Setting posts below frost

[dustinfox]

So.... If I need to set some posts at 48 inches deep to get below frost level, and in one of my holes I hit a boulder at 36 inches, but I know for sure that boulder is more than 12 inches thick, would that point be considered acheiving the 48 inch depth requirement?

 

[lman]

Are these fence posts?

 

[Roadworthy]

I think you answered your own question when you said the hole with the boulder is thirty six inches deep. The post cannot go past your anticipated frost level even though the boulder may.

 

[CH4Ohio]

Depends -- Is a building inspector going to be looking in these holes? If it's for a pole building or deck, for example, I'd count the boulder as part of my foundation and overall depth.

 

[deezler]

for a building... no.
Anything else, sure.

 

[dustinfox]

I think you answered your own question when you said the hole with the boulder is thirty six inches deep. The post cannot go past your anticipated frost level even though the boulder may.

Yes that was my logic but I just wanted some reassurance that my logic was not flawed. Since I would have gone down 48 inches and then poured a concrete tube, and since we know the boulder is not going to freeze, then my thinking is that essentially the thickness of the boulder will fulfill the remaining 12 inches.

and no, this is not under a building... It's for added support under some deck stairs.

 

[PILOON]

For added insurance you might drill that boulder so as to accept some rebar B4 pouring any concrete as that way the boulder and post will one otherwise frost could grab your post and still heave it upwards.

 

[JJT]

Yes that was my logic but I just wanted some reassurance that my logic was not flawed. Since I would have gone down 48 inches and then poured a concrete tube, and since we know the boulder is not going to freeze, then my thinking is that essentially the thickness of the boulder will fulfill the remaining 12 inches.

and no, this is not under a building... It's for added support under some deck stairs.

I'm in the same frost belt as you, for deck and porch stairs I simply pour a 4" slab at ground level. There's no need to excavate below the frost line for a stair footer. In 30+ years my slabs/stairs have not moved. If this was for a deck post, or any type of structural foundation, yes, 4' would be my minimum.

 

[dodge man]

My understanding is the frost grabs a hold of things so to speak and can move it up or down. In your case it could upheave the post and pull it away from the boulder. That’s in theory and in your case for what you doing it will be fine.

 

[Cycledude]

Styrofoam