Garage Slab-Level on Perimeter, Sloped Interior?????

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Old 09-29-13, 06:41 AM
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Garage Slab-Level on Perimeter, Sloped Interior?????

In the process of purchasing a 28' x 24" two car garage with 1/2 story storage area above. This is a unit that comes in two halves (front and rear) and is put together on-site. I've been to the plant where these are made and the craftsmanship is excellent.

County Permits dept. is already giving me grief about the drawings, but I should be able to overcome their concerns except the concrete slab. They want the slab sloped from the rear toward the garage doors and claim floor drains are not permitted although the building code (IBC 2012) the County follows allows for either floor drains or sloped floors. I'll be calling out the Permits Dept. on that one to see if they have a local amendment that specifically prohibits floor drains.

I have spoken to one concrete contractor who deals with this garage builder and he says they just usually slope the entire slab.

In effect, this will put the entire building 3" out of level from rear to front if a 1/8" per foot slope is utilized. Even I can see a 3" out of level building if viewed from the side, so this option isn't very appealing to me.

My question to the concrete experts is the following:

Is it possible to pour a slab where the perimeter (8"-12") is level and the interior is sloped?

This would allow the garage to be level and still address the interior slope mandate.

I can see an issue at the garage door opening where the transition from level to sloped would have to be very abrupt so the bottom of the garage door would sit flat.

Just FYI--The building is not designed to be lifted by crane. It is positioned by use of a wheeled motorized dolly that has to have a flat (Not necessarily level) surface to operate on, so I can't pick it up to sit it on level foundation walls and pour a floating sloped slab inside of the walls.

Thanks for any comments/suggestions.
 
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Old 09-29-13, 07:37 AM
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Is this going to be a stem walled, monolithic, or slab on grade foundation?
All of them are going to be level on the outsides edges.
There's 0 reason if done right the building it's self is going to be out of level.
 
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Old 09-29-13, 07:41 AM
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It will be a stem wall. Contractor stated he pours the slab on top of the stem wall, but slopes the entire slab so it meets a requirement to have a sloped floor.

Even though the stem wall would be level, the slab slopes which means the structure on the slab would be out of level. Not what I want.
 
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Old 09-29-13, 10:24 AM
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Your contractor is taking the lazy man's approach. Probably because no one has ever questioned him on it before--with the "it" being he likes the entire garage to be built on a slope. A definite drawback doing it that way, because the bottoms of the wall plates will never dry out. Even if preservative treated, they will turn black with mold and start to stink before too long. Saw something similar happen in a high-end home built two doors down from us in SW Colorado--the builder was a Texan who had never built in snow or rain country before, and he chose to build a flat floor, with no curbs or slope at all. What a mess.

The correct way to do it is for the curbs to be variable-depth while having their tops level, and the floor sloped downward towards the vehicle door. For a 1/4" per foot slope on a garage that's 28' deep, this means the curbs will be 7" taller at the vehicle door than they will at the opposite wall (I think the 1/8" per foot slope you mentioned is too flat to be effective at moving floor water, as a lot of concrete finishers can't finish that accurately). If your contractor refuses to build integral curbs, find a different contractor.

Or offer to show him how it's done, since it sounds like he doesn't know how to form and pour integral curbs. For a significant fee adjustment, of course.
 
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Old 09-29-13, 10:29 AM
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Can't have curbs

Thanks for the reply, but I can't have curbs, since there is no way to lift the building onto them.
 
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Old 09-29-13, 10:38 AM
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Actually, you can have curbs. Simply lay down level, temporary ramps for the motorized dolly to run on. Heavy timber planking, with variable-depth blocking, can be used. They would transition outside of the garage down to driveway grade.
 
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Old 09-29-13, 11:02 AM
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A stem wall (block or concrete) with a slab poured on the prepared grade inside the projecting stem/curbing is the best way for several reasons.

1. You can have the wood portion of the garage above the soil to prevent rotting and water entry.

2. You wall have a curb (6" or 8" thick) above the interior finished floor height that makes it many times easier to wash or sweep clean. The wood frame structure bears directly on the level stem curb.

3. The floor can be contoured as you wish (slope to front door or even slope to 1 or 2 floor drains) and still have a level place for the door to close against with a slight apron slope away from the garage.

Dick
 
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