subfloor over concrete

Old 03-08-05, 06:06 AM
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subfloor over concrete

Hi folks. I need some advice. I am planning on converting my garage to a workshop. My local home centre recommended that I lay a 6mil polyethylene sheet or 15lb felt paper directly over the concrete slab, and then nail down 2x3 sleepers (on 16" centres). I want a warm shop so they suggest laying 1" rigid foam insulation between the sleepers and lastly they suggest screwing down 4x8 sheets of OSB followed by another layer of 4x8 sheets of plywood (running opposite to the OSB).

I am pretty handy, but have never built a subfloor before. Does the above plan make sense? Any other ideas and suggestions are welcome (and appreciated).

Thanks in advance.
Old 03-09-05, 02:51 PM
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Don't forget the garage concrete floor is likely to be pitched. You may want to correct that with the new subfloor.
Old 03-09-05, 03:23 PM
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How much room do you have to allow for raising the floor. Since the slab is probably sloped, you'll probably want to level the floor. The easiest way is to install Pressure Treated 2x6 floor joists at 16" o.c. and support them on short pieces of Pressure treated 4x4's (cut to length to make the joists level). 2x6's will span 6 ft, so you just need to have the 4x4's every 6' under each joist.

Over the top of this, 3/4" T&G Plywood with the face grain perpendicular to the joists will provide a nice sturdy floor. Rigid insulation below the joists or between them may help, but you will more likely have cold air coming thru the walls and the garage door than up thru the plywood.

Be careful what method you use to heat your shop, depending on what you use it for. Sawdust is highly combustible and can cause explosions if exposed to an open flame or spark.
Old 03-13-05, 06:43 PM
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Not to jump anybodys thread, but Im also converting my garage into a useable room. My room at its widist point is 14'. I nave already install 2x6 joists spaning the distance. The 2x6's sit approx 1/2" - 1'' off the old garage floor. ( I used a 360 degree laser to get everything level ). My question is: Can I use 2x6 pieces of wood every six feet to brace the joists? I was thinking about nailing the pieces to the face of the joist and having the bottom of the wood (2x6 pieces) resting on the floor, or possibly making a "L" ( out of scrap wood)and nailing to the face and use tapcons to secure the bottom piece to the floor. Or sould I use plastic shims under the joists? I'm up for anything that keeps my floor from sagging.
Old 03-13-05, 07:00 PM
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Hello Mike, it's ok to jump in. Whatever shims you use will be held in place from the weight of the floor and gravity. Plastic, cedar or treated shim pieces will work, no need to over engineer this, along as the floor is level and solid, nature will take care of the rest. Good luck.

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