Sonatube foundation design questions

[willywilly2020]

Hello friends!

I am building a shed roof cabin in the north cascades. Much fun! Am finalizing my foundation plans and wanted to solicit feedback.

I am building a 12x20 cabin with a 4ft deck. So this would be a 12x24 footprint. The shed roof high/low side is on the 20ft sides, and the sonatubes would be supporting beams along the 20ft side.

I am planning a foundation of 8 sonatubes. This would be a sonatube on each corner of the cabin, sonatubes on the deck corners, and a mid-point sonatube under the cabin. 6x8 beams would be used on top of the sonatubes.

Some questions for the squad:
(1). What diameter sonatube would you use? I was thinking 12'', but wondering if I should use 16''?
(2) I am uncertain about the position of the "mid-point" sonatube. Should I have it at the mid-point of the cabin footprint? Or should I have it at the mid-point of the deck-inclusive footprint?
(3) Would you use pressure treated plywood for your subfloor?
(4) How deep would you put your sonatubes into the ground? I was thinking 18'' below ground, with 18'' above ground.

Any other feedback?

Thanks!

[paul s]

I think frost depth may be a concern.

Lateral bracing may be issue  also,

See what others say.

I am considering a similar size.

Paul

[MountainDon]

I think frost depth may be a concern.

Lateral bracing may be issue  also,

See what others say.

I am considering a similar size.

Paul

Yes, check frost depth.

Piers of any kind can have lateral stability issues. The piers are only ties together at the top and may move indeopendently of each other. That is why Pier foundations are not found as a prescribed, approved foundation in the IRC. But if piers must be used the larger the diameter the better one cam resist lateral movement.

A 6x8 beam is likely too small for a 10 foot span, even with a 12 foot width. But calculatikns would need to be done.

I wonder about a shed roof, usually a low oitch roof, in the Cascades. Does not the Cascades get a lot of snow?

I would build the porch as an add on to the shed. I would not extend the main beams from under the cabin out the end for a porch if that is the idea. That exposes the end of the beam under the porch to the weather which eventually will lead to rot. How much rain is there?

[akwoodchuck]

Don't do it...your proposal is what I like to call "designed to fail".....get help from a local contractor for the foundation at least....

[paul s]

I have done the following, and not had problems in areas up to 36 inches  frost depth. I just do not know how good it is?, or perhaps bad it is.

The site is well drained, with or without  vegetation

I lay out where I will place solid cement blocks, 4 inch  thick,  8 x 16 inch.

The come in contact with the buildings skids/beams
At each  location I dig out 16 x24 inches. Removing about 9 inch  deep, leaving firm soil

I then put In about 9 inch  of 3/4 inch stone and tamp to firm and level

Install solid block, level with all others

I cut asphalt  shingle to fit on top of block between  cement block  and skid or beam

This takes time, my last was 8 x 16 feet  and took a few days

The weakness of this way is less resistance to up lift.  again I am not sure how good this is

A similar is to form concrete blocks and cast in place, connecting skids or beams to cast blocks

Paul
.