Frost depth for garage set into hillside

[jgct]

Hi everyone,

This is my first post here after being a long time reader. I've waited 15 years, and I'm finally moving ahead with building my garage. It will be a 24x32' 1.5 story cape style set into a hillside. The soil is 6 feet higher in the back, so the first floor is essentially a walkout basement. I'm in CT with a frost depth of 42".

How is the frost depth treated in this situation? The designer I hired to draw my plans has drawn a stepped footing that makes a couple steps down to stay 42" below finished grade, but as this will be an unheated structure, isn't finished grade essentially the basement floor? The way he has it drawn the slab is sitting directly on the footing at the back wall. I feel like there is potential for heave in this location, or am I completely wrong?

Thanks!

 

[johninct]

It sounds ok to me. I also am in CT. I have very good gravel/ drainage where my garage is. My garage is also set into a hill. I should have had step footings but don't. In one corner, the top of the footing is literally exposed and I have not had a problem.

 

[billconner]

You might ask this question on thebuildingcodeforum.com. On regular is a CT building official and an architect.

I agree johninct but don't think it is quite code compliant. I built mine into a hill - just 3' at back - and did 60" all around under walls. (I did use a rubble trench foundation.)

 

[firebirdparts]

I think you’ll be okay, but removing the steps would not be a huge deal. Definitely would cost more.

 

[Rusted Nut]

A lot of buildings here in the PNW are built into hillsides. Many of these have unheated crawl spaces with the bottom of the footing exposed on the inside. No backfill or anything covering the footing. I have asked numerous building inspectors, geo-tech engineers, etc… and everyone just shrugs..hmmm. I don’t really understand it, but it’s never seemed to be a problem.

 

[billconner]

A lot of buildings here in the PNW are built into hillsides. Many of these have unheated crawl spaces with the bottom of the footing exposed on the inside. No backfill or anything covering the footing. I have asked numerous building inspectors, geo-tech engineers, etc… and everyone just shrugs..hmmm. I don’t really understand it, but it’s never seemed to be a problem.

Aren't those heated buildings? Crawl space seems unusual for a garage with concrete floor?

 

[Rusted Nut]

Aren't those heated buildings? Crawl space seems unusual for a garage with concrete floor?

Yes, heated buildings; but unheated crawl spaces. Not slab on grade buildings.

 

[ConCretin]

If you've got good soils and drainage, you should be fine. Even though the building is unheated, frost has a hard time developing from the inside. The structure traps the natural heat from the ground enough to keep frost from getting in. It might be good idea to ask your designer why he did it that way.

Make sure you insulate your perimeter walls from the slab elevation to frost depth to keep the frost from getting in through the walls.

 

[jgct]

Thank you everyone for the comments.

@billconner I'll take your advice and try posting to the building code forum. I'm in a small town and at the moment our building department is without a building inspector. Our town in borrowing an inspector a few hours a week from a neighboring city, and I don't believe he's even a building inspector. As a result, I fear I won't have anyone looking over my shoulder, so I want to make certain I get the details right.

Related to what ConCretin said about insulation, I'm also considering a frost protected shallow foundation. This seems to be a great option for a building that will be unheated. I was already planning to insulate the slab and walls, so for the cost of a some extra gravel and foam, I might save excavating so deep. Has anyone else built their garage with a FPSF?

Thanks!

 

[billconner]

I think a FPSF is a terrific idea. You won't even have to add much foam where grade is high, though may want to put foam on that wall for future.