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Old 11-24-2009, 01:33 PM   #1
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Garage Conversion-Flooring


We are converting our attached garage into a bedroom/bath. The current garage sits about 3' lower than the rest of the house's main floor. We are going to build the subfloor directly on top of current concrete floor (so it basically will rest on the floor).

My question is, how should I prepare the concrete floor for this? I planned on using a water proofing paint and then covering the floor with thick plastic to act as a moisture barrier. I would then lay 2x8 (to give myself enough spacing for plumbing) atop the plastic. I would like to be able to use regular lumber to stay away from pressure treated due to cost, but I didn't know if there was another "best" way.
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Old 11-24-2009, 05:21 PM   #2
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I assume you mean placing the 2x8s on edge as joists. I would either hang these off ledger boards bolted to the walls or toenail them into 2x4 PT sleepers tapcon screwed to the floor.

It's likely your floor isn't level. Most garages slope to shed water. You would have to do a lot of shimming if you set the joists on the concrete floor and would also need to use PT joists. Even using sleepers I'm thinking you'll need to do a lot of shimming to level but then you won't need PT joists.

What are you doing about closing up the doorway? This can be a source of water problems if not done right.
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Old 11-24-2009, 07:50 PM   #3
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Quote:
Originally Posted by jogr View Post
I assume you mean placing the 2x8s on edge as joists. I would either hang these off ledger boards bolted to the walls or toenail them into 2x4 PT sleepers tapcon screwed to the floor.

It's likely your floor isn't level. Most garages slope to shed water. You would have to do a lot of shimming if you set the joists on the concrete floor and would also need to use PT joists. Even using sleepers I'm thinking you'll need to do a lot of shimming to level but then you won't need PT joists.

What are you doing about closing up the doorway? This can be a source of water problems if not done right.
My building inspector said I could use an 6 mill poly and then use untreated lumber. The garage is old, so I'm not sure how badly it slopes....I'm guessing its not too bad. My first plan was to just build the perimeter of the floor, then level, and add shims where needed, and attach to the existing wall studs.

As far as the door....I'm not sure exactly. I will either build a 2x6 box and pour concrete six inches high and build from that, or just use treated lumber and frame it in from the ground up. Suggestions?
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Old 11-24-2009, 09:15 PM   #4
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I have yet to see a garage floor that is level and flat. I would suggest you frame your floor a little off the floor. This way you have room for any inconsistancies in the floor and also if you are having plumbing in the area which if I remember correctly you said you were, you can run any waterlines under the joists instead of having to drill. This also can help with the wiring.
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Old 11-25-2009, 08:08 AM   #5
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Fastening to concrete


This may be helpful. http://www.confast.com/articles/choo...te-anchor.aspx
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Old 11-25-2009, 08:23 AM   #6
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Quote:
Originally Posted by cibula11 View Post
We are converting our attached garage into a bedroom/bath. The current garage sits about 3' lower than the rest of the house's main floor. We are going to build the subfloor directly on top of current concrete floor (so it basically will rest on the floor).

My question is, how should I prepare the concrete floor for this? I planned on using a water proofing paint and then covering the floor with thick plastic to act as a moisture barrier. I would then lay 2x8 (to give myself enough spacing for plumbing) atop the plastic. I would like to be able to use regular lumber to stay away from pressure treated due to cost, but I didn't know if there was another "best" way.
Shim ( and gasket ?) your PT and frame up off the floor.

There is no way your garage floor is level.

Besides, I'm not sure you want to place 6mm poly and trap it to the floor with studs over concrete without any air flow. Maybe it depends on where you live?
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Old 11-25-2009, 09:51 AM   #7
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Quote:
Originally Posted by RobQuillin View Post
I have yet to see a garage floor that is level and flat. I would suggest you frame your floor a little off the floor. This way you have room for any inconsistancies in the floor and also if you are having plumbing in the area which if I remember correctly you said you were, you can run any waterlines under the joists instead of having to drill. This also can help with the wiring.

Would you suggest using 2x10's and anchoring the rim joists to the current walls? They would span approx 13 feet. I just don't want to give up head room.... since the ceiling will be vaulted.
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Old 11-25-2009, 09:52 AM   #8
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Shim ( and gasket ?) your PT and frame up off the floor.

There is no way your garage floor is level.

Besides, I'm not sure you want to place 6mm poly and trap it to the floor with studs over concrete without any air flow. Maybe it depends on where you live?

