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Floor joist direction for raised floor in 12 x 24 garage

Floor joist direction for raised floor in 12 x 24 garage

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  #1  
Old 10-11-13, 07:23 PM
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Floor joist direction for raised floor in 12 x 24 garage

Hello all,

Thank you in advance for any help!

I am converting my attached garage into interior space, and need some initial guidance on building up the subfloor.

The garage is 12 x 24, and the garage door is on the 12' span (of course), and the easiest access to HVAC and plumbing is on the other 12' span that is adjacent to the kitchen. The floor slopes about 2.5" it entire 24ft length, and the highest point in the floor, adjacent to the kitchen, is 8" below the rest of the floor in the house.

My plan was to use a Ramset to nail down pressure treated 2x4 sleepers, and then lay the joists on top of these.

I am trying to figure out which way to run the floor joists. If I run them widthwise, then I will have to somehow shim each one more than the next to achieve a level floor. This way would also present some difficulty in running HVAC from the kitchen area on the 12' span.

If I run them lengthwise, then it will be easy to build the sleepers up widthwise to level the floor, and easy to run HVAC, but the 24' span of floor joist will likely have to be 2 pieces of lumber butted together, as 24' lengths of 2x6 seem either unobtainable or expensive.

Which way would folks do this?

As well, use Pressure Treated for the floor joists, or just the sleepers?

As well, use 6 mil poly below the sleepers, or above?

If I were to use option 2, and run the sleepers widthwise, and build them up as I go toward the garage door, how should I build them up? I have a table saw and can rip, or should I shim them, and if so, with what?

Many thanks!
 
  #2  
Old 10-12-13, 04:15 AM
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Welcome to the forums! Definitely 6 mil poly on the floor. BUT, driving a kazillion holes in it with a ramset will just negate the purpose. I would make a box frame of the joists of PT, allowing them to stand on edge. They will be supported by the boxing effect of cross members. THEN you can go back and shim the entire floor as necessary, creating your level subframe. Lay down your HVAC, etc and cover with Advantech. It won't really matter which direction you lay the joists, and the box effect will tie in the wood so the joints won't matter.
 
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Old 10-12-13, 06:54 AM
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Before all that how do you plan on closing up the old over head door area?
The biggest issues I see all the time is water coming in under the wall, and the front looking like there was a door there in the first place.
It's best to remove the old apron and build a stem wall with block to get the bottom plate up off the slab.
Siding and sheathing should never be any closer then 6" to any grade or solid surface.
Got any pictures?
 
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Old 10-13-13, 01:07 AM
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Hello,

Thank you for the responses!

Joe: I plan to pour a concrete "curb" that would match the rest of the foundation, and then build the new exterior wall, in place of the garage door, on top of this curb. Obviously this wouldn't be a load bearing wall, so I would frame it up simply, side it with the existing 8" shiplap siding t o match the rest of the house, and put a window in it. These conversions are not uncommon in the area, the house next door has the same.

The house is currently a 1948 2br 1ba 900 sq ft, with a detached 400 sq ft garage built in 1985, on a 5000 sq ft lot. The original attached 12 x 24 garage is ripe for conversion to interior space. To make the house "family friendly" with a 3rd bedroom and a second bath adds about a 100K to the value here, and I'm planning to sell and move imminently.

I am in the Berkeley area. With a 2" gap below the garage door, and a driveway sloped toward the garage, I still have never seen water in this garage in 2 winters here.

Chandler: Im so sorry, but your post was like a foreign language. I don't know what a "box frame of joists" are. Entry points for HVAC are limited, and if the floor joists run the 12ft direction, then there's no way to get a 6" or 8" duct to where it needs to be. The current slab is 8" below the rest of the floor in the house, so the floor joists are going to nearly sit on the slab. Not sure how to run a large duct through a 2x6 joist. If the floor joists run the long direction (24'), then the "sleepers" are only 12 ft long, and I imagine I only need 3 of them, making the floor joist span at 6ft. Or 2 sleepers with a floor joist span of 8ft. How many Ramset pins do you need for a 2x4x12 on a smooth slab? I'm sure I'm missing something here, please advise.
 
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Old 10-13-13, 03:53 AM
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I'm missing something here
Just a little. Don't drive anything through your vapor barrier. Your frame can be built with the lumber standing on edge, connected by additional lumber to form a "box". This will keep your framing from falling over while you level it with shims. In essence you will level the entire floor with the framing put together as a unit. I don't know that ductwork 8" is necessary for such a small space. You won't need the 2x4's laying flat on the floor, just the 2x8's as joisting.

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  #6  
Old 10-14-13, 11:06 AM
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Hi,

Thank you for the clarification. I see now that you are talking about making two 12x12 boxes. I researched Purlins, and it seems like these simply act as beams below the joists.

I guess I'm wondering what the difference would be between the purlins and the sleepers I had originally mentioned. They both go below the floor joists at spacing that works to allow the proper span for the size of lumber I am using.

So, to clarify your plan, I would use 2x8x12s as floor joists going the long direction. At the high end of the floor, the 2x8 will have to sit directly on the floor (vapor barrier), as this will be the proper floor height to match the rest of the house once 3/4" subflooring is laid on top. If this isn't preferable, I'll have to use 2x6 lumber, and raise the this end as well.

At the garage door side, where the slab is 2.5" lower, I will rip a 12 foot piece of lumber (the purlin) at 2.5", and lay this under the floor joists in order to level it with the other side. In the middle of the span, I will rip a ~1.25" piece of lumber (purlin), which will support the middle of the span at the right height. Of course I will level the floor as needed, and rip the purlins to the right height, and not just use the 2.5 and 1.25" assumptions. Should I use 2 purlins in the middle of the span and butt the floor joists as in your drawing, or use 1 purlin and overlap the joists on the single purlin? The long span is actually 23' 6". Does the purlin need to be taller, with channels cut in for the floor joists, or can I just lay the floor joists on top?

Should I search out 2x8x24' lumber, or OK to use 12' lumber, and butt/overlap it?

With such a small purlin, I see that running the HVAC will not be an issue.

Should I use PT lumber for the Joists, or just for the Purlins?

Finally, it sounds like this floor will not be attached to the concrete slab, is that correct?

Many thanks!
 
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Old 10-14-13, 01:44 PM
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Poly below the sleepers, pt on sleepers only. Holes in a vapor barrier are fine, not in an air barrier. The Ramset holes won't make any difference in the vapor barrier, other than 1/4" each, lol. Use 2x6 joists and shim them anywhere- as needed on the full slab below. Attach the framing to the slab- in a seismic area as I. Check local code on HVAC in its own room, not bedroom closet if gas. Egress window/door required.

Gary
 
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Old 10-14-13, 06:45 PM
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Gotta wrap my head around that one, Gary. Holes in a vapor barrier won't matter??? I seem to remember if I stick a pin in a balloon full of water.......Help me out here. I the VB is so important, why punch holes in it. If the holes don't matter, why have the VB??
 
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Old 10-14-13, 08:33 PM
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Yes, many get confused on that, here are a few links, I know you'll get it on the first one, just don't get to use these links very often, lol.; pp.5; "Magnitude of vapor diffusion..."; http://www.buildingscience.com/docum...researchreport

Paragraph below Photo 6; BSI-003: Concrete Floor Problems — Building Science Information

Podcast; Air Barrier or Vapor Barrier? - Building Science Podcast | GreenBuildingAdvisor.com

Gary
 
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Old 10-15-13, 10:47 PM
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May not need/require pt wood with the poly vb, check locally as CA has it's own state code; eg. #3- Chapter 3 - Building Planning

Gary
 
  #11  
Old 10-31-13, 05:51 PM
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??

To me there's a lot of mis-information being passed on in this thread. Why would you place poly on the cold concrete and create a breeding ground for mold? Concrete is porous and absorbs and releases moisture. If you take a basement wall and turn it on it's side we have the exact situation that is being described. In my area when this is the case (which is the same as having a covered deck as it is a structure with a roof) You can use whatever building product below that is necessary. The sheathing acts as the vapor barrier (which has been an issue that has changed theories over the years) and if it doesn't in your area I'm sure that the poly is to be placed on the warm side of the floor which would be the top (and in a basement stud frost wall directly behind the drywall/sheetrock).

Regardless, the course of action I would take if I were to be doing this in my own house would be to use PT 2x8 for the first 12' place sleepers underneath fastened to the floor with PL and a concrete anchors. I would have the joist butt into each other as you initially intended to keep working space for running necessary mechanical and electrical as easy for myself as possible. Simply toenail the joist together, add a row of bridging and glue your subfloor down and be done with it.

To go one step further I would place rigid foam insulation on top of the concrete slab and fill the joist spaces with batt insulation to keep the floor warm as it's going to be a cooler area without making sure there's something between you and the frost. Spray foam the joints where your cuts aren't perfect form that vapor barrier with the foam and insulate at the same time.

I've built a cabin in northern Alberta where temperatures get to -40 degrees with 2" blue rigid foam R-28 batt insulation in a 9 1/4" (2x10) floor and walked on it with my socks at the coldest temperatures using this building method......laying poly and building into where you intend to run mechanical through will create no advantage for you.

The only thing with this method is that you're relying on your slab for the floor structure. Adding the load to from your living space shouldn't be more than your vehicle in my humble opinion and if there's no visible cracks and heaving and you don't require an engineers stamp of approval of building permit I would carry on as described. Here I would have to pull permits and get stamps for a job of this nature.....adding electrical, HVAC etc. usually requires one.

I'm no engineer and it's just my opinion.
 
 

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