Shed floor (OSB) question

[Rob Blaustein]

Does anyone have any suggestions for a good finish or paint for OSB ply? I'm building a shed from a kit and the floor is 2x4 frame construction (pressure treated) with an unfinished 3/4" OSB plywood floor nailed to it. I didn't use any sort of vapor barrier and was wondering if I could both protect the floor and stop moisture buildup from below by painting the floor with a durable paint or finish that would also provide a good vapor barrier.

[glenn bradley]

For LOML's shed I used a heavy plastic barrier over the floor frame (steel channel in her case) and then laid the OSB. If your OSB is already down and the shed is assembled, this is obviously not an option.

You could seal the interior surface but the exterior (towards the ground) surface will still absorb moisture. If the exterior is exposed there may be some wisdom to not finishing the interior as this would allow the absorbed moisture to evaporate as opposed to remaining trapped under the sealer.

[Jim O'Dell]

I would think a few coats of a good primer/sealer would work ok here. You are talking about painting the underneath side, right? Don't just paint the top side, or the moisture will destroy the OSB from underneath. If it's already nailed in place, well.....you have 2 options. Pull it up now, paint and/or use 6 mil plastic for a vapor barrier, or let it go and plan to replace in a year or so (could be longer depending on your climate). If nothing else, the OSB will wick moisture out of the pressure treated lumber until it dries out. 3/4" pressure treated plywood would probably be a better choice for this application. Then after it dries for a year, put the OSB back down on top of it. Jim.

[David G Baker]

What Glenn said and/or what Jim said.

[Rob Blaustein]

Thanks for the suggestions. Yes, I realize I probably should have put the barrier down first, but we were working from the instructions in the kit (which didn't mention a barrier) and just plowed ahead. I'd rather not undo the floor. Another possibility is laying down a barrier on the ground and covering with gravel. And/or painting both sides of the ply. I agree that painting only the top side wouldn't make sense.

[Rob Blaustein]

I just had a thought--what about laying down a barrier on the underside portion of the framed floor (ie on the bottom of the joists) and attaching it using 1x pressure treated nailed around the perimeter?

[Tim Morton]

maybe upgrade the OSB to something like advantech in order to get better moisture protection?

[Jim O'Dell]

I just had a thought--what about laying down a barrier on the underside portion of the framed floor (ie on the bottom of the joists) and attaching it using 1x pressure treated nailed around the perimeter?

Rob, if you can still get to the bottom, putting a vapor barrier on might be your best bet at this point. How wet are the 2X4s that the OSB is sitting on? Remember you will be trapping that moisture, which could still pose some problems with the OSB and with mold and mildew. That is the reason it is best to have the plastic between the joists and the subfloor. I would do the plastic and gravel on the ground under the building if you can, just to keep the moisture from building up under the structure, but I wouldn't use it as a substitute for a proper barrier between the joists and the sub floor.
I think it all boils down to when you pull the OSB up and do it right. Now, or later after there are problems with the sub floor. I know that's not what you want to hear, but doing it now, as long as you don't have walls up yet, will probably save you time and a lot of headaches in the long run. Don't forget to use some flashing around the perimeter between the subfloor and the walls to shed water away from the subfloor also. Cheap insurance. Jim.

[Jim Becker]

I'm surprised that the kit has OSB for the floor rather than PT plywood. The one kit shed I did quite a number of years ago had the plywood and it's still going strong at my previous residence. Consider replacing that OSB now 'cause over time it's going to soak up a lot of moisture even with a vapor barrier down under the unit. IMHO, of course.

[Gary Keedwell]

From what I have gathered when my house was being built, ( I had a rather heated discussion with the builder) they are making OSB with exterior grade glue. If you go around to the construction sites, you will see OSB, that is being used for flooring, sitting out in the rain.
Gary

[Bill Wyko]

I recently got a similar kit, 12x20 shop. I ended up replacing the wood with a cabinet grade ply from HD. It was around 29 bucks a sheet. I also added blocks between the floor joists to help them stay inlign. I did this because I have several heavy tools. I ended up using the material for a loft above for lighter storage. IMHO I felt concern for longevity. I would imaging a good coverage would help considerably

[David G Baker]

Gary,
In some of the poorer rural parts of Michigan there are a lot of homes that have OSB exteriors and have been that way for years. They are not painted just weathered gray looking. The OSB that is glued with exterior grade glue is designed to last for quite a while exposed to the elements and if it is vertical where water does not sit on it it will last for quite a while as demonstrated by all those Michigan houses.

[Gary Keedwell]

Gary,
In some of the poorer rural parts of Michigan there are a lot of homes that have OSB exteriors and have been that way for years. They are not painted just weathered gray looking. The OSB that is glued with exterior grade glue is designed to last for quite a while exposed to the elements and if it is vertical where water does not sit on it it will last for quite a while as demonstrated by all those Michigan houses.
David

Don't know about the sheathing but they have a "brand" name for the OSB flooring and they use it alot up here in the Northeast. They deliver it to the construction site and don't even worry about it. In the old days they used anything they could get their hands on for sheathing. Osb would be an upgrade. LOL A friend of mine stripped some cedar shingles off an early 50's house and found drywall for the sheathing. LOL
Gary

[David G Baker]

Gary,
I have about 20 sheets of that OSB flooring that is sitting in the upper area of a small barn that I have that is waiting for me to get the energy to floor the barn's upstairs. Mine is 3/4" and has a tongue on one side and a groove on the other. If I was going to put it over dirt I would use a moisture barrier and think about putting foam insulation under it as well if I was going to heat the building.
I too have seen some real scary sheeting on buildings. Quite a few use house wrap for exterior covering. Bet they freeze their buns in the Winter. On some of the very old buildings I have seen a lot of slab wood that seems to have held up for 100 years or more. Quite a few house trailers that were made in the 50s are still around as well.

[Bart Leetch]

I recently got a similar kit, 12x20 shop. I ended up replacing the wood with a cabinet grade ply from HD. It was around 29 bucks a sheet. I also added blocks between the floor joists to help them stay inlign. I did this because I have several heavy tools. I ended up using the material for a loft above for lighter storage. IMHO I felt concern for longevity. I would imaging a good coverage would help considerably


Bill: I wonder if I understand this right did you use a interior type plywood for an exterior type application?

[Ray Knight]

If the floor is already on and unfinished below, be sure the area underneath is ventilated well, and that you don't get standing water under it. If you do the osb won't last. Be sure the roof water drains away from the shed. I have seen these shed built over ground, covered with heavy plastic, and layer of river rocks, with no standing water, well ventilated and you should be okay. Ray Knight

[Rob Blaustein]

If the floor is already on and unfinished below, be sure the area underneath is ventilated well, and that you don't get standing water under it. If you do the osb won't last. Be sure the roof water drains away from the shed. I have seen these shed built over ground, covered with heavy plastic, and layer of river rocks, with no standing water, well ventilated and you should be okay. Ray Knight

We ended up doing pretty much that approach since I wasn't keen on redoing the floor. The area is pitched to allow drainage and we put down 2 layers of 6 mil plastic covered by gravel. There will be a few inches of space between that and the floor so there should be reasonable air flow. Boy, it's amazing how many bags of gravel you need to cover even a small area (6' x 6').

[Kirill Kaminsky]

hey guys, i know this is a really old thread, but I think this is the perfect thread to ask my question... I am building a shed in my side yard, on top of concrete. The area is narrow (5ft wide) and is bordered by my neighbor's shed on one side and a wall on the back side. Needless to say, I am worried about water problems. I was initially planning on just laying a few pt 4x4s and then building the floor framing right on top with pt 2x4s and then plywood floor on top. Should I be using a pt/exterior plywood for the floor? Do I need to put some type of moisture barrier between the 4x4s and the floor framing? How about between the floor framing and the plywood floor?

I actually got a suggestion to put concrete post brackets into the concrete and put the 4x4s on those, thereby no wood actually sitting on the concrete... This seems very expensive and I would love to avoid this, but moisture is a big concern as I need this shed to be as maintenance free as possible...

BTW, while on topic with the moisture barriers, you guys don't put this between the wall framing and the exterior plywood sheeting (the plywood that has the lines routed out)?