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Old 04-10-2019, 12:28 AM   #1
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Garage Floor & Foundation Questions

Here is what I think is true. Please let me know if you agree and if not, please tell me where I went wrong.

1. Foundation walls are notched for garage doors. The height of the notch is 4" below finish grade because the outermost edge of the garage concrete floor is 4" thick and it overlaps the notched foundation wall and sits on top of it and thus the top of the garage concrete floor is exactly at finish grade level where it will eventually meet up with a paved driveway. Is that all correct?

2. Trenches are dug for foundation footings and walls. The tops of the footings are at or below the frost line. After the footings and walls are poured and have cured, everything that is enclosed by the foundation walls is covered in washed stone and that includes the trenches that are on the inside of the foundation walls. Is that all true? How deep is the washed stone layer (other than where it fills the trenches where it will be much deeper)?

3. What is usually given to excavation/foundation subs in the way of plans and specs to get a quote on the job? Is a simple plan that just shows the location and length of the foundation walls and how high the basement ceiling should be above the concrete floor and how wide openings should be for garage doors enough because the subs usually know all the details such as how deep to place the footings, what dimensions the footings should be, how thick the foundation walls should be, etc? Or are very detailed plans now given out for quotes where every single dimension is specified (widths, heights, thicknesses) and everything is located on the plot plan to show exactly where it goes? Again, I'm only asking what is needed to get excavation/foundation quotes, not necessarily what is needed to do the actual building. Right now, I'm just trying to cost out building a house. I haven't built a house since the 80s and back then, we used only the simplest of plans and I have no idea if things have gotten a lot more detailed these days.

Thanks very much for your help.


Last edited by audioresearch; 04-10-2019 at 12:38 AM.
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Old 04-10-2019, 06:46 AM   #2
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Re: Garage Floor & Foundation Questions

There is different rules and local codes so I will just run thru what we do here.
You want o organics or certain clay's under the slab or the footing.
In my years of building we have only worked in trenches a couple times because it was sandy soil. the rest of the time the hole is dug out at the depth of the footing.

Working space at depth so the dirt can't fill back in is 5 ft wider than the building, so a trench for a small garage would not be much dirt left in the middle.

The sides of the hole is covered with poly to protect it from rain and washing dirt in the hole.

Our standard footing is 8" x 18" for a 8" foundation wall, your area may have a different spec. for that.

We build the footing form out of 2x6 and lift it to 8" when we level and stake it in place. That leaves room under it if the ground is no perfectly level, all is good.
You need to talk to the contractor to see how level they need the ground and what lumber they use.

Almost every job we do the ground is inspected by and a geo tech engineer and he tells us how wide to make the footing if he is not happy with the dirt on the bottom of the holes. That may not be needed where you are. The city asks for the engineer if they know the soil is questionable in the area.

You want the foundation to be 8" above ground when done so yes you do cut outs for the door.

The man door, if the cut out will only be 8" down from the top of the concrete we just add 2" like you would in normal framing but if it will be much deeper we we make the hole another 3" wider so we can build with a treated 2x one each side of the door.
We leave the height for the door cut out much higher so you can deal with the slope and put the floor slab and fill in the door framing after the floor height is figured out.

The main garage door, we trim the finished opening with a 2x? trim. so we make the opening 2" bigger than the door and then when we finish with the 1 1/2" on each side of the opening the finished hole is 1" smaller than the door.

We leave the cut out much deeper than the floor, so if the slab was to settle the foundation does not cause a hump at the door.
We put blocking just inside the forms to make the foundation to match the wood wall that will be built above just to allow for the door guys to put their track down to the floor with out running into concrete.

Again we put the beam up high in the framing and build it down to door height after the floor is poured.

Inside the building can be filled with sandy soil if that is what came out or we use sand or rock chips or what ever that is clean and compactable.
Best outside is the damp proof the foundation below ground level, perimeter drain beside the footing, drain rock for the first foot. and fill it up, but best with sandy soil that does not trap water.

BTW when forming up for the slab, at the man door come out to the outside of the foundation, the edge will be covered with the threshold.
At the main door stop the slab at the inside of of the foundation so the seam with the driveway is under the door.
landfillwizard likes this.
Do not use flat bottom gables with scissor trusses.

Last edited by Nealtw; 04-10-2019 at 06:52 AM.
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Old 04-10-2019, 02:33 PM   #3
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Re: Garage Floor & Foundation Questions

3. The more specific the drawings, the better the bid. Give verbal or crude drawings now, and you get a ballpark with cushion. Return with detailed drawings and most likely the price still goes up. Eliminate guesswork with a dimensioned plan, section of garage/crawl, and section of basement. Things like thickened slabs, post footings, porch haunches, wider/notched walls for brick veneer, will change the price a bit too. A foundation plan is the easiest to do and notate.

1. I prefer to specify the top elevation of foundation wall. Indicate notches from there (minus). A better detail for an overhead door is notching down 12". Then the slab is thickened over the wall. This allows, if desired, for an inset for the door, rebar to the slab or for the drive flatwork, and slope of the slab but still maintaining min. 4" exposed wall inside.

2. "Trenches" can mean different things. It can refer to a straight sidewall dig for a shallow monolithic pour. In the field it's used as general reference to a deeper foundation dig too, but not done with straight sidewalls. The dig slope is based on soil makeup and available room to not cave in, and the backfill on the exterior is not done with gravel beyond the drain tile (if done for hydrostatic pressure that would require shallow lifts of both soil and gravel at the same time). For a basement interior, the 4" thick gravel base is usually placed on top of the footing, so the extra required to fill the space next to the footing is moot compared to dropping soil from above, shoveling and compacting that. For a garage, there's easy access for the bobcat to enter, and the amount of subbase fill will depend on how much was removed for the foundation walls (whether for slope stability or to eliminate the top organic). Then minimum 4" gravel.

Last edited by 3onthetree; 04-10-2019 at 02:36 PM.
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