How I Replaced a Shed Subfloor

[James Combs]

About 5 years ago I put up an 8'x12' utility shed. On of those Lifetime branded molded plastic jobs. I didn't have a level grade to install in on so I built an 8x12 deck for it. I used PT 2x6s on 16" centers for the floor joists and then played stupid and use OSB for the decking/sub-floor. Needless to say it didn't last very long. The building had is own plastic floor and it was to the point that it was about the only support there was for the floor between joists. You felt like you would go through it at any time if you didn't step carefully. I fully expected to open the door one day and see my rider mower sunken into the floor and astraddle a couple joists.

Before the fix.
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Well enough babble, here is how I fixed it.
This shows some of the condition of the sub-floor. The OSB was completely disintegrated and had moved the building slightly due to swelling from absorption of water.
If you touched it, it would flake off like a loose pile of leaves.
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You can see some of the plastic floor here and a piece of plywood that I was using to park the rider on.
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I decided that maybe I could build another temporary deck beside the original and get the neighbors to help move the building onto it. I attached four joist like members to the side of the original deck and after emptying the building and for a lark I tried sliding the building onto them. I figured I was wasting my time and would probably have to remove the roof to lighten it a little as well as get the neighbors to help move it. Well I put my shoulder to one corner and gave it a push and WOW, it moved 6" without any major effort, now I am starting think that what I new was going to be a 2 or 3 day job may just turn into a piece of cake. Within 5 minutes I had it sitting on the temporary deck structure. Looking at the old decking/OSB it was easy to see why it moved so easy. It was slimy rotten which made it very slick. Kind of like sliding on snot.
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(more to follow)

[James Combs]

Removing the old decking also turned out easy. I used a 2x4 as a pry-bar, stabbed it through the OSB between each joist and pried up on it. I did't have to remove a single screw to remove the OSB.
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Here I have installed 3 sheets of PT 3/4" plywood which is what I should have used in the first place. The length of the building is actually 12.5' so I used a 1x6 piece of PT decking for the last 6 inches. I had to rip a little of one edge and also dado the edges that rested on the joists to get it down to the same thickness as the Ply.
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The project also gave me a chance to shim up a low support pillar.
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After I got the new decking/sub-floor down I slid the building back onto it's new deck. It was a little harder this time without the snot but still not too bad even for old coot like me. All that is left here is replacing everything back into the building and getting rid of the scrap OSB. This 3 day project turned out to be about 7 hours.
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Thanks for looking, comments welcomed.

Edit: Just noticed my misspelling of OSB to SOB, probably applies but it wasn't intentional.

[Phil Thien]

Clever repair.

So the OSB was covered with plastic, and still rotted.

Is that because water (rain) runs down the side of the building, and then wicks between the plastic floor and the subfloor?

[James Combs]

Clever repair.

So the OSB was covered with plastic, and still rotted.

Is that because water (rain) runs down the side of the building, and then wicks between the plastic floor and the subfloor?

Right on both counts but not only between but also into the OSB

[Ken Fitzgerald]

Nicely done James!

I had a friend who took his family on a 1 month vacation to determine if they wanted to live in a foreign country as missionaries. The insurance investigators figured the cold water line to his kitchen sink in his home here broke a couple days after they left. The kitchen was on the 2nd floor of their tri-level home. They had to remove all the OSB used for subflooring as it too became saturated with water.

[Phil Thien]

Right on both counts but not only between but also into the OSB

I wonder if it would be possible to retrofit the building with a sort of drip-edge to protect the plywood.

Obviously the plywood will last longer than the OSB. OTOH, there is a lot of end-grain all around the plywood, and it isn't rot-resistant.

I suppose you could always just seal the plywood thoroughly to slow the process.

[James Combs]

Nicely done James!
I had a friend who took his family on a 1 month vacation to determine if they wanted to live in a foreign country as missionaries. The insurance investigators figured the cold water line to his kitchen sink in his home here broke a couple days after they left. The kitchen was on the 2nd floor of their tri-level home. They had to remove all the OSB used for subflooring as it too became saturated with water.

OSB is not great for much of anything but it is cheap, and I mean that in every since of the word.

I wonder if it would be possible to retrofit the building with a sort of drip-edge to protect the plywood.
Obviously the plywood will last longer than the OSB. OTOH, there is a lot of end-grain all around the plywood, and it isn't rot-resistant.
I suppose you could always just seal the plywood thoroughly to slow the process.

Phil thanks for the idea. I may have to see what Lowe's has that might seal it.

[Phil Thien]

Phil thanks for the idea. I may have to see what Lowe's has that might seal it.

I would seal it, plus when you are at Lowes, see if they have any plastic or vinyl drip-edges that are normally used on roofs, etc. I'm attaching a picture. I'm thinking if you were able to shove this stuff under the building, around the perimeter, and then run a thin bead of silicone caulk on top between the drip edge and the building, you may never have water touch the floor again. And it might look okay too as it would hide the wood edge.

Just something to think about.

I don't want to detract from the awesome job you did. It really is very impressive.

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[Joseph Tarantino]

great tip on side tracking the shed. i may need to re-orient an 8X8 shed, and this technique looks like it's just the ticket.

[James Combs]

I would seal it, plus when you are at Lowes, see if they have any plastic or vinyl drip-edges that are normally used on roofs, etc. I'm attaching a picture. I'm thinking if you were able to shove this stuff under the building, around the perimeter, and then run a thin bead of silicone caulk on top between the drip edge and the building, you may never have water touch the floor again. And it might look okay too as it would hide the wood edge.
Just something to think about.
I don't want to detract from the awesome job you did. It really is very impressive.

Thanks for the comments Phil, I like the idea except that I have one problem. If you look closely at the wall/floor interface you will see that the wall fits(actually snaps) into a grove along the outer perimeter of the floor.


Water can still run down the wall and into the floor area at the interface area. Yes I could calk it but I am don't think I would like the looks of calking on it although I may have to consider it. The one thing the drip edge would do would keep the water from the raw edge even though I would still get water on top.

[James Combs]

Sorry, an accidental post.

[Joseph Tarantino]

can that flashing be place between the plywood and the building so water runs over that "1/4" round at the base of the shed? this should protect the end grain of the plywood while not letting water onto the surface of said plywood.

[Chris Padilla]

Interesting and a nice job, James. When we bought our place back in 1999, there were two 8x12 Tuff Sheds on the property. One was a legitimate tool shed, plywood floor, etc. The other became a school-house for the home-schooled children. The mother decorated it nicely and all but the biggest problem was the MDF they used for the floor. Fast-forward to today and it is still okay but it has bubbled up in places. I'm surprised it has lasted this long. Replacing it won't be fun in the least.

[Robert Payne]

Along the lines of what Phil suggested, I would add a "Z"-shaped piece of flashing above the plastic floor edging that would extend out over the plywood to convey any water away from and over the wood parts. Old school teachings: if wood gets wet, it will rot. Use an exterior white window or flashing caulk on the back side of the flashing where it is attached to the vinyl siding to keep water out of that seam. The flashing can be vinyl or aluminum. My .02

[Dave Ray]

Jim, your plan worked great. Now I wonder if you will double the size of your shop? Seems like you have a good start.