I recently bought a house that already had a (~2 year old) 12ft * 10ft shed in the backyard. Currently the shed just sits on some gravel. I plan on turning the shed into a 2 room sauna/changing room and want to secure it on a more permanent, level foundation. After much thought and research, I think the best approach would be a concrete pier/sonotube foundation. Seems simple enough... but I have a bunch of questions that perhaps someone could help with:
How many and how far apart should the piers be spaced? The shed is currently sitting on 5 4x4 runners running the long way. Am I correct in the assumption that I should have piers supporting each runner? If the shed is 12 feet long - how many rows of 5 piers would I need?
From inside the shed it appears that the floor is slightly sagging on the left side (you can kind of see it in the photo as well). Should I expect that to level out once it's properly supported on piers?
I'm in PA, so I understand that the piers must go below the frost line - however, do I need to pour footings under the piers as well or will a shallow bed of gravel suffice?