CDX plywood for shed platform?

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Old 11-15-16, 08:31 AM
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CDX plywood for shed platform?

I've got a plywood question for all you wood pros out there...I've built a wooden platform to hold one of those resin shed kits. I built the platform on-grade, digging small-ish holes in my yard and adding ~4 inches of gravel, and then stacking solid concrete blocks until level. I used 2x6's 16" OC with a double rim joists. The platform is 8'x10', so I used 2.5 sheets of plywood to sheath it, and screwed it down about every 8 inches. The shed kit is coming this week and I'll build it on top of the platform.

My question is this: is pressure-treated CDX plywood OK to use as the sheathing? The shed will sit on top of the platform, but there will an inch or so exposed to the elements all around the perimeter of the shed. I'm worried about rain runoff and rotting the plywood. Home Depot only carried CDX (23/32" thick), and the label says that it is rated for ground contact. My Google searching has given me some concerns that the CDX is not truly rated for permanent outdoor construction, and that it is meant to be covered by housewrap or shingles.

Should I just leave it? I'm only storing some miscellaneous items and kids' bikes etc. to clean out my garage. If it lasts 3-4 years I'll be happy. I'm not sure if I want to flash it, because it will just be an "L" shape that might actually cause more water to flow under the shed and onto the wood.

I've even considered cutting ~6 inches off the perimeterof the plywood, all around, and replacing it with PT 1x6 boards all around...which should hopefully last longer than the plywood, and protect the plywood from rain water. None of the plywood would then be exposed to rain...hopefully.

Thanks in advance for any advice you guys can offer.
 

Last edited by CT_guy; 11-15-16 at 08:59 AM.
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Old 11-15-16, 09:09 AM
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How would flashing allow water to get under it? You can caulk the bottom plate of the shed to the flashing and leaving the other end of the flashing stick out a little from the joist will allow the water to fall away from the joist.
 
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Old 11-15-16, 10:12 AM
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I had thought about doing that, but I'm nervous about the caulk failing and water seeping in. Additionally, I can't angle the flash away from the shed/platform since it will be sitting on the platform...so it will be horizontal, at best.

Who knows, maybe I'm over-thinking this.
 
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Old 11-15-16, 10:21 AM
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IMO, unless the shed installation instructions told you to do it that way, it's a big mistake to have the base be 1" larger than the shed in each direction. If anything, it should be a little smaller so that water from the shed is inclined to drip off. As is, it will run into the shed something terrible.
 
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Old 11-15-16, 10:35 AM
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I completely agree. The shed instructions called for particular dimensions, and I intended to build it ~1/4" smaller all around for that very reason. But as I screwed everything together I ended up being dead on to the shed instructions' dimensions. I won't know for sure how much of an overhang or underhang I have until the shed kit comes later this week.

I really don't want to rebuild the whole platform. One other thought that I had is to cut off about 1/2" from the plywood, all around, and then add flashing. The 1/2" will hopefully allow the flashing to be under the shed (instead of proud of it) for the water runoff reasons. And it will allow me to angle the flashing away from the shed platform.

But back to my original question...is CDX plywood acceptable for this use? Even if a little bit is exposed to water, would it be OK for a few years?
 
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Old 11-15-16, 04:16 PM
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CDX is full of voids in the core, would have been far better off using Advantec as a subfloor!.
I agree floor should have been built slightly smaller than the shed and at least 6" above grade or there's going to be issues.
 
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Old 11-16-16, 06:51 AM
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Thanks Joe. Is Advantech an OSB? I couldn't find any PT variety of it at Home Depot online.

Any thoughts on the validity of the Ground Contact rating on the CDX? It's going to be screwed down like crazy so I'm not too concerned about delamination...but I am concerned about rot. I forgot to mention that the plywood will be a minimum of 8 inches above grade.

I just viewed the shed assembly videos online and I don't think it would've mattered if I built the platform any smaller. It's a Lifetime brand shed, where the wall panels snap into slots in the floor panels. From what I can tell, the slots go all the way through the floor panel and water could likely flow down the wall and through the slots. The platform would need to be ~6" smaller all around to avoid this slot water, which is a bad idea because it would then no longer support the walls.
 

Last edited by CT_guy; 11-16-16 at 07:13 AM.
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