Sona tube size for 12'x16' "saltbox" bathhouse-cabin ?

by JB

I intend to build a "saltbox" style guest cabin-bathhouse between our main cabin and the lake. This is an off the road place (Alaska) and I'd like advice on whether the six, 6" Sona tubes I hauled in this winter are sufficient for a pier type foundation. The building will have a metal roof, a woodstove, shower pan and screen, and moderate furnishings. It will not have a kitchen or toilet facilities. Any suggestions or references are welcome.


Comments for Sona tube size for 12'x16' "saltbox" bathhouse-cabin ?

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Frostproof shed foundations
by: John - Admin

Hi JB,
Thanks for your question. I would say that the tubes you have at the moment aren't sufficient.

It does however depend a little on the ground conditions you have (if there is rock fairly close to the surface then you may be OK). It's just a guess but are you building some type of pole building? If so the general rule is that the Sonotube diameter is about 3 times wider than the size of the post you are using. For a typical cabin of this size a 4"x4" corner post would be used so a minimum tube diameter of 12" would be appropriate.

The second point regards the number of 'piers that you would use. This is slightly dependant on teh size of floor timbers you are using but I would have thought that you would be looking at at least 9 and possibly 12 piers.

I hope that this is useful let me know if you have more questions.

All the best.


PS The location of this cabin sounds beautiful.

Frost Heave
by: GreenBoy

I have no experience of Sonotube's however i would have thought one of the major considerations of building foundations in your area is frost heave, and therefore as much as you need the diameter to be great enough to take the loadings - you would also need the depth to take the pile down 2 - 3 feet past the frost line.

Sounds an interesting form of construction if you are looking to produce a pole barn. The alternative is just to use the concrete foundations as micro piles and use some sort of metal bracket fixed to the top of the concrete to bolt the timber sub floor to.

Post some pictures of the location and make us all really green with envy!

Good points
by: John - Admin

Good points GreenBoy - you are getting better at this than me! Would you fancy doing your own article occasionally?


by: GreenBoy

I would need to dig out my college note - haven't worked in construction for over 10 years... and even then I specialised in Mech and Elec Quantity Surveying - so what do I know about shed engineering...

but thanks for the flattery!

Thanks for the help
by: JB

Thanks John.
Very helpful thankyou.

I am building a small cabin, that will serve as a guest
cabin and 'bathhouse' since we just don't have a good place to put a
shower enclosure inside the main cabin. There is about 3'-4' of loamy
soil and then fist-sized angular rocks (glacial till). The main cabin is
on 8x8 treated 'greenwood' pilings that are crossbraced with treated
I'll get bigger Sona tubes for my new cabin and use the six-inchers for
an addition onto my wood shed (a pole building) or some such.

Thanks again.


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