Suggestions on sonotube size for a 12' x 20' shed on unlevel ground

by Zvander

Hi John, I'm thinking about putting up a 12'x 20' shed. The ground where it is going is sloping and (at this point anyway) very uneven. I was thinking about using Sonotubes for the footings (the shed will be built on 4x4 pressure treated runners). I was thinking about using 4 or 5 tubes per runner (w/4 runners total).

What size (diameter) Sonotubes would you recommend I use?

Any help is appreciated


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Sonotube shed foundations
by: John

Hi Zvander,

Thanks for your question.

I would have thought for a 12’ wide shed that you would need three runners and 6No. diameter Sonotubes per runner.

The decision as to whether to go for 10 or 12 inch diameter tubes will depend on the ground conditions where you intend to build your shed.

If your site is on sandy or gravelly ground then 10” diameter would likely be sufficient, for a more clay soil then 12” diameter would more likely be adopted. 8” diameter tubes are rarely used except when bedrock is quite close to the surface.

The size and spacing of the Sonotubes will ultimately be down to you and your towns code enforcement officer.

The depth that the tube goes below natural grade is a function of where you are and the depth of the frost line in your local area. The base of the Sonotube should be below the frostline so that it is not subject to seasonal movements from the freezing and thawing of the ground.

Overall the use of sonotubes should save you a lot of time and effort rather trying to level the site and give you better access beneath the floor should you want to install plumbing, wiring or insulation.

I hope the project goes well and please let me know how you get on.

All the best


Width supports.
by: Lindsay Richards

My home shed is 10 by 16 built on 4 by 6's and I used 4 in the 16 feet. It has been about 5 years with no visible shifting or sagging.

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