New Garage Slab - Foundation questions

[Shriner]

Due to the large expense and permanent changes after this is done; I want to double check something before moving forward. As part of our fire rebuild, we ran into a dilemma. We have a four car garage but half was part of an addition. We wanted to replace one slab which was cracked and heaved (original portion of the garage) since the house was emptied out due to the fire. The County said we could not replace it since there is no 6" gas curb into the home. Well, I figured I can kill two birds with one stone and replace both slabs while lowering them 12" which would give me room to stack cars on a lift and then comply with the newer gab curb code.

Fast forward...Both slabs are currently removed and the garages have been dug out to the necessary depth. One thing I did not think in my head, not anticipate, was that there would be a foundation wall under the front of the slabs where the overhead garage doors are. In Chicagoland area, I imagine this wall was four feet down. I came home yesterday and the concrete contractor notched out the sides with a concrete saw and then busted up roughly 12" of that wall so the new slabs will now be 12" lower and fit OVER the old foundation wall. Any structure concerns from this? Heaving concerns? Keep in mind the yard slopes away from the front of the garage.

This will be a typical floating slab pour (4" concrete over 4" gravel, #10 welded wire mesh and 6 mil vapor barrier). We may toss PEX tubing in there in case we heat it later. I have a picture which I can try to upload if that helps.

 

[Shriner]

 

[jkuro]

It's not a floating slab as it sits on the house foundation.

Being in Chicago what is your frost depth. Lowering the front 12 inches may compromise that. I would check

From the pics, you may have a water problem in front of the garage after lowering it. I would consider a french drain across the front.

 

[ConCretin]

This is a common detail although the slab is typically thickened where it passes over the foundation wall. You may get some cracking where the slab transitions from being firmly supported by the wall to being supported by soil. At the very least, I'd add some rebar to keep the cracks tight.

Another option would be to stop the slab at the inside face of your overhead door and place a sloping approach apron outward a couple feet. I'd place the apron a little lower than the slab to create a small step behind the doors, which would help keep water out.

 

[Shriner]

Thanks guys. Water definitely should not be an issue as the asphalt driveway will be up against the slab and we have a huge slope away from the house on this side (side loader garage). Good advice on the rebar and cool idea on the apron. I was more concerned with the foundation wall being weakened since we have living space above. Are garage slabs normally poured over a foundation wall where the entrance is?

 

[wssix99]

+1 The way you have things now are OK. The foundation wall underneath the garage door doesn't carry any loads because there is nothing above it. It serves its purpose to hold the rest of the wall together, provides some insulating benefits, etc.

It is a common practice to pour garage slabs over the foundation wall so it meets the driveway at the outer edge of the house's envelope. However, this approach does present the cracking concerns mentioned above. It also creates a thermal pathway for heat some to escape the garage through the slab. (If you have a heated garage.)

Facing these concerns, I poured a separate threshold over my foundation wall, separate from the garage slab and driveway. You can see that starting in post #4 here: https://www.garagejournal.com/forum/threads/slab-thickness-at-door-thresholds.491191/

This approach adds a good bit of expense because it requires a separate concrete pour. (If you can manage that with your other concrete work, you may be able to escape that extra cost.) In my case, I went this way to avoid the cracking concern and also eliminate exposure of my heated slab to the outside air. (I have a layer of insulation between my threshold and heated slab right where my garage door comes down.)

 

[billconner]

I agree with wssix and did basically same. Apron and slab on top of stem wall was separate pour - along with some unrelated patio so not the cost of a short load. If you do pour it all at once, I think a control joint in line with interior of wall would make sense.

 

[billconner]

Just curious what jurisdiction still requires the step up - gas curb- from attached garage to house? Gone from most codes.

 

[Shriner]

Just curious what jurisdiction still requires the step up - gas curb- from attached garage to house? Gone from most codes.

DuPage County, Illinois. They would not let us replace the garage floor even though the house was built that way. They would let us leave the slab (grandfathered) but it was cracked and heaved so we figured now was the time to replace.

 

[wssix99]

Just curious what jurisdiction still requires the step up - gas curb- from attached garage to house? Gone from most codes.

We don't have auto inspections in Illinois. We really don't even enforce actual laws anymore. (As long as a gun isn't involved.)

The chance of a leaking P.O.S. being parked in one of our garages is very high.

 

[ConCretin]

Are garage slabs normally poured over a foundation wall where the entrance is?

Yes. If you placed the slab against the wall, it would leave the top of the wall exposed, which in addition to being unsightly, makes it less likely that water will drain away from your overhead door. It's much neater to have the slab extend to the face of the foundation.

They would not let us replace the garage floor even though the house was built that way. They would let us leave the slab (grandfathered) but it was cracked and heaved so we figured now was the time to replace

I'd want to know why the original slab "cracked and heaved" before I invested in a new slab. Is the base adequate? Is frost getting into the base material? Is water an issue? Maybe it was just a poor quality install but the heaving has me concerned.