Garage Conversion Raised Subfloor

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Old 04-10-19, 04:13 PM
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Garage Conversion Raised Subfloor

Hello,

I am working on a garage conversion (into a living space). Currently the garage is on a concrete slab and it is about 6.5" lower than the rest of the house. Garage floor also slopes towards its door. In terms of framing, I have already beefed up the studs and rafters from 2x4" to 2x6" to accommodate proper insulation and also framed the windows and skylights. My questions are about how to raise the floor and create a subfloor:

1. What is the process to follow for this?
2. What member do you fasten to what? I don't exactly understand the terminology in the code.
3. Do you lay a vapor barrier or use treated lumber or both? What type of vapor barrier?
4. Also, the garage is 10'x20' and I would like to frame the floor joists lengthwise (parallel to 20' wall) as HVAC will be going through the subfloor and I cannot cut each joist to lay the ducts. How do I fasten the joists to make a 20' one as I don't think I will be able to find a 20' lumber? I don't think the joist span tables are valid for my case since the tables are for strength of joists on their own. In my case, they would be already be supported by concrete floor underneath so there is no worry of bending.
5. How do I address the floor slope? It is not a minor slope so I don't really want to deal with concrete leveling unless I have to.

Thank you!
 
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Old 04-10-19, 05:16 PM
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I'm no expert on garage conversions, but IMO you aren't going to have room for ductwork unless you are using 2 1/4" rectangular ducts. (since you should really have 2" of foam insulation below your joists) Drop hvac through the ceiling if possible.

Here's how I'd do it.

1. Poly vapor barrier.
2. Tapered sleepers, 24" on center, perpendicular to your joist layout, tapconned to the pad every 4 ft or so. This gives your garage a level surface.
3. 2" foam lays on top of the tapered sleepers... where the pad is level and the sleepers taper to nothing, the 2" foam will lay right on the concrete pad in foam adhesive.
4. 2x4 perimeter ledger and 2x4 floor joists, ledgerlok lags through the floor joists and into the sleepers as needed.
5. R13 insulation between joists.
6. 3/4 tongue and groove subfloor on joists.


That equals 6 1/4" of height from where the pad is level to the top of your current floor joists. 2" + 3 1/2" + 3/4".
 
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Old 04-10-19, 06:57 PM
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Thank you XSleeper. Unfortunately I cannot run the HVAC from the ceiling as it is a cathedral ceiling and rafters run perpendicular to the HVAC direction. So considering that, if I want to run the joists lengthwise, what would you recommend? Is it possible to sister two 2x4s somehow to make a 20' long joist? OR would I put another ledger right in the center to attach the joists to it from both sides?

As for the foam insulation, I am a little confused about where it goes; joists sit right on the sleepers, how does the foam sit on them as well? I think I am getting it completely wrong.
 
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Old 04-10-19, 07:02 PM
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I thought that was clear. Sleepers... foam... joists. The foam is between the sleepers and the joists. Where the cement pad is level and there are no sleepers you only have foam over the concrete and your joists on top of that.

They sell 20' 2x4s at real lumberyards. You could also use the ledger between 2x4x10s.
 
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