Expert: Daniel Humphrey - 10/23/2008

Question
Hi Daniel
  
I'm planing to build a cabin in a remote part of Alaska. it will be 24ft. by 24ft. with a loft. I am using power poles for the foundation. They will be 8ft. on center with 6ft. of pole above ground.
How deep do I need to put the poles in the ground? What size does the beam on top of the poles need to be? The floor joists on top of the beam to be 2 x 10, 24 in. on center

Answer
Dear Hal,

You didn't say whether your building site is on permafrost or not; it makes a big difference on how far you go down. In permafrost, you want to make sure the pilings stay in the frozen part, and usually that means 10 or 12 feet at minimum, to make sure the pilings don't melt out the ground supporting them. You can help keep the ground frozen by covering the area under the house with insulation during the summer months and opening it up during the coldest part of winter.

Of course, if you are not in permafrost, the ground should be kept from freezing, and about 6 feet down is sufficient for that. Additional care must be taken to allow the surface to slip past the pilings should it ever freeze; that way the posts don't get jacked out of the ground. Creosoted pilings are notorious for rotting out right at ground level in about 25 years. I don't use them at all any more because of the noxious fumes that never go away. I use 8" culvert these days, with concrete poured inside once the pilings are in place and all the boards are bolted on. The culvert is a lot lighter and rot proof.

As far as beam sizes, as a rule of thumb you need to have it about the same depth as your joists and a width that is equal to what the joists they support would be if they were all side-by-side. In your case, that would make it 8 inches wide and 10 inches deep.

Daniel