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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:

  1. 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?

  2. 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?

  3. 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?

shed

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  • Do you have a plan for pouring new piers under the floor where the 3 4X4s run under the shed? FWIW, I would place the piers every 4 ft part under each 4X4. If you don't raise the floor to make it flat, it will not happen all by itself.
    – Jack
    Commented Aug 30, 2018 at 6:07
  • @Jack There are 5 4x4s actually, but yes - I plan to move the shed away from where it's standing now, level everything, dig the holes and pour the piers. Then I will raise up the shed onto the piers. 4ft apart would be 4 rows of 5 piers then for a total of 20 - seems like a bit overkill, however, can a 4x4 safely span 6ft unsupported if I only do 3 rows? Regarding the floor - my hope would that once the 4x4's are sitting on level piers it would force it back into shape.
    – Yev
    Commented Aug 30, 2018 at 14:41
  • batsplatsterson has the right idea about the beams below...
    – Jack
    Commented Aug 30, 2018 at 14:53

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Digging a little deeper and setting the piers on gravel seems to help them heave less, but if you go below the frost line they should be pretty solid.
Digging to that depth by hand is slow work, the first foot is cake, the second foot a little slower, the third foot quite slow, the fourth foot is like digging with a table spoon.

You could support the five 4x4 rails with a post / pier at either end, and one in the middle; that's 15 piers, a lot of work. It will be practically like digging three 10' footers.

You could use six piers, four in the corners, plus one in the middle of the 12' sides, and build three 10' beams on 12' rim joists. That's a manageable amount of digging and building piers but you have to frame the joists / beams and the shed's going to sit higher, you'll need to build a ramp.

I think because the shed was made to set on a level surface, you're going to be better off pouring a pad.

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    I agree. Being a shed, the sub-floor design did not need to meet building code standards for strength and support. Putting it on piers without it having been engineered for that is not something I would do. I'd pour a slab for it. You didn't ask, but in my opinion this is a poor choice for a sauna. The exterior walls are usually press board and not likely to hold up under heat and excess water vapor. But it's your shed...
    – JRaef
    Commented Aug 30, 2018 at 20:08
  • @J.Raefield the exterior walls are T1-11 which from my understanding is similar construction to plywood.
    – Yev
    Commented Aug 31, 2018 at 5:11
  • @batsplatsterson what size lumber should I use for the 10' beams and 12' rim joists?
    – Yev
    Commented Aug 31, 2018 at 5:24
  • @yev - I am getting 3-ply 2x8's to span 10' for the beams, so 2x8's for the rim joist. The rim joists would not be doubled, the beams are carrying the weight, the 4x4's rest on the beams. The rim joists kind of just hold the beams in place.
    – batsplatsterson
    Commented Aug 31, 2018 at 9:41
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If i was doing it. Jack it up were you can get logs or rollers, under to pull out of way. Dig or have hole dug ,pour a footing. Then add block wall.Have trenches also for ...water and electric. fill in with back fill. Form on the outside 4 to 5 inch slab. let it set up. Then pull back on to slab , 4x4 on slab ,the are not going to bounce sag solid . Drill into slab anchor shed. As you pour fill block cells . I think trying to dig tubes,, then setting shed it is going to be tough and shed broken. plus joist under sized still have to fix that... hope this helps.

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