Help/advice for installing a sub floor


Old 03-01-16, 02:14 PM
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Help/advice for installing a sub floor

Bought a home in what used to be a commercial building built in 1947. One story on a thick concrete slab. Many feet thick. Two rooms still concrete floor that I want to use for a bedroom and small den/TV room. The floor goes in several directions from there being a drain on one area. See the picture for what I'm up against.

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The height of the floor may be off by around 3" or so. I want to install a sub floor of joists, lay down 3/4" plywood and have it wall-to-wall carpeted. Should make for a cozy, warm two rooms come next winter.


Can I use 2 x 3's for the framework?

I heard about not having wood touch concrete. Never heard of this before. Please educate me as to why this is (now?) required and what do you suggest I use as a "separator"? Will asphalt roofing shingles work OK?

Do I need to cover the concrete with plastic sheeting as a moisture barrier before laying down the framework?

Thanks for the help.
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Old 03-01-16, 03:45 PM
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Got some real issues there.
No way unless there was some specialized machinery in there that slab is 2' thick.
If the floor is that far off then something has to have happened under that slab to caused it to sink.
It would take mud jacking to lift it, Google it.
Not going to find 2 X 3 pressure treated lumber.
Any wood in direct contact with concrete has to be pressure treated of it will rot out.
Old 03-01-16, 04:21 PM
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Just thinking out loud here.... you'd almost need to use self leveling cement to match the floors.
Old 03-01-16, 04:43 PM
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You could use regular spf 2x3's and make a framework, shim it up every 2' using treated shims you cut on a table saw and construction adhesive. I'd recommend you shoot the floor with a rotary laser and write all your measurements down on the floor where you have chalked your layout lines. That way you can easily figure out what size shim goes where. To secure it to the floor, lay treated 2x4s between the joists and shoot or screw them to the floor. Then nail your joists to that. Weight of floor should keep it down but you should attach it anyway. Hope you have thought about all your door headers being too short if you raise the floor. It will also affect any stair risers.
Old 03-02-16, 08:57 AM
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Appreciate the replies but let me try to address some of them and see what we can come up with.

As to the floor thickness... this used to be a milk processing plant back in the early days so yes there was heavy machinery, tanks and such. In fact, out that door in the picture is 65' of concrete loading dock some three feet off the ground.

The slanting is for the wash water or milk or whatever to run off to the lowest point where there sits an 8" round drain. Not sure what this has to do with my issue, though?

This notion of rotting wood comes as a surprise to me in that my first home, which was wood built in the 1940's, to the best of my knowledge, has a frame sitting directly on a concrete foundation. So does the two car garage out back. Still going strong and no rot. Is this a new building requirement?

So if no pressure treated 2 x 3's can be had...

I need to use regular 2 x 3's as long as they are off the concrete. You mention using pressure treated 2 x 4's (laid flat?) and this may work for the higher elevations but can I also use asphalt shingles for the spots I need little "lift". They do not rot and will not flatten over time.

Door headers...

Behind where the camera was sitting taking this pic is the door to the rest of the house. That floor has already been raised up some 6+ inches and a wood floor laid. There is a much higher ceiling in that area. All I will be doing, here, is not quite meeting up with what is there, now.

As to the door at the back of the pic, the one to outside, I plan to leave a section of the floor as it is and place a rubber mat there for when you come in with wet boots and such. There are three stairs to the right that lead down to a bathroom and laundry room. So there will be a small step up to enter the area... no bid deal, really.

No one replied RE: laying down plastic sheeting first. Is this a good idea, or actually required, to keep the moisture out?
Old 03-02-16, 02:08 PM
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How about simplifying your carpet solution. Why not use glued down carpet squares on the sloped portion of the floor like a kind of "border" and then transition into normal wall to wall carpet in the balance of the room. Certainly simpler than creating a joists system on a slab. Another option is to jack hammer the slope and re-pour level with the rest of the floor.
Old 03-03-16, 07:37 AM
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Would rather proceed with the sub floor if I can get some useful replies to some of my questions. Need to get going on this rather soon.
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