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PretendBuilder
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# Posted: 24 Jun 2025 08:36am
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I am starting to build a very simple garden cabin in our backyard. See design attached.
It will be for occasional fun use - naps - maybe grandkids sleeping overnight - not full time anything. It is 12'x12, with three rows of 24" concrete piers using sonotubes, 6' feet apart and treated 6x6 posts, 2 feet high at the most.
Question - would I be ok with a single 2x10 rim joist all around and across the middle? I will notch the 6x6 posts and they will be attached with lag bolts. Thanks!
btw - last year I built (overbuilt) a 14x27 deck for my daughter and a 20x24 carport for our Transit camper van. SO I have a little experience. : )
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Brettny
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# Posted: 24 Jun 2025 10:15am
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Why use sono tubes at all? 12x12' can be done with 2 girders right on patio blocks.
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PretendBuilder
Member
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# Posted: 24 Jun 2025 10:56am
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We get some mighty strong storms sometimes and I want the structure to stay put. Last week we had 70mph winds and two huge branches came down and narrowly missed my new carport. I feel solid into the ground is best - for me.
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gcrank1
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# Posted: 24 Jun 2025 02:27pm
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So given that wind issue how about sonotubes at the corners and blocks in the rest?
In a 12x12 (I had a toolshed that size once) your floor loading isn't going to be massive, take a look at the span tables for 2x's to calc the width.
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PretendBuilder
Member
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# Posted: 24 Jun 2025 04:27pm
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I will for sure do the sonotubes on the corners, but I am considering using Camo plastic deck blocks with 6x6 posts in several places, including the middle. No digging and peace of mind by adding some extra support.
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Brettny
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# Posted: 24 Jun 2025 07:14pm
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Quoting: PretendBuilder We get some mighty strong storms sometimes and I want the structure to stay put. Last week we had 70mph winds and two huge branches came down and narrowly missed my new carport. I feel solid into the ground is best - for me
Get mobile home ground anchors.
I would not do sono tubes for a 12x12' building and I'm on nasty clay in the northeast.
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DRP
Member
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# Posted: 24 Jun 2025 07:22pm - Edited by: DRP
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If you want the structure to stay put, the 6x6's are the weakest link in that chain.
Thinking about the "didn't stay in place" potentials.. Uplift, sinking, sliding, overturning.
In the sonotube, 6x6, girder, soup of connections overturning of the "soft story" in this case the 6x6's, is the most common. Run the sonotubes up to the underside of the girders with a cast in place bracket, that removes 2 hinges in the "column". it is still poorly braced as far as overturning, so the building code limits it to small structures with the floor no more than 18" above grade. It needs to be on a level site (which they are kind of baking into that to help avoid slope related problems).
Not knowing particulars I just figured what a #2 SYP (southern yellow pine) 2x10 could carry with a 6' span, ~2000 lbs if uniformly distributed along the length.
If 2 girders, then the area tributary to the beam is half the 12' joist span (one end of the 12' joist bears on either girder) so 6' of that load x the 6' span= 36 square feet. Call it 10 lbs per square foot dead load and 30 lbs/sf live load, 40 psf x 36 sf=1440 lbs.
In that scenario it could possibly carry the floor but we haven't talked about walls and roof... snow, wind.
Although, if there were "columns" in the walls over the piers and another 2x10 notched into the studs at the top, then a top plate cap. That embedded beam is available to carry the roof. If tall sheathing osb ran from rim to top plate I don't believe you could hurt that assembly. Just another way but more work and the same materials as a double rim... which would play out at... around 4500 lbs capacity. so up to the 35 lb snow load on the roof range with double 2x10, #2 SYP rims.
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