Building a subfloor over concrete


Old 08-15-07, 07:28 PM
Thread Starter
Join Date: Aug 2007
Posts: 1
Building a subfloor over concrete

I am converting my attached garage (21x22) into a great room. The floor is concrete and is slightly sloped. This floor was poured 16 years ago and there has never been an issue with moisture, and there is only one small crack.

Does anyone know if I can build the subfloor like this:
Lay 6mil poly for a vapor barrier
Styrofoam board over the top of the poly about 3" thick to achieve R-value
Lay 2x4 pressure treated creepers flat to be used as nailers for the T&G subfloor, then hardwood over the top.

Does anyone have any thoughts....Thanks!
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Old 08-15-07, 07:48 PM
Join Date: Feb 2005
Posts: 67
your idea is well thought out, but very unconventional. insulation is a nice idea, but i would be concerned that it would crush over time. Floors are ususally installed over concrete slabs in three layers:
-2x4 Pressure treated sleepers at 24-48" o.c. flat on the slab attached with power actuated fasteners (Hilti Ramset or other)
-2x joists at 16" o.c. - rip joists to eliminate slope (as much as possible), pick a joist depth to match new floor to existing floor level and to hide HVAC.
-floor sheating

insulation is only an issue at exterior walls; if at all. install blocking between joists at 8' max spacing. you can eliminate the 2x4's flat if you use Pressure treated joists. typically garage slabs slope from all walls which will be a test of your skills to overcome.
Old 08-17-07, 06:39 PM
Join Date: Aug 2007
Posts: 7
Need more info

Is your house floor level with the garage or do you step down. What is the intended purpose of the new room. Have you thought about radiant floor heating for that room. Do you ever plan to re-convert. How about windows and the header and sill height to attain egress. Like the previous reply, the standard MO is to lay flat 2x4 P.T. on 16" center and attach with powder actuated tool. then (depending on height) joist out the room. Make sure you use a P.T. rim against the stem wall. Insulation is an option, but it has to breathe. 3/4 " plywood will do the sheathing. Frame in the garage opening.Check for attaining necessary permits and check your CCRs. Have fun and be an asset to your neighborhood OB

Last edited by twelvepole; 08-22-07 at 08:12 AM. Reason: Not necessary to quote entire post in order to reply. Quote deleted.

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