Garage Foundation Question ?

[James Nipper]

I want to build a small garage at a vacation cabin in the mountains. Several years ago, I had a spot cleared at a spot that I intended as a level parking spot. I had some rock delivered, so that the spot was level. So, I have an area of about 25 feet by 35 feet, where the rock (because of the land slope) is about 1 foot deep on one end, and about 6 feet deep on the other end. (That is quite a pile of rock, and has been a good parking spot).

Here is my question: I know that foundation footers should be dug to below the frost line, which in this area is about 3 feet. This is because of frost heave, as I understand it. But, it is very unlikely to me that I would experience frost-heave in an area where I have 3 to 6 feet of rock, right ??

I can't remember the type of rock, but I think it is limestone. It is 2-3 inch size. It has some "fines" in it, so that is has bonded together very well. It is VERY hard , and stable.

Questions:

1. What type of footer would I need, and how deep should I did it ?

1. I am assuming that I would have a footing around the perimeter of the garage, and then the concrete floor of the garage would just be poured 4-5 inches thick, right? Is that what is called a monolithic slab? Would I still need some pea gravel and/or sand, to be put over the rock described, before the concrete is poured ?

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I know these questions are basic, but I don't want to sound too green to my contractor.

Thanks for any advice !!

--james--

[Matt Whiting]

Correct, if the stone has adequate drainage.

If it is too many fines and has hardened into almost a concrete like state, then it may not be porous enough to drain the water fast enough to prevent heaving.

You may not need one at all, just a slab.

If the drainage is good, you likely need little else on top. If the drainage is poor, then you'll need to dig footers to below the frost line, which could be rather deep on the deep side of your rock pile.

Matt

[willshak]

On 1/3/2005 9:12 PM US(ET), James Nipper took fingers to keys, and typed the following:

Check with your local building inspector. See what he will accept. I don't think that having 1-6 feet of stone piled on top of the ground meets the requirements of a 3' frost free footing.

[HotRod]

I know several people including my Father who have large shops on what's called a "floating slab" there are not any footers, the whole slab is designed to move and "float" as the frost comes and goes. My Dads shop even has a hoist in it without any problems in cold Ontario for over 15 years, not even a single crack...

[bill a]

Some areas (inspectors) understand floating slabs, and some don't. They work great, however. Another alternative I've used on shop buildings is to have the structure supported by piers (sonotube, whatever) that go well below the frost line, and then an ordinary slab floor. hth bill

[Lil' Dave]

A monolithic slab is a one-piece unit. It consists of beams on the perimeter, and a much shallower surface within the beam perimeter. How much stone, road base, or other fill depends on what you intend the slab to support. Yours will probably require at least one beam down the center as well. You will need steel (1/2" rebar) in the beams and to support the surface of the slab.

The garage wall bottom plate should be secured with 1/2" J-bolts embedded in the slab's perimeter. Where exactly these should NOT be depends where you intend to put the garage door and any other entry door. So you need to come up with floor plan now.

[3D Peruna]

Frost-protected shallow foundations.

Google it.