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Old 10-14-2013, 07:56 PM   #1
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Garage Conversion - How to build a raised subfloor


Hello all,

I'm trying to collect some ideas on how to create a subfloor for a garage conversion, with a couple of specific questions.

The garage is currently a smooth slab that is 8" below the subfloor of the rest of the house. The rest of the house has 5/16 top nailed red oak for flooring. The garage is 12x23' 6", and the actual garage floor slopes downward toward the overhead door about 2.5" over the 24 ft direction.

An HVAC duct will be run from the crawlspace, and must go the long direction to get to where the vent needs to be. After collecting some ideas from different places, my plan for this floor is to
  • Lay down 6 mil poly
  • Fasten down three 2x4x12 PT sleepers widthwise across the slab with a Ramset, adding shims on 2 of 3 sleepers to make sure the floor is level considering the 2.5" slope.
  • Lay 2x6x12 floor joists lengthwise so that the floor joists run the 24' span. This will allow the HVAC duct to be run through the channel.
A couple of questions: Should the floor joists be butted together on a double wide sleeper in the middle, or should they overlap side by side? Should I try to find 2x6x24 for the floor joists to eliminate this issue? Should the floor joists be 12" OC to compensate for the 12ft span, or add more sleepers to reduce the span?


One suggestion was 2x8 lumber, but then one end of the floor joists would have to sit right on the slab in order to create the matching final height, and the sleeper in the middle would only be 1.25" or so.
Also...pressure treated for the joists, or just the sleepers?

Many thanks in advance!

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Old 10-14-2013, 09:15 PM   #2
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Garage Conversion - How to build a raised subfloor


I would stick with 2x6 joists. 12' span is far too much for 2x6, even at 12 OC. I would use four sleepers, to give you aprox 8' joist spans. Technically, you could probably span the joists 24 OC, but that's too tight to code for me. I'd do 16"OC. You only need the sleepers to be PT, particularly since you're laying down a vapor barrier.

But my most important advice is: (a) get a permit, (b) check with your HOA, if these apply to you.

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Old 10-14-2013, 09:31 PM   #3
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Garage Conversion - How to build a raised subfloor


A 24' joist will be stronger because each span acts like a cantilever over the sleeper, in essence reducing the span opposite the first span. (weight of one span tries to lift the opposite span) It may not be needed for strength but it reduces bounce. An engineer could tell you technically how it works.
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Old 10-14-2013, 09:48 PM   #4
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Garage Conversion - How to build a raised subfloor


Thank you for the input!

I'm curious what is the best way to shim the joists? Looks like the end of the garage will need around 2.5" of shim. I could rip the sleepers out of 4x4 pt stock so that each sleeper is the correct height to provide a level floor, or use some other wood underneath. Suggestions? I'm curious about the size and type of shimming material that is preferable.
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Old 10-15-2013, 06:40 AM   #5
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Garage Conversion - How to build a raised subfloor


Quote:
Originally Posted by malgyver View Post
I'm curious what is the best way to shim the joists? Looks like the end of the garage will need around 2.5" of shim. I could rip the sleepers out of 4x4 pt stock so that each sleeper is the correct height to provide a level floor, or use some other wood underneath. Suggestions? I'm curious about the size and type of shimming material that is preferable.
I would, as you suggested, rip 2x4s and 4x4s to the needed sleeper height, but I wouldn't shim. I would instead cut birds mouths in the joists. These will be very slight notches, as the pitch in a garage is minor (typically 1/8" per foot). Believe it or not, code allows notches at joist ends up to 25% of joist depth. The bird mouth will require much less than that.
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Old 10-15-2013, 08:14 PM   #6
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Garage Conversion - How to build a raised subfloor


No pt required with poly vb, #3; http://publicecodes.cyberregs.com/ic...9_3_sec017.htm

Where are you located? Heating climate, add rigid foam board on poly between sleepers to uncouple joists (thermal bridging) from earth temps; http://www.buildingscience.com/docum...n-crawlspaces/ Add 2x4 blocking along top edge to prevent rollover at laps--- though sandwiched between side walls......

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Old 12-15-2013, 11:00 PM   #7
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Garage Conversion - How to build a raised subfloor


Hi Malgyver,

I'm in the same boat as you are and in the process of converting my one car garage to a room and need to build a raised subfloor. What did you end-up doing to level your floor to compensate for the slope?
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Old 12-15-2013, 11:13 PM   #8
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Garage Conversion - How to build a raised subfloor


I'm thinking doing something like this in my case. Instead of using sleepers, I was thinking of using 4x4 posts and attaching floor joists to them via joist hangers. This way I can cut 4x4 to match the garage floor slope and even out the floor. Would something like this work and meet the code?

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Old 12-16-2013, 08:33 PM   #9
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Garage Conversion - How to build a raised subfloor


Neither your concrete or beams are large enough to support the point loads. Run it by the local building department to see if it will fly....

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Last edited by Gary in WA; 12-18-2013 at 02:06 PM. Reason: comp.
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Old 12-16-2013, 11:32 PM   #10
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Garage Conversion - How to build a raised subfloor


Hi Gary,

Thank you for the response. I'm not sure what "nor beam" is. I'd like to make this conversation a somewhat less permanent addition that can easily be removed if I decide to sell, that is why I'm trying to build the floor as a platform/deck within the room without attaching it to any of existing walls or beams.
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Old 12-18-2013, 02:06 PM   #11
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Garage Conversion - How to build a raised subfloor


Thanks, I cleaned it up. Check locally.

Gary

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