Hello,
I have a 12ft x 20ft shed with a concrete slab on top of 6-12 inches of well tamped gravel. Site is sloped and heavy, wet clay. I am adding an 8ft x 18ft lean-to to the shed with 3 or 4 posts. Should the posts be on sonotubes or just packed gravel, like the slab?
Thanks, Eric
PS: I live in vermont with a 48 inch frost depth.
Replies
Has your existing slab shown any signs of movement from frost heaving? If not, it's probably due to the thick gravel base underneath, which provides some flexibility.
If it's working for the existing slab, then the same arrangement will probably work for the lean-to.
The effect is similar to a method sometimes used in frost-prone areas called a "grade beam". This is where a perimeter trench is dug and gravel (rounded is better than sharp) is packed in the bottom, then a grade beam is poured on top of the gravel. Frost movement from below is absorbed and deflected by the cushion of gravel.
Footings on top of the gravel will move with the existing slab. However, I would add spikes of some sort into the ground to prevent lateral movement. Posts on top of footings, anchored to prevent slippage.
~ Ted W ~
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See my work - TedsCarpentry.com
Now for something completly different:
Before the price of scrap metal went to $300/ton, (and probably now that it has dropped) when I did a lean to like yours I just used 2 posts. Dug 7 ft deep holes and dropped the chassis of an old car into the hole, welded a couple of 2x2 angle irons to the hulk to bolt to the posts. Sluice backfilled the holes. Good and solid 5 years later anyway, only cost was the scrap and diesel to run the backhoe.
an' dont tell me that aint GREEN????