Just looking for some positive reinforcement. I live in Wisconsin. I have a 1930’s house that has a short gabled kitchen extension on the back (maybe three feet). It has a window and a door. I wish to push that profile out into the back in order to have (at least) a three season porch, maintaining the house profile and have it look as if it had been integral when the house was built. Contractors around here like to sink poured walls four feet deep for such things, but I’d prefer having concrete Sonatube pillars supporting it, as I’d like to get under the porch for maintenance rather than have a walled off space I can’t access. Does this make sense? Would I have a fight with the designer, do you think?
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If it will be occupied living space then you'd need full frost protection, whether it be pier foundation (Sono tubes like you propose) or regular foundation walls. Porches and decks are usually not subject to the same rules (check with your jurisdiction to be sure). If it were my place, I'd go 4 ft deep and pour pad footings with rebard dowels out, and then Sono tubes to above grade. Then I'd know it isn't going to heave and make my work above look like hell.
.. your local building dept may have something to do with the way contractors build in your area ... here (frosty Canada) they want anything attached to the house to be on a foundation as deep as the one it is attached to ..
.. sonotube structures need maintenance, poured "inclosed" foundations don't ..
Submit what you want to a local residential structural engineer. He should draw the plan and the contractors will be obligated to follow. He will consider all factors like frostline conditions, loads and exisitng structure.
"I'd like to get under the porch for maintenance rather than have a walled off space"
If it's open underneath you've to think of making sure that the insulation is protected from critturs...probably plywood here, and with vents along 2 of the sides.
All the best...
To those who know - this may be obvious. To those who don't - I hope I've helped.