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pnwnp
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# Posted: 9 Aug 2021 03:03pm
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Hey all,
I'm second guessing myself and could use some advice to help reinforce my decisions before it's "too late" and I've found this site to be a great resource so far 
I'm building a 10'x14' cabin. It's in a heavily forested area on uneven ground (though only about 2' elevation difference between high and low points).
I decided to go with pre-cast piers (ez tube) since water and equipment access are an issue. The piers are 22" at the bottom and 11" at the top with a tie rod running through them. I've dug 6 holes 2' wide and 3' deep. It's a dense clay at the bottom, which I know can be problematic and there's a lot of annual rainfall.
I'm going to put a layer of gravel in the holes and set the piers on that, which will then have 6"x6" pt posts attached with offset brackets.
The plan is to have 2 rows of 3 piers along the 14' side 6' OC, with 8' spacing between rows. If you're doing the math, this leaves roughly a 1' cantilever on all sides.
I was planning to use 3 2"x10"s as built up beams across each row of posts.
Single pitch roof 8' plate height low side 12' on the high side.
Does that seem adequate to support the load? Any adjustments that could save my skin? I'd like to fair on the safe side given the soil conditions. I debated on the 1' cantilever. I could shrink that a bit to bring the wall load in closer to the supports, but it's a pretty small cabin as it is, so space is meaningful.
It's only going to be used seasonally, mostly in the better weather months. Not full-time living.
Any input appreciated!
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NorthRick
Member
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# Posted: 9 Aug 2021 03:57pm
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What kind of snow load do you get?
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pnwnp
Member
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# Posted: 9 Aug 2021 04:08pm - Edited by: pnwnp
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Quoting: NorthRick What kind of snow load do you get?
Not much. A few inches at most, a few times a year. No real deep freezes.
The ground snow load recommendation is 15psf
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gcrank1
Member
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# Posted: 9 Aug 2021 04:52pm
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Consider going 12x16
Old farmer adage,"nobody ever builds a barn big enough". I think that goes for garages, pole buildings, storage sheds And cabins.
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NorthRick
Member
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# Posted: 9 Aug 2021 06:04pm - Edited by: NorthRick
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RQuoting: pnwnp The piers are 22" at the bottom and 11" at the top with a tie rod running through them. I've dug 6 holes 2' wide and 3' deep. It's a dense clay at the bottom, which I know can be problematic and there's a lot of annual rainfall. I'm going to put a layer of gravel in the holes and set the piers on that, which will then have 6"x6" pt posts attached with offset brackets. The plan is to have 2 rows of 3 piers along the 14' side 6' OC, with 8' spacing between rows. If you're doing the math, this leaves roughly a 1' cantilever on all sides. I was planning to use 3 2"x10"s as built up beams across each row of posts. Single pitch roof 8' plate height low side 12' on the high side. Does that seem adequate to support the load?
I think it is. Here's a rough calculation:
10' x14' = 140 ft2
(10lbs/ft2 floor dead load + 40lbs/ft2 floor live load + 10lbs/ft2 roof dead load + 15lbs/ft2 roof snow load) * 140 ft2 = 10,500 lbs
You have six supports so 10,500 lbs / 6 = 1,750 lbs per support (I know it's not evenly distributed but we'll neglect that for now).
Each support is 2.6 ft2 at the base.
1,750 lbs / 2.6 ft2 = 673 lbs/ft2 of pressure on the soil.
Most soil bearing capacity charts have clay soils as being able to withstand 2,000 lbs/ft2. 673 lbs/ft2 is well below that.
However, I agree with gcrank1, make it a bit bigger. 12x16 makes better use of sheet goods that come 4' x 8'.
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pnwnp
Member
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# Posted: 9 Aug 2021 09:38pm
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Thanks NorthRick! That seems more accurate than my estimates.
I'll think about making it a bit bigger. There are a couple factors that come in to play there though. One is cost. Even a little extra right now costs a bit with lumber prices, but they do seem to be improving slightly.
Using your calculations, I get 923 lbs/ft2 on the soil for 12x16. That still seems to be within reason.
Keeping the pier spacing the same (those holes were a bear to dig out by hand) would leave the cabin floor with a 2' cantilever out from the piers/beams on each side. Would that be an issue since the load isn't even with the 12' high wall on the long side parallel to the beam?
I was concerned that might put too much force on one side, but maybe with everything adequately strapped together that doesn't matter as much?
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Brettny
Member
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# Posted: 10 Aug 2021 06:13am
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It's only 10x14? Put it on skids on blocks. It's small enough.
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WILL1E
Moderator
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# Posted: 10 Aug 2021 07:59am
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Not sure where you are located but here in WI anything that cantilevers beyond the outermost beams cannot extend more than the depth of the joist being used. I'm guessing you are using 2x8's, so that means you can't cantilever more than 7.25".
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gcrank1
Member
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# Posted: 10 Aug 2021 09:16am - Edited by: gcrank1
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Iirc, its because the roof loads are carried straight down by the walls, thus the need for adequate support at/near the wall base.
With less snow loads than WI, and that small a structure, it may not be an issue practically speaking, but for code/regs probably. If it is an 'under the radar' build you still want to use good building practices. There are a lot of old cabin builds that grew beyond the original conception either as the build proceeded or later.
Youve already dug big holes. Ummm, consider a new set of holes on one side only so you can go wider?
Thing is as I see it you dont need to cantilever if you use the proper lumber for the box frame/joists for the span between the walls.
Fwiw, Im a fan of small buildings on 'floating' blocks but with your 2' diff on site that probably is not practical.
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Brettny
Member
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# Posted: 10 Aug 2021 10:24am
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Building any building on a slope that drops 2' in 10' is prob a feat. Is there any flatter place to build? Dollars spent on site prep now will be alot cheaper than doing it over again later with a building over the area.
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gcrank1
Member
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# Posted: 10 Aug 2021 10:41am
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Good point; this summer we have had some threads on old cabins built on slopes with serious shifting downhill off the piers issues.
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Brettny
Member
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# Posted: 10 Aug 2021 12:11pm
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That's where blocks can help. If/when it shifts you just jack it back up and re stack the blocks. You can add anchors to keep it from sliding but that wouldnt help if what your anchoring to also slides.
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gcrank1
Member
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# Posted: 10 Aug 2021 12:49pm
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Lol, cable/chain it to an uphill tree and just set it on blocks.
I like it!
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pnwnp
Member
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# Posted: 10 Aug 2021 01:06pm
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Appreciate all the input.
I'm in WA and under 200 s.f. I don't need a permit, but I do want to do things as "up to code" as possible.
The site is reasonably flat, just a bit "rolling" and no great way to level it easily. The difference in height between the top of the upper piers and lower is probably only 1'. The bottom of the floor joists will likely only be about 2' or so off the ground.
There are some rather large tree stumps that prevent me from adding another row of piers, which is why I'm trying to see what I can get away with within that foundation spacing. I chose the best site I could without having to excavate with heavy machinery, since the road can't accommodate that without some....other heavy machinery.
Quoting: WILL1E I'm guessing you are using 2x8's, so that means you can't cantilever more than 7.25".
Thanks. That seems like one of those good rule of thumbs to keep in mind.
Another option I considered was to go 12x12 so the side walls sit over the piers and then the front and back cantilevered portion could be reinforced using exposed roof beams that sit directly over the piers to shift the load down, but that seemed like overkill for 144 sf.
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Brettny
Member
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# Posted: 10 Aug 2021 01:30pm
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If 200sqft is the max..I would build it 200sqft. Tree stumps still in the ground is another reason to put it on blocks. There going to rot and cause a sink hole.
If the site is accessible with a sheet of plywood its accessible to machines with the ability to do site prep.
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gcrank1
Member
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# Posted: 10 Aug 2021 01:45pm - Edited by: gcrank1
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Hmmm
If your cantilever sides will not be a 'load bearing wall'; ie, the ones the rafter ends sit on, the cantilever wouldnt be near as much an issue.
Remember, back in the day 'pre-machines', all digging was done by by hand labor, and still is in many parts of the world today. Yeah, it is hard work, but not so bad if you dont have a whip-cracker behind you.
Fwiw, my wife and I, in our mid-fourties, hand dug a 12' dia x 3' deep koi pond off our front deck; shovels and a wheelbarrow.
You have a head start, youve already dug some good sized holes in there!
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ICC
Member
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# Posted: 10 Aug 2021 05:45pm
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Quoting: pnwnp I'm in WA and under 200 s.f. I don't need a permit
I don't know where in WA state you are but I do know that in some parts of WA state the permit department states the following.... "Permits shall not be required for the following: One-story detached accessory structures used as tool and storage sheds, playhouses, and similar uses, provided the floor area does not exceed 200 square feet....."
That wording that I placed in boldface means you cannot build a legal habitable building without a permit, no matter the size. PLus the use of the phrase, "accessory structures", means that to build the under 200 sq ft structure there first must be a properly permitted building on the site.
Perhaps your county is different and does allow a habitable building of less than 200 sq ft to be built without a permit? Just want to be sure you are correct in your beliefs. Here in NM I know of some who mistakenly believed they were okay in building an under 200 sq ft cabin without getting a permit.
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pnwnp
Member
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# Posted: 11 Aug 2021 10:47am
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Quoting: ICC Perhaps your county is different and does allow a habitable building of less than 200 sq ft to be built without a permit?
Hmm, thanks, I hadn't considered the definition of "accessory". My sister did something similar down the road and they said it was fine, but her primary residence is also on the property...might make a difference.
I'm gonna do it and plead ignorance either way, but good to be aware of 
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ICC
Member
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# Posted: 11 Aug 2021 11:42am
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Quoting: pnwnp primary residence is also on the property...might make a difference.
Does make a difference, depending on the level of enforcement. Don't piss off any neighbors.
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gcrank1
Member
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# Posted: 11 Aug 2021 12:36pm
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Imo, being silent is way better than pleading ignorance when one is not
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