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I wand to close in my back deck can I put plywood over the deck board the wood is still in good shape

Donald Mobley
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  • Closing a deck in may be against code in your area and may also not be advisable if it is a wet climate. – P2000 Mar 06 '24 at 04:19
  • Will this laundry room be closed off from the rest of the house? Will it be HVAC'ed? Insulated? – Huesmann Mar 06 '24 at 13:07
  • You'll want to seriously consider if this is a good idea as a shortcut to doing it properly. You'll have to get plumbing in there under your current deck, you'll want to insulate underneath, and basically make it a finished space. If this isn't a covering of your complete deck, then I highly suggest not doing this. We have a screened in porch that covers part of our deck, and they put the porch walls on top of the deck boards. Now, I can't replace the worn out deck boards as the porch walls are resting on them and supported by the deck boards. – Milwrdfan Mar 06 '24 at 15:13

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Yes, you can lay the plywood over the decking. The decking is a legitimate subfloor, and your plywood can be categorized as underlayment.

The deck boards themselves are good for their own self weight and the full 40 psf live load of both a deck and a laundry room. Add a sheet of plywood with a span rating for your joist spacing, and that will also be sufficient for its own self weight and the full 40 psf live load. Together? Good for the total self weight and the full 40 psf live load. Realistically you can remove the span rating requirement. A building official should be fine without it.

Technically a deck's framing could have been designed for 5 psf dead load, whereas your laundry room may deserve 10 psf. 1/2" plywood plus tile, for instance, would push a light deck assembly's dead load up to 10 psf (more like 7 psf to 9 psf). Again, a reasonable building official shouldn't split this hair. Just be sure to mind your manners.

In response to Jim's comments:

Tables 2304.8(1) and 2304.8(2) under 2018 IBC 2304.8 prescribe minimum thicknesses and grades for lumber subfloors. The OP's deck boards will certainly satisfy the thickness and grade constraints. The guy in your video seems to be talking about using a higher-than-standard stiffness subfloor without underlayment below hardwood flooring. In the OP's case, he already has a subfloor and is preparing to install a layer of underlayment. He needs to install the underlayment because of the board side-seams without tongue and groove connections and without blocking.

On Matt Risinger, he's a salesman. I don't mind getting introduced to unknown products by a salesman, but I trust nothing from a salesman without verifying it independently (and I've met many salesmen with engineering degrees). There's no such thing as interior grade OSB. It's all Exposure 1 rated. Comparing OSB to Exposure 1 plywood (CDX plywood is Exposure 1 rated, for instance), my money is on the plywood's longevity when the two are subjected to identical moisture conditions. Unrestrained plywood sitting on a stack will curl up and become annoying to install if exposed to water and allowed to dry in the sun. OSB will still lay flat under its own weight. I expect that this is why Risinger prefers OSB. Or maybe a sheet of plywood cartoonishly slapped him in the face once after he cut a pallet's banding. Boi-oi-oi-oing.

But maybe the conventional wisdom unfairly tars OSB with particle board's reputation. Putzing around the internet, I see lots of qualified answering both ways. Sounds like short term exposure impacts OSB more than plywood, because it takes much longer for the OSB to purge the water. For what it's worth, you can look at OSB's Exposure 1 moisture testing framework under NIST PS 2-18's 7.16.3.

popham
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  • @Jim, for a given thickness, it should all perform the same. If strength or stiffness was critical, I would shop for span rating instead of thickness to be safe. No need for that Sturd-I-Floor stuff in this application. – popham Mar 06 '24 at 20:12
  • @Jim OSB has a specification with performance metrics. There's not going to be a brand making "the best" OSB. It's a commodity. The brands are interchangeable. I don't like your video's 7/8" OSB, because all I see is the APA treating it as legit under a 60/32 span rating. I don't see the stuff in any AWC documents like Chapter 9 of https://awc.org/wp-content/uploads/2022/01/AWC-2018-Manual-1810.pdf (technically it's section "M9"). I've extended my answer with a response to some of your comments also. I'd like to remove the OSB stuff if you can delete them too. They sound like spam. – popham Mar 07 '24 at 21:11