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# use of 2x4's for joists for sub-floor

#1
10-23-15, 08:13 PM
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Join Date: Nov 2007
Location: USA
Posts: 199
use of 2x4's for joists for sub-floor

My son owns a house that had a one-car attached garage that was converted to a room by a former owner. Since the walls are finished, the only evidence that the room was a garage is the concrete floor, which has some slope, which is typical for garage floors.

We discussed the idea of putting a false wood floor over the concrete. To maintain a high ceiling, keeping this sub-floor as low as possible in desired. The joists will be about 9'4" long.

We know 2x4's are not normally used as floor joists, but I am thinking they will fine as long as a support is place in center of each joist. By doing so, the effective span for the 2x4 joists would be about 4 1/2 ft. Does this sound feasible?

If two supports were equally spaced under the 2x4's, then the effective span would only be about 3'. I'm wondering whether one support would be sufficient. The joists will be spaced 16" apart, center to center. We would probably put 23/32" OSB panels down for the sub-floor.

#2
10-23-15, 09:19 PM
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Join Date: Mar 2006
Location: USA
Posts: 32,950
If you want to take the slope out you should run them length wise. Much easier IMO. Run you joist with adequate blocking on the low end so it is level then you can easily figure your blocking every four feet just by measuring from the floor to the bottom of the joist. Do one joist on each side and verify you have level cross wise before filling in the joists between. The only problem will be the crown so choose straight 2x4s.

#3
10-25-15, 09:13 AM
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Join Date: Nov 2007
Location: USA
Posts: 199
Thanks for your reply. After installing some joists, I see that a slight variation from perfect straightness can cause differences in height at the crown (center point of joists), as you mentioned. I think I might need to use some shims on top of one or more boards to provide even support for sub-floor panels installed on the joists.

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