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Is it OK for OSB to get rained on?

Your framing lumber and the oriented strand board (OSB) are going to be fine. The glues used to make OSB are water-resistant because the manufacturers know that virtually no one can build a home that won't get wet before the roof and siding is applied.

chicagotribune.com - Temporary exposure to rain won't hurt your framing lumber

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DEAR TIM: I'm having a new house built and it's been raining for a week. The roof is partially framed and every piece of lumber and all the flat OSB is soaked. My builder says this is normal and I still get my 10-year warranty. Is it OK for a house to get wet while it's being built? Will it hurt the lumber? What can be done to prevent mold and rot that probably will happen? What did you do to protect the homes you built? --Seth P., Titusville, Fla. DEAR SETH: Here's the good news. The lumber that is being used to build your home came from trees that were out in the rain all the time as they grew. I'll grant you it had a protective layer of bark that works very well to protect the wood in the center of the trunk. But realize that it takes quite a bit of time for lumber to start to decay from water. You'd probably be surprised to know that lumber that is submerged in fresh water can be in great shape and rot-free for hundreds of years. Timbered logs are routinely salvaged that never made it to sawmills over a hundred years and these logs are prized pieces of wood. The wood rot you're familiar with happens in the presence of water and oxygen. Your framing lumber and the oriented strand board (OSB) are going to be fine. The glues used to make OSB are water-resistant because the manufacturers know that virtually no one can build a home that won't get wet before the roof and siding is applied. Houses I built got wet routinely during the framing process and I never had an issue. Wood rot can and will start when the moisture content of the wood reaches 20 percent. But wood rot is a slow process where the fungi start to grow and feed on the cellulose in the wood. The wood needs to remain damp and wet all the time for wood rot to advance. It's much more likely that you'll start to see mold growth in as little as 48 hours as mold spores are everywhere on your home construction site. To minimize or eliminate any chance of damage, you need to get rid of the water as quickly as possible. This means the job site needs to be clean. All scraps of lumber, sawdust, etc., need to be off the OSB. The OSB floors should be swept clean each day after work concludes. This debris can trap water and hold it against the OSB. You want all the lumber to dry out as fast as possible after each rain. Standing water on the OSB needs to be swept away. Some OSB panels are created that have drainage slot in the tongues to help get rid of water. Lumber on the job site that's in piles needs to be covered and up off the ground. You don't want to wrap the lumber like you would a sandwich with plastic. It needs to breathe. It's key that the lumber is up off the ground at least 4 inches so that air can get under the stacked lumber. Try to create a storage method that mimics a rain fly on a camping tent. You want the sides of the stacked lumber exposed to the air but protected so rainfall can't hit the lumber.

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When I was building, I used to spend extra money and apply a clear penetrating water repellant to the plywood and OSB I used on the floors. I never worried about the sheathing on walls or roofs because water rapidly ran off and never collected there. Before any walls were built on a subfloor, I'd blow off all saw dust, pour out the clear water repellant and apply it with a large paint roller on a pole. It only took about 30 minutes to do an average subfloor. The water repellant prevented swelling of plywood and untreated OSB. Once your house is under roof and there's little chance of the lumber getting wet, you may want to do some first aid and preventative care. If you do have mold growing on the lumber, you can clean it easily with an oxygen bleach solution. You can also use chlorine bleach, but many complain about its fumes and toxicity to nearby plants and vegetation. You mix up the oxygen bleach and apply it with a garden hand-pump sprayer. It has no odor and will do an excellent job of cleaning mold. Once the lumber is clean and dry, you can then decide if you want to prevent wood rot in case the wood somehow gets wet after you move in. This can be accomplished now while the framing lumber is accessible by spraying it with borate solutions. Borate chemicals are not toxic to humans or animals, but they are very toxic to many species of wood rot fungi and termites. You spray the lumber and OSB using a the same garden sprayer. (Need an answer? All of Tim's past columns are archived for free at www.AsktheBuilder.com. You can also watch hundreds of videos, download Quick Start Guides and more, all for free.)