How important is the frost line for smaller projects?

[dave11]

I'm planning two projects that will require post holes--1) a trellis, and 2) a small roof for the wood stacks. The frost line in my area is 36 inches, though I've heard from the local builders that it's very hard to get there because of the very rocky soil we have.

Neither of these projects would require an inspection or permit.

Thirty six inch deep holes seems excessive for the trellis, though some up of the uprights are going to have to be 6-7 feet, so sticking to the 1:3 rule, they would otherwise require 36 inch deep holes. The roof for the wood would be 16 feet long by 2 feet wide by 6 feet above ground.

I keep hearing that the frost line is set artificially deep, to cover the very rare times the ground will freeze that deep. But the stability of the posts in wind is a separate issue.

Just curious what others who have done this before would do, in terms of the depths of the holes.

 

[stee6043]

What kind of soil are you building in? Sandy soil will drain better typically.

For what it's worth I set 17 posts for my deck install. All at 42" deep +/-, rather sandy soil. Zero problems with my deck. I set 4x4's for all of my wood rows at 12-16" deep. Most all of them developed a pretty good lean after a year or two and I don't believe it's 100% because of the weight of the wood.

I'd go as deep as you can if you want longevity. Think of how mad you'll be if things start moving three or four years down the road...

 

[dave11]

Our soil here is dense, rocky clay. I can't imagine the posts would move even at only two feet deep, but since I don't have a lot of experience with it, I can't be sure.

 

[smokinj]

Our soil here is dense, rocky clay. I can't imagine the posts would move even at only two feet deep, but since I don't have a lot of experience with it, I can't be sure.

If you can get 8 inchs of nothing but clayyour golden!

 

[gpcollen1]

I try to get to code/frost line but it is not always possible. Do it if you can. What I do in holes where it gets too rocky is to then bang some rebar in to the bottom a foot or so if i can. If not, wing it...

 

[midwestcoast]

Even if you don't get down to frost line make sure your holes are straight-sided not sloping in as you get deeper. If you're pouring a footing make it straight sided as well or even better to taper it out wider at the bottom. This all gives the frost less of a grip to push your posts up.
In real hard ground I'd probably just do a surface footing for the wood cover & straighten it out when needed...

 

[dave11]

Even if you don't get down to frost line make sure your holes are straight-sided not sloping in as you get deeper. If you're pouring a footing make it straight sided as well or even better to taper it out wider at the bottom. This all gives the frost less of a grip to push your posts up.
In real hard ground I'd probably just do a surface footing for the wood cover & straighten it out when needed...

I was planning on using gravel to fill the holes, and treated lumber. People seem to favor that over concrete. And less likely to heave (?)

The trellis is never going to have much weight on it, and could easily be fixed if it moved, though I'd rather not have to go back and fix it.

Bought the Fiskars post hole digger today, and really like it. Using my 8 foot pry bar to break up the bottom, I got down to two feet without too much trouble. Might try for three feet tomorrow.

 

[btuser]

When I built my deck I went 52" with a flared bottom and 12" footings. When I built a lean-to for my wood stack I dug "enough" so the wind wouldn't blow it away. Its not a real structure, and if it heaves/sags who cares? I'm not planning on siding the thing and moving in, yet.