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cdrew23
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# Posted: 30 Jun 2014 10:14am - Edited by: cdrew23
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Hello -
I am currently planning to build a small 12' x 12' cabin to live in temporarily as my wife and I begin the longer process of designing and building a permanent residence nearby.
The cabin will be located in a dense forest with many mature trees, so in building the foundation I want to disturb root systems as little as possible. The cabin site is not level, and roughly drops 3-4 feet in the 12' footprint.
My main question is: are deck piers with 4x4 posts on the downhill side a bad idea since the lot slopes so much? It seems if i could put the blocks on a bed of gravel and concrete paver it would disturb the woods less than digging many holes and pouring concrete.
I also wondered if i could use concrete blocks on gravel for the uphill side, then a beam spanning the downhill side anchored in to poured concrete below the frost line? Is a hybrid foundation like this a bad idea since one end may move a bit with frost, and the other would be rigid?
The cabin is located in a somewhat windy place, but nothing crazy. No tornadoes or anything like that, and no snow load to speak of really. Soil drains very well, is somewhat rocky so could be hard to dig proper footers.
I thought i would build the floor with 2x8's 16"OC, then go up with 2x4 framing. Shed roof style, with 8'height on low side, sloping up to 13' ish on the high side, with a small loft for sleeping.
I want to build something that can hold up a long time, even though we will only be living in the cabin temporarily. I also want to disturb as little of the forest as possible!
Thanks for any input/suggestions in advance
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cdrew23
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# Posted: 1 Jul 2014 11:03am - Edited by: cdrew23
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Well, attached is a rough sketch of what I am thinking:
2 sets of 3 piers with a built up beam running the 12' length. Sides of cabin to be cantilevered 1' off each side. Dotted line represents potential deck expansion.
Questions:
What size beams should i build up? would 3 2x8's suffice, or should i go 2x10? How about a single timber 4x6? For the downhill side, will a 4x6 PT post provide support?
Floor joists? 2x8 16" OC ok with this span (10') or should i beef these up as well?
How about sonotube sizing? 8,10,or 12"?
I would like to make a sturdy long lasting structure, but use materials as efficiently as possible...
Thanks for any advice
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Truecabin
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# Posted: 2 Jul 2014 12:41am - Edited by: Truecabin
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If u dont want to disturb the forest use just 4 supports and it will be easier too If you have a rocky ground thats hard to dig thats great. You are building on a slope maybe use long 6x6 posts for cornerposts and shearnail them into the wall sheeting. Then they will stay vertical. 12x12 is a small structure dont waste time and cost treating it like a large structure.
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Nirky
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# Posted: 2 Jul 2014 12:53am
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I'd dig out the footprint to an appropriate depth, put down gravel, put down 4x6 or 4x8 pressure treated skids and then build the flooring over the skids. Could use 2x pt and double or even triple up if the 4x's aren't available.
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cdrew23
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# Posted: 3 Jul 2014 09:47am
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Thanks for the input, i think the site is too steep to dig and level for skids, but I like the idea of PT 6x6's - you think i can just sink these into the ground below frost on solid rock with no concrete involved? What size beams would i need to span the 10' if i skipped the middle post supports?
Getting excited, started making the trail to the site yesterday!
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Don_P
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# Posted: 3 Jul 2014 06:45pm
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Beam size depends, ... on span (12'), snow load(?) and tributary area loading the beam. You mentioned a possible deck, this might contribute to the trib area. Is it going to be loading the beam or will it be built on its' own self supporting structure? If the floor rim is cantilevered over the beam it pretty much precludes supporting the deck off of it.
Using what you have given and the assumption of no deck support;
Floor:
12' span x 6' trib width (half the 12' width of the cabin)=72 sf
72 sf x 50 lbs per sf =3600 lbs
Roof, assuming 1' overhang = 84 sf x 30psf (a very little snow)= 2520 lbs
Total=6120 lbs uniformly distributed along the beam
Quadruple 2x12 of average #2 material or a triple of very good material... say a #1 SYP. Span takes serious lumber.
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cdrew23
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# Posted: 3 Jul 2014 08:50pm - Edited by: cdrew23
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Should I just put the 3rd post support in for each beam and use a smaller beam? Triple 2x12's is no joke!
Thanks for the help.
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Don_P
Member
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# Posted: 3 Jul 2014 10:17pm - Edited by: Don_P
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Let's see what happens, I suspect it'll be pretty suprising, the load will half and the span will half, bending moment on the beam will quarter.
Floor;
Tributary area will be 6'x6', 36 sf x 50psf=1800 lbs
Roof
trib area 6'x7', 42 sf x 30psf=1260lbs
Total load = 3060 lbs (Max bending moment 2295 ft-lbs, in the previous example the bending moment was 9180 ft-lbs)
A single 2x12 in #2 SPF or equivalent
A double 2x10 would be stronger but about the same stiffness
A double 2x8 in very good material would be the lower limit
You would have a far better structure if the posts are in the plane of the wall and run from foundation to top plate, pole barn fashion, with the beams on inturned hangers attached to the full height posts. Then the wall sheathing acts to laterally brace the posts.
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