Depth is code, not preference
Footings above the frost line get jacked upward by freezing soil — the classic cause of leaning decks and heaved posts. Your county building department publishes the required depth; call 811 before digging regardless.
Setting the tube right
Set the tube on 6 inches of gravel, brace it plumb, and rod or tap the concrete as you fill to release air pockets. Slope the top of the concrete away from the post so water sheds instead of pooling against the wood.
FAQ
How many bags for a 12-inch Sonotube 4 feet deep?
About 7 bags of 80-lb or 10 bags of 60-lb mix. The tube holds 3.14 cubic feet.
Do I need gravel under a Sonotube?
Yes — 4–6 inches of compacted gravel under the tube provides drainage and a stable base, and it's what most codes expect below the footing.
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