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We recently purchased a barn-style 8 x 24' pre-assembled shed. When it arrived, the plywood floor was not even. The edges weren't aligned, so the floor is 1/8 inch higher in some places. I'm wondering what the best practice for evening the floor surface is?

I researched different ideas, but none of what I saw seemed to apply perfectly to my situation. The floor is OSB I think (Does it make a difference whether it is OSB vs plywood?). I'm looking for the most economical solution. Inside our house we have a "floating floor" with some foam sheets of something covered in shiny gold mylar-like material, with a laminate wood floor on top. I'm just trying to have a workshop in my shed, not a fancy floor, so I'm not sure if that gold foam layer would even be applicable.

I've seen blogs about using some kind of pourable concrete or mortar, but that solution seems to be about $100 for 4 square feet (perhaps I'm exaggerating there). For the time being I'm out of work and looking for the lowest cost longest-lasting solution.

What I'm really looking for is a way to prevent the floor from sagging or getting holes in it over time. I live down in the humid deep south and that is a real issue here. It also needs to be durable to take things being dropped on it (hand tools, wood, bricks, etc.), and it has to be flexible enough that when it gets to around freezing (yes, it freezes in Alabama) that the temp change doesn't undo the "solution".

MY PROPOSED SOLUTION

Based on everything I've read and experienced, here is a multi-faceted solution strategy I'd love for you to weigh in on, or offer a better solution without addressing mine at all is fine too:

  1. Paint the floor with Killz 2
  2. Caulk the edges where the wall meets the floor (should I use clear silicone? Will this cause problems down the road?). This should help with temperature maintenance and reducing critter/bug access.
  3. Nail (or should I use construction glue??) 1/8" plywood sheets on top of the OSB. This will give a smooth surface.

I'm not sure what kind of putty, paste, cream, or other material I should use under the thin plywood to make the floor level. Is this solution even practical?

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    Have you tried taking a 2-3 foot section of two-by-four (to spread the force) and just beating on the high spots with a sledgehammer? Beyond that, 1/8" plywood will be unsatisfactory as a floor if there are any voids below it. At a minimum, I'd suggest 1/2" ply (not OSB). Jan 10 at 22:33
  • "I'm wondering what the best practice for evening the floor surface is?" Regrettably I don't think there's a one-size-fits all answer to this, because the causes of this issue can and will vary. I don't think it'll be down to the OSB as in my (not extensive) observations of it is uniformly very flat and quite rigid. So my assumption is this is an installation thing. We could really do with some pictures showing the floor for better input. But assuming no simple fix is doable (like just tightening a lot of the screws!) one relatively quick fix would be to sand the high spots down.
    – Graphus
    Jan 11 at 7:53
  • If you do end up going ahead with the plan for skinning the floor, I suggest you find a good quality tempered hardboard (e.g. Masonite) and use that in preference to any plywood. Ply of this thickness tends to have a wafer-thin surface veneer that wouldn't be durable at all. Hardboard on the other hand is a uniform material through its thickness.
    – Graphus
    Jan 11 at 7:56
  • @AloysiusDefenestrate, why ply in favour of OSB? I actually think it makes a better flooring material (or a flooring substrate) than a typical plywood of the quality chosen for this application. It is gaining wide acceptance here for this application. I'm sure it's the same in NA.
    – Graphus
    Jan 11 at 7:58
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    I'm thinking the root cause is more significant than the materials here. Why/How is the existing floor uneven? Is it at the seams/joints, or in the middle of OSB sheets? Is it uneven at the periphery of the shed floor, or in middle places? Does the flooring flex/bend at the high spots? Jan 12 at 17:27
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Here are a few ideas that might be helpful regarding your proposed solution from my experience with OSB/wood floors:

OSB vs plywood

I made good experiences using OSB instead of plywood as it is made from stronger wood and will be more durable. For flooring and subflooring I usually use the type of OSB that comes with tongue and groove. enter image description here

There is no need to glue those in place and they can expand and contract to accommodate changes in size due to temperature. Just make sure to not squish them together with too much force, especially if it’s relatively cold when you install them, otherwise they might bulge during the warmer month.

Painting the floor

I paint the OSB sheets from both sides (one coat for the underside and two coats for the visible side). One thin coat of paint on the underside helps to prevent moisture getting in from underneath and helps to prevent the OSB sheets from bulging up.

Depending on how much effort you want to put into your floors you can get them to a very smooth and shiny finish by using wood filler and lots of sanding before you apply the paint on the visible side (after you installed them).

Levelling the floor

If you have an uneven surface underneath I can recommend using a dry levelling compound (filler substrate), this is a fast and easy method that is relatively cheap (at least the stuff you get here in Germany) and looks similar to this: enter image description here Some of these levelling compounds might help with insulation as well.

Caulking the edges

I have never caulked the edges of my OSB floors, but I don’t think that it will cause problems down the road, especially since you can relatively easily remove it if you use silicone. I personally would not use clear silicone as it is the most prone to discolouration (yellowing) and rather go with something closer to the color of the OSB or paint. Alternatively you could also use a baseboard to prevent bug access (but I guess this won't help much with insulation.

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  • Thanks, Steffi! Although yours was the only actual answer, it was perfect. I loved your suggestion about painting the underside, unfortunately since it is on a premade building that won't be possible. Based on your tip I will probably use white caulk around the edges. Loved the images. Oct 7 at 19:50

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