OSB is a really strong type of board that stands heads and shoulders above other manufactured woods. Stronger than MDF and even Plywood, OSB (otherwise known as Oriented Strand Board) is often used for outdoor construction.
They can handle moisture and rain surprisingly well too, all thanks to the water-resistant glue used to bind all of those wood shavings together.
Still, water resistance is a far cry from being completely waterproof. And while OSB can easily shrug off a few drops of rain, can OSB board get wet?
OSB board can get wet, but only up to a certain level. OSB grades 3 and 4 can soak in enough water to give them a moisture content level of up to 21% (without falling apart). However, you will need to wait for any wet OSB board to dry out first, (with its moisture content level falling below 19%), before using it for construction.
And just what do we mean by ‘Moisture Content’? Well, moisture content simply refers to the amount of water in a piece of wood.
Green wood, (which is wood that has been freshly cut from a tree), has a moisture content of roughly 100%. However, the moisture content found in kiln-dried pressure treated wood is 19% or less.
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So, as long as the moisture content of that wet OSB board isn’t too high, you should be good to go. Right?
Yet, what if your OSB boards got caught in a heavy rain shower? Or, worse still, that OSB panel isn’t drying out? And does the grade of OSB even factor at all?
Well, if you’re asking yourself any of these questions, then you’ve come to the right place. Because we’ve got the answers to the top 7 things you need to know about that drenched OSB board…
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No. 1) Is OSB OK To Be Left Out In The Rain?
OSB is perfectly capable of handling a bit of rain. That water-resistant adhesive holding all of those wood strands together was designed to shrug off rain showers and humidity.
Which makes sense since most outdoor constructions are rarely built in a day. Which means that for weeks on end, that OSB roofing or siding could be left exposed to the elements. So a bit of autumn rain (or summer humidity) shouldn’t be a problem for this manufactured wood.
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No. 2) Are OSB Boards Waterproof At All?
OSB boards are not waterproof. They are water resistant up to a moisture content level of 21% at most.
No. 3) Is OSB Exterior Grade?
OSB doesn’t have an official ‘Exterior’ classification like plywood.
However, OSB does come in 4 standard grades; OSB/1, OSB/2, OSB/3, and OSB/4.
Yet not all of them are suitable for ‘exterior’ construction.
The least water-resistant grade of OSB is OSB/1. This grade is an all-purpose board suitable for indoor furniture, baseboard, and fittings. This grade is meant for interior use only.
The next grade up is OSB/2. Designed with strength in mind, this OSB grade can handle the stress of interior loadbearing duty. This grade is generally used for interior use.
The next grade along is OSB/3. This grade is able to handle moisture and humidity since this OSB grade uses water-resistant glue as a binding agent. This grade is generally used for outdoor structures thanks to its high strength.
The strongest and most water-resistant grade of OSB is OSB/4. This grade is designed to last for years under tough, humid conditions. This grade is used for buildings, roofing, siding, and even attic flooring.
In fact, OSB/4’s water resistance is just one of a few reasons it can make for great attic flooring material.
OSB’s superb ability to handle structural loads works in its favor, too. Our post explains why OSB’s often used for attic flooring; read it here: Why You Should Use OSB (Not Plywood) For Your Attic Floor
No. 4) Is OSB More Water Resistant Than Plywood?
Yes, OSB is more water resistant than plywood. And this is pretty much down to the fact that plywood soaks up water so quickly and easily.
In fact, OSB/3 and OSB/4 are more water resistant than even Exterior Grade Plywood.
This is because OSB takes a good long while to soak in any moisture (but can take equally as long to dry out afterwards too).
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No. 5) Will My OSB Board Be OK Outside On A Humid Day?
If it is Grade 3 or Grade 4 OSB, then that OSB board will be fine outdoors. These two specific OSB grades have been made to withstand high-humidity environments.
No. 6) Will This Wet OSB Board Ever Dry Out?
Yes, given time OSB will eventually dry out in the right conditions. Those conditions being a low-humidity space with plenty of air flow.
However, this is more the case with OSB/3 and OSB/4 grades. With OSB/1 and OSB/2, once they absorb water, they’re more likely to end up permanently water damaged, than ever truly dry out.
The best way to waterproof OSB (especially the lowest OSB grades) is to seal them up as best you can. We dive more into this topic in our post What’s The Best Way To Go About Sealing OSB For Exterior Use?
No. 7) And How Long Does It Take For OSB To Dry?
It can take up to 2 months for a well-soaked OSB panel to dry out completely (at least without any help).
But, if you where to dry that OSB using the help of dehumidifiers and fans for air-flow, then you can cut that time in half to as little as 4 weeks.
At the end of the day, OSB can shrug off the odd rain shower or two with relative ease. However, it cannot handle a deep soak.
But as long as the moisture content of those OSB panels do not rise much above 20%, you needn’t worry about them getting wet.