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Installing a basement sub floor over broken concrete

Posted: Thu Nov 14, 2019 3:27 pm
by ReedArcher
Hi
My basement has concrete floor that had 4x4 studs imbedded in and a floor attached to them. As one can imagine the 4x4s rotted. I pulled up all the wood and have been left with a broken sloping floor. I was planning on floating a sub floor above it all with flat PT 2x4s on 16” centres with cross pieces every 24” or so and exterior grade 5/8 plywood and a vapour barrier under it all. I will need to level the floor by 0-4” above the concrete. Is this a good idea? The room is 21 ft by 12 ft. Anyone have any other ideas?
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Re: Installing a basement sub floor over broken concrete

Posted: Thu Nov 14, 2019 7:03 pm
by Shannon
I would suggest you place the supports closer together . 2x4 s lying on edge supported only every 24” may be a little spongy.

Re: Installing a basement sub floor over broken concrete

Posted: Thu Nov 14, 2019 7:56 pm
by A. Spruce
That concrete is really topsy-turvy, it will probably be easier to remove the concrete and start over. You can pour a new slab flat and level,to seal off the dirt under the floor, then build your new floor.

Yes, absolutely use pressure treated materials against the concrete. I'd keep spans to 16" to avoid spongy floors as Shannon noted.

Re: Installing a basement sub floor over broken concrete

Posted: Thu Nov 14, 2019 8:13 pm
by ReedArcher
Yes the best would be to remove the concrete and pour a new one but that would be more expensive then the house is worth. So that is why we want to put in a sub floor.
I am currently thinking of 12 ft 2x4s laid flat across the width of the room with support underneath every 3 ft or so. Then put some cross pieces between the 2x4s.

Re: Installing a basement sub floor over broken concrete

Posted: Thu Nov 14, 2019 8:22 pm
by A. Spruce
I'm not following your method of laying 2x4's, it sounds like you'll have 3 layers of 2x, and I don't think that's what you mean.

I built a deck many years ago over a similar surface that was a little larger than what you are dealing with. I scribed PT 2x to the irregular surface so that the top was perfectly level and the bottom followed the contour of the surface. Then, I laid the decking over the level framing, directly on the scribed 2x.

If this area is captive on all 4 sides, then there isn't a big need to secure it to the concrete, or you can place dabs of adhesive along the 2x "stringers" OR secure blocking into the concrete next to the "stringers" to secure it down.