I'm planning to build a 14ft x 16ft A-Frame on grade in Vermont this summer. The frost line here is 4ft. My site is at the top of a small, wooded knoll. I know from some exploratory digging that The first 10 – 12in or so of soil is somewhat gravelly forest loam. Below that is layer of fine sand. I dug 18 – 24in into this sand, but presumably it continues deeper.
Because the structure I want to build is relatively small, I am considering digging deep enough to rest the foundation on the layer of sand described above. The idea would be to pour nine footings on the sand layer that would serve as the base for short concrete piers to support three carrier beams that would be the base for the structure.
My hope is that the sand will provide adequate enough drainage to prevent freezing and heaving, even though the foundation would be resting above frost line. I'm also hoping that, given the small footprint of my cabin, the entire structure would float up and down if the ground heaves.
Have others built small structures without getting the foundation below frost line? How has this worked out for you?
Does the foundation I've described sound reasonable given what I am planning to do, or are there modifications I should make?
Best Answer
Sand (as opposed to larger material with larger pore spaces - e.g. rubble trench foundation, train track ballast rock) will hold enough water to freeze and move.
If your cabin has no plumbing or electrical going into the ground, I have several 8x12 sheds supported by pressure treated lumber piers that are simply set on the surface of the ground (removing the leaves/humus, but not otherwise "digging") which have been fine for 20 years now in a cold climate.
If you do have plumbing or electrical going into the ground, movement can be a problem. Cheap foundation option is basically to go "pole building" style and set treated posts into the ground below frost depth in postholes - they don't need to be concrete piers, but they do need to be below frostline.