do want to use exterior OSB that is already rated as being water-resistant.


Now that you know exactly how to waterproof oriented strand board, you can get to using this type of engineered wood for virtually any type of outdoor building purpose.