I thought about air floor, but how else do I keep moisture from getting into the insulation (anytime there is a heavy rain or high humidity, the floor is damp. I live in Iowa. I would use 2x6's but I'm afraid that wouldn't give me much wiggle room for plumbing. All fixtures will be along one wall...so the need for running pipes through joists won't exist.

Last edited by cibula11; 11-25-2009 at 09:55 AM.
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Old 11-25-2009, 09:52 AM   #9
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If you frame out the door right on top of the slab you'll very likely get moisture intrusion and the sheathing at grade level will likely rot. If you pour a 6" concrete "curb" on top of the slab you'll get the sheathing clear of the grade but you'll still have a potential problem of water migrating between the slab and curb in to rot your joist bottoms or create odor/mold.

If it was me I'd cut the slab at the opening even with the inside of the current garage stem wall, drill in some rebar to the current stem wall on the sides and stem wall/footing below. Then form up and pour concrete to match the rest of the stemwall.
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Old 11-25-2009, 10:00 AM   #10
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If you frame out the door right on top of the slab you'll very likely get moisture intrusion and the sheathing at grade level will likely rot. If you pour a 6" concrete "curb" on top of the slab you'll get the sheathing clear of the grade but you'll still have a potential problem of water migrating between the slab and curb in to rot your joist bottoms or create odor/mold.

If it was me I'd cut the slab at the opening even with the inside of the current garage stem wall, drill in some rebar to the current stem wall on the sides and stem wall/footing below. Then form up and pour concrete to match the rest of the stemwall.
I thought about doing that. If I used a PT 4x6 and placed that on the bottom as a plate, would that suffice? My building inspec. just said I could build straight from the slab.
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Old 11-25-2009, 11:31 AM   #11
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I'd worry about water getting in but I'm a worryer about things like that. I guess there are houses framed right on slabs all over the country that don't have problems. Maybe it depends on how far the slab sticks out beyond the wall, how big the roof overhang is and whether the grade brings water too the slab or away.
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Old 11-25-2009, 12:08 PM   #12
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I'd worry about water getting in but I'm a worryer about things like that. I guess there are houses framed right on slabs all over the country that don't have problems. Maybe it depends on how far the slab sticks out beyond the wall, how big the roof overhang is and whether the grade brings water too the slab or away.

The slab extends about 6 inches to the outside and the eve hangs over by a good 16". I think at the very least, I'll use a treated 4x4 just to give the sheathing and studs an extra 4" just in case.
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Old 11-25-2009, 12:09 PM   #13
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Would you suggest using 2x10's and anchoring the rim joists to the current walls? They would span approx 13 feet. I just don't want to give up head room.... since the ceiling will be vaulted.
I'm not exactly sure so ask someone else or make sure to find the carry loads yourself, but that's exactly what I'd do.

Maybe over 13ft, you can get away with 2 x 8's? CHECK!

2" is ALOT and I know every bit counts in a garage!
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Old 11-25-2009, 02:17 PM   #14
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I'm not exactly sure so ask someone else or make sure to find the carry loads yourself, but that's exactly what I'd do.

Maybe over 13ft, you can get away with 2 x 8's? CHECK!

2" is ALOT and I know every bit counts in a garage!

I looked at the garage and I would need 2x10s 16" o.c. Then raising to clear the slope in the concrete I'd be well over 1' loss. I don't think that will work with the ceiling height (I'm having to remove the old 2x4 ceiling joists and add collar ties to give me more heighth)

Is there another option where I could paint concrete, lay 6mm poly, and lay my joists over? That way I can add insulation to fill the 2x8 cavity.

Is there a way to create ventiliation, or do I even need it since the poly will act as a moisture barrier.
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Old 11-25-2009, 03:54 PM   #15
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"My building inspec. just said I could build straight from the slab." -------- Really? Usually, they want a footing and stem wall on garage conversions at the big door opening. And possibly a vapor barrier plastic and rigid insulation, if building off the slab.

The exterior wall must prevent water penetration- IRC703.1 (Not caulking a p.t.4x4.

Concrete or masonry wall to extend 6" above finish grade - 404.1.6

The floor system needs to be engineered for the permit, or at least drawn out and approved.
You need a second window or door minimum egress from the bedroom: http://www.taunton.com/finehomebuild...s-windows.aspx

The roof system needs engineering if cathedral. Rafter ties required or a structural ridge beam, not collar ties.
Your County's Health Department should be notified with the bathroom addition.
Is there a furnace or HWT in the location? One conversion I did required a 24" high shear wall across the garage, under the floor for shear flow. Be safe, Gary
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