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Small Cabin Forum / Cabin Construction / Novice Builder Questions; foundation, skids, insulating floor
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PBC312
Member
# Posted: 15 Nov 2020 12:24
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Hello all, I'm learning so much from this forum. I've never built anything in my life but plan to build a simple, small, guest cabin next spring and have (many) questions. It will have electricity but no plumbing. My tentative plan is to build a 10 x 14 cabin on skids. (Narrow site, could maybe go 12 feet wide if there were big advantages.)

My area gets harsh winters (northern Iowa). For skids I thought I would laminate PT 2 x 6s (or 2x 8s?). I've seen posts suggesting using two skids - no middle skid - because of heaving and lack of access. That makes sense to me. Assuming I use two skids, I was planning on laminating 3 2x6s. Thoughts?

For a base I plan to lay down some gravel for drainage, then cement blocks on top of the gravel to get it off the ground. The skids will sit on the blocks. I was thinking the blocks should elevate the skids about 8-12 inches off the ground. (Does it matter how high?) Then 2 x 6 joists (2 x8s?) sitting on top of the skids. The joists will overhang the skids by one foot on either side. So the skids will be 8 feet apart. Thoughts? Do the joists need to be PT?

For insulating the floor I plan to insert 4 inches of XPS foam board between the joists, supported by small wood blocking, and foamed to fill in any cracks. If this isn't enough I could go to 6 inches. Have no idea if this makes sense or if there are better alternatives. Fiberglass batts seems like it could be a haven for bugs and critters. If I go this route what else do people do to seal up the underside of the joists? Vapor barrier? I want to whatever I need to do to keep bugs and rodents out. Some kind of metal screen attached to the underside? Is it feasible to install plywood to the joists, then flip the frame upside down so the joists are sandwiched with solid material?

Not sure about a roof yet. A one-slope shed roof seems the simplest but I'm not sure I like the look. If I do a one-slope roof I'd like it to overhang the frame by at least one foot on all sides. I'm not sure how much pitch I need. Thought I'd just make the "high" side wall 10 feet high and the "low" side wall 8 feet. Thoughts?

Once I settle on a final design, are there web sites out there where I could plug in my dimensions and material choices and get a cheap set of plans? It's not that complicated but I like the idea of looking at plans as I build to help me (hopefully) avoid dumb mistakes. And the plans would, I would think, give me a framing materials list I could give to the lumber yard without me having to calculate it all.

This is already way too much for one post so I'll stop for now! It's great to learn from the collective wisdom of this great forum.

Thanks,

frankpaige
Member
# Posted: 15 Nov 2020 13:24
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There are a TON of free shed sites around for the size you are looking at.
I like your thoughts on insulation for the floor.
General throw out question for you and others. Why concrete blocks? It adds to height, adds wind to the equation, room for rodents. If your doing gravel? Maybe add concrete blocks/pads to ground first, then gravel?
One sloping roof? Maybe cabin oriented for solar or sun during cooler months? I like "normal" (whatever that is) roof style. IMHO. But, dang it, it is your cabin.
Enjoy the process, even the planning.

gcrank1
Member
# Posted: 15 Nov 2020 15:32
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Well, your adventure is about to begin!
I suggest you consider something like 12 x 16 or 20, or 16 x 20 or 24; basically multiples of '4' as in 4x8 plywood sheets to minimize waste/max $ spent. And 12x12ish size 'rooms'/floor spaces inside do make decent living spaces. We ended up with 3 such 12x12s; a kitchen and dining on one end wood stove in the back center and living room on the other end. The 12x12 loft was the bedroom until it became more like storage and we used a big fold out futon couch/bed deal downstairs. After having those spaces Id find smaller to be a bit cramped.
Take a look at your local big box 'builders supply' store in the books section for a book on building sheds and such, lots of useful stuff to review, materials, techniques, spans and capabilities of the lumber chosen. The 'cabin' books Ive browsed through have all been much fancier than I would have wanted, ymmv.
Prep the site for drainage and potential ground moisture depending a lot on soil type. I had sand and gravel so my 1983-84 12x24' build went on 'silo blocks' as ground pads, no foundation or in ground piers at all. I never had to even re-level it!(we just left it this past Sept 1). My site was no level ground so the the 'high end had only one height of a solid cement block, like a half thickness of the normal holed blocks and the other 5 varied. Most of it I could crawl on my back beneath if needed, like if I had wanted to 'run stuff' underneath, but I never did. I still like the idea that I could get under if I had to even though it means having some steps up to the door or deck.
Ive since heard that I should have put down some kind of a moisture barrier.....I never had moisture issues.
P-treated? Maybe use something between the cement block and the joist wood, but if I did it again Id do it mostly the same way; basic 16" oc box framing, not pt, no 'skids' jacking the height up.
Once my box frame with joists in place was done I put furring strips 1" down from the top (used a pair of tacked together simple jigs to located the furring strips) and cut strips of 1" blue DOW foam board to sit down on em (no gap filling foam) then 3/4" staggered deck plywood. The floor was never really cold even in the bitter south central WI winters with only throw rugs down. I gotta wonder if the thickness you are intending is overkill?
I considered the simple shed roof then gave it up and made a 12x12' loft with a high peaked roof the narrow way; it looked like a Swiss chalet from the front/back. It got bigger and heavier than I ever intended but the base, blocks and all held it fine. Btw, somewhere in our late middle years that loft with ladder access became a problem to get to/out of, and in the summer heat stacked up there, had to put in a turbine vent to make it remotely usable and in the winter all the heat from the wood stove went up there first and had to work its way down to us. Off grid, pre solar (and woods anyway) so no power fan to kick it down.
I suggest considering that 'shed roof' with the slope to the back and thought to the future connections when you put on the complimentary front half of it to cover the front deck you will want. Yes, plan for it in the site work you do......
There is more, Sooo much more. You will be back, I know

PBC312
Member
# Posted: 15 Nov 2020 16:07
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Here's a photo of essentially what I had in mind for a foundation, though I don't think I need it this beefy for the small cabin I have in mind. From a poster on this forum named Owen Christensen.
Skid Foundation
Skid Foundation


Brettny
Member
# Posted: 16 Nov 2020 06:59
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That is your typical shed foundation. I wouldnt mother with laminating PT for the skids and just use 6x6 for runners. As for how to insulate the floor foam is best, not sure you need 4in of foam though. You can build the floor and walls like normal then lay down foam ontop of your sub floor and go from there.

We have a 10x14 shabin..its not big enough even with bunk beds. If your going small I suggest 8ft side walls and a tall peak roof. A gambrel roof gives the most head room for a loft and you dont need really long lumber.

PBC312
Member
# Posted: 16 Nov 2020 11:46
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I’m intentionally building a small cabin as this will be more of a guest cabin for overflow sleeping and short visits. (And because I don’t want to bite off more than I can chew.) It’s on the same property as a larger modern cabin that my wife and I use. The 10 foot width is dictated by limits on the specific location. I could maybe push it to 12 but it would be tricky.

I’ve already come around to the idea of a traditional gable roof instead of a shed, just an aesthetic preference. Not a fan of gambrel roofs even if they are functional. I don’t need or want a sleeping loft.

For skids, my thinking was that laminated 2x6es would be less likely to warp and twist over time compared to 6x6es, and not a big deal to make. Could make whatever length I want. Might want to extend them out for a small porch.

Will probably scale back XPS foam to two inches. That would be a n R value of about 10.

gcrank1
Member
# Posted: 16 Nov 2020 18:37
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Best advice I heard years ago on insulation was to build like a Thermos bottle remembering that heat rises. The ceiling the most important needing to be the thickest, then walls then floor.
Last I heard on 6x6 beam cost was pretty big bucks and yes, I too think lam planks would be more stable. Thing is, are either really necessary?

PBC312
Member
# Posted: 16 Nov 2020 18:44
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I think the rationale is if you omit a center skid you should go extra beefy on the two skids and the joists. I’m not sure what is “enough” for a 10x14 cabin (leaving an 8-foot joist span with the skids in one foot on the perimeter.)

gcrank1
Member
# Posted: 16 Nov 2020 19:44
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There are specs on such in any 'builder's book, online in an inquiry too; ie, 'how far can a 2x8 span?', quick & slick answer.
Of course it gets more complicated in a shed build to hold tractors vs 'light stuff', heavy vs a 'potting shed'.
I remember breaking into this stuff back in 1983 when I started my first cabin build, I went to the library then got my own 'construction book', invaluable that. But now when have the i-net, all at literally your fingertips. I also remember being in 'cabins', really shacks, with bouncy floors.
So I built the bottom like a small addition on a house and was never sorry. Iirc it goes like this, the span is 1.5 x the width of the plank.
2x6 spans 9'
2x8 spans 12'
2x10 spans 15'
On a 10x (whatever) 2x6 would be good out to the middle 9' then cantilever the ends. You could use a larger cantilever a bit and shorten the span and be even more solid.
Use the 3:4:5 squaring method, be extra sure it is level then be sure to stagger the floor decking. Dont chinz out on the decking.

willywilly2020
Member
# Posted: 17 Nov 2020 13:06
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I would highly recommend getting a book on framing. I went through a handful, and really loved "The Very Efficient Carpenter" by Larry Haun. Internet can be an overload of information at times if you don't know what you're looking for.

Would second the recommendation for even multiples for sheets of ply in your walls/roof if possible.

I build a shed roof cabin, and it was quite fun, but I did beef up my rafter joists much more than if I had build gabled trusses. You'll also need beefier rafter joists if you want to fit a fair amount of insulation into your ceiling. I know a couple folks who swