foundations and frost line

catvet · Feb 18, 2007

Don't know much about building but have been reading up on foundations and such. Have a quick question.

Where I'm thinking about building the current grade is about 3 feet below what would be finish grade so would be bringing the foundation up that high and filling. Our frost line is 5' down. Does that mean I need to dig 2 feet down and with the 3 feet of fill I'd have 5' from the surface, or do I still have to dig 5' down?

Thanks as always.

Sberry · Feb 18, 2007

I believe you are right but foundation depth is something that can be very particular to local codes. Your building dept will be able to answer this exactly.

Tig · Feb 19, 2007

Around here the spec refers to depth below finished grade. That means three feet deep and two feet of back fill on top would be acceptable as five feet. BTW, our local code only calls for four feet.

hockeyhead · Feb 19, 2007

Ours is 4' also, our building dept will supply genaric sketches of exactly what they want.

catvet · Feb 19, 2007

When I talk with builders they always are saying go down 5 feet so I'm assuming that is the frost line. As far as building department afraid it doesn't exist. No zoning.

Farwell · Feb 19, 2007

In my area of Michigan it is 42 inches. I had a retaining wall built that was approximately 36 inches high. The dirt side had dirt approximately 4 inches below the wall top. My contractor put in a 12 inch footer below the wall making the base of the wall 2 inches below the frost line if my math is correct. I have had it about 3 Winters and so far all is well.
David B

WVBill · Feb 20, 2007

If what you are building will be heated in the winter to a minimum average temp of 64 deg. F. and you want to use a concrete slab-on-grade, you might want to consider a Frost Protected Shallow Foundation.

http://www.nahbrc.org/Docs/SubsystemNav/Foundations/3808_NAHB_fpsf.pdf?TrackID=&CategoryID=1802&DocumentID=3808

That's what I'm planning to use building a new Studio for my wife's quilting business. Here in WV, our frost line is 30" so a FPSF will save considerably in concrete.

WVBill

catvet · Feb 20, 2007

WVBill,

Good website looks like a possiblity. I keep facillating on type of foundation and whether to sub it out. Seems like if I have a tractor and backhoe I should do it myself but not having the experience I hate to screw it up.

Never really worked with concrete so the thoughts of a huge truck showing up and starting to pour out massive quantities of a liquid which will turn into something that will never move again is sort of scary.

WVBill · Feb 22, 2007

Me too.

I would really like to rent a backhoe and do it all myself. But, I expect this building to outlast me and my wife and I don't think this is something I want to practice on.

At my age, I find I'm making more and more decisions to leave certain things to the younger guys and the experts.

WVBill

scott_vt · Feb 22, 2007

Mornin Jerry,
Your about 100 miles north of me so I could see where you need to go 5ft deep. When we put our front porch up I dug four post holes just over 4 ft deep for the sonotube. I was told frost could possibly go 4ft deep here. That was a few years ago and havent had any movement yet !