Garage slab elevation questions

[Jayhem]

I'm starting a design build of my garage. I've drawn up the plans myself going by local code as best I can. My question right now has to do with the elevation of the slab floor in relation to surrounding grade. I will NOT be cutting grade between the garage and house, instead I will be installing a 24" foot wall on footings as my foundation for the stud walls and then pouring a floating slab within the wall.

Looking at the front of the garage (24'W) the natural grade is 18" lower on the left then on the right. My house is 10' off the right side of the garage wall. Would you make the slab the same elevation as grade on the right wall or just a few inches above that?

I'm also installing a french drain 12" off the garage on the right side to divert surface runoff downslope around garage to daylight.

In this detail render I've made the black sliver on the left under the slab is compacted fill I'll have to add to bring up that side for a level slab. My footing on the left side is also going to be quite a bit deeper to maintain 15" minimum below natural grade.

Any advice is appreciated! I would like to start digging the footings this month!

 

[Jayhem]

Here's my foundation wall detail.

 

[astroracer]

I would raise the slab 6" above grade. Dig your footings extra wide to provide room for your drain and never have to worry about water in the building. 6" is a decent step up into the shop from your grade\sidewalk.
24' is pretty narrow with 2 doors. 9' doors in your drawing? I ALWAYS preach this when designing a shop or garage. Pay attention to the actual room you have alongside the vehicles for storage or workbenches and even opening the car doors fully.
Your 24' building is only 23' on the inside. Minus the 18' for the doors leaves you with 5' to split three ways for wall clearance. That's just over 1 1/2' from the edge of the door to the inside wall. Going to be pretty tight in there. Just take this as an observation on my part. You build what you can build, BUT if you could go 28' that would be better.
Mark

 

[Radix2]

You want the finished floor at least 6" above the finished grade. It is also desireable to have your block and sill at least a couple inches above that. Since you have a trapped area there, it is even more important not to let it get too low. And make sure the natural slopes take the water around. The buried drain tile/French drain is not to be trusted.

 

[readhead]

Most building departments will require 6"-8" from framing to finish grade.

 

[GMCGarage]

Here's my foundation wall detail.

If you use this detail, extend the rebar into the footing more and have at least a 90degree bend at the end to fully develop the bar.

 

[Jayhem]

I would raise the slab 6" above grade. Dig your footings extra wide to provide room for your drain and never have to worry about water in the building. 6" is a decent step up into the shop from your grade\sidewalk.
24' is pretty narrow with 2 doors. 9' doors in your drawing? I ALWAYS preach this when designing a shop or garage. Pay attention to the actual room you have alongside the vehicles for storage or workbenches and even opening the car doors fully.
Your 24' building is only 23' on the inside. Minus the 18' for the doors leaves you with 5' to split three ways for wall clearance. That's just over 1 1/2' from the edge of the door to the inside wall. Going to be pretty tight in there. Just take this as an observation on my part. You build what you can build, BUT if you could go 28' that would be better.
Mark

You are right. Bigger is better. I can get 26'W x 32" deep footprint...any wider and my grade on the left gets really bad as it drops off at 20% grade just after the 26' mark.

If I can get it 26'W (25'W inside) with a single, 18 or 20' x 8' door It will certainly be better. I'm waiting on a truss revised quote extending it to 26' W.


So with 6" slab height above the grade in front of the doors, would you recommend adding a concrete ramp? Initially I was just planning on bringing in crushed gravel to build my ramp up to the slab and installing a rubber sill under the doors to prevent rain from being pushed under the doors by wind. My driveway is gravel but after the garage is done and the budget comes back I plan to concrete an area in front of the garage the width of the garage by 24' long for additional parking.

 

[Jayhem]

Most building departments will require 6"-8" from framing to finish grade.

From framing? I'll be at least 12" from framing to grade in the highest spot which is the right/rear of building. That was my reason for the 24" block wall foundation, gets me up out of the dirt without having to level the entire area and worry about fill settlement cracks in my slab.

If you use this detail, extend the rebar into the footing more and have at least a 90degree bend at the end to fully develop the bar.

Will modify the plan to do so, that makes sense.

 

[theoldwizard1]

I would raise the slab 6" above grade.

ABSOLUTELY !

The OP's drawing show a small berm along the right side of the garage. That is a big NO-NO !

Many municipality now require that foundation be pour so that there is a 6" curb above the slab/floor. Pouring a slab/floor that is NOT ABOVE GRADE ON ALL SIDES is asking for a problem.

 

[astroracer]

Build your driveway out of the #57 gravel when you do the base. This will save the cost of excavating it when you get the coin to do the concrete. Build your ramp to the floor with crushed limestone as this can be incorporated into the driveway base when the time comes.
Mark

 

[readhead]

Yes, from framing. The wall could be waterproofed and dirt placed to within 6"-8" of framing.

 

[Jayhem]

ABSOLUTELY !

The OP's drawing show a small berm along the right side of the garage. That is a big NO-NO !

That is showing gravel that extends from the top of the french drain to the foundation. I'm planning to line the french drain and up to the foundation with poly liner under the gravel to intercept any runoff coming from the house area. Is that not advisable?

Many municipality now require that foundation be pour so that there is a 6" curb above the slab/floor.

Could you explain this more? Is it not ok for my footings to be below the slab height and then a block stem wall extended above that with the slab fully inside the block wall, 6" above grade at the garage door openings at the highest grade side? What do you mean by a "curb"?

 

[Chris705]

Jayhem - will you be bringing in an excavation contractor to clear away all the top soil from the building pad? I would adjust your section so the floating slab is entirely above your footing/grade beam. Also I think you will want some continuous reinforcing bars in the footing. I'd run two continuous #5 bars. Bring your vertical bars down to these horizontal bars and tie them together, then align your anchor bolts with the vertical bars. Yes until you are ready to pour your apron or approach slab stop your floor pour at the centerline of your overhead door. The door will come with a rubber weather strip on the bottom if your buying a new OH door. Establishing your slab height is important, so once that gets fixed, I'd be sure to get all the topsoil out of way of your footprint, including the footings. That may force you to make the footings deeper. Fill (gravel, crusher run or whatever is used in your area (which would be good to know) can be brought in to level out the subgrade before pouring the slab.

 

[Jayhem]

Jayhem - will you be bringing in an excavation contractor to clear away all the top soil from the building pad? I would adjust your section so the floating slab is entirely above your footing/grade beam. Also I think you will want some continuous reinforcing bars in the footing. I'd run two continuous #5 bars. Bring your vertical bars down to these horizontal bars and tie them together, then align your anchor bolts with the vertical bars. Yes until you are ready to pour your apron or approach slab stop your floor pour at the centerline of your overhead door. The door will come with a rubber weather strip on the bottom if your buying a new OH door. Establishing your slab height is important, so once that gets fixed, I'd be sure to get all the topsoil out of way of your footprint, including the footings. That may force you to make the footings deeper. Fill (gravel, crusher run or whatever is used in your area (which would be good to know) can be brought in to level out the subgrade before pouring the slab.

Yes, I will be having a backhoe dig my footings and will be using a loader to remove the rest of the top soil under the slab.

So you are saying my floating slab should be above the upper face of my footings and just floating along my block stem walls? I'm probably going to go with a 4-course block stem wall above my footings which will give me plenty of room to keep my footings deeper. The footings at 18" below grade will be rested on sandstone bedrock for the most part as my auger boring indicate digging refusal at that depth.

Yes, I plan to use horizontal runs of rebar in the footings, good idea about tying them to the vertical rebar through my stem wall. I'll also have to put a 90 degree bend on the bottom ends of my vertical rebar to anchor them in the footings.

I was talking to a friend today who used to be in construction. He recommended ending my slab on the INSIDE of the OH door centerline and adding a 18-24" wide skirt 1" below that for the doors to rest on which prevents wind blown rain from pushing under the doors. Kinda like this:

SKIRT_____||-----------------------SLAB
...............DOOR

I'm in full agreement that setting the slab height is one of the most important parts of this project as the entire rest of the building is based on getting that part right!

 

[Chris705]

Yep. That OH door detail is good, create a little bit of a vertical lip from inside to exterior apron slab....again not sure of your location but if you ever wanted to heat or cool your garage I'd say end the interior slab at the OH door center line until you pour the apron sometime in the future. That way the door can come down onto the slab edge, you can then add 1 or 2" of rigid insulation at this face of slab edge before pouring the apron at a slight incline and 1/2" lower than main garage slab. Bevel top edge of insul at a 45 and you will have isolated heat/cold transfer along this edge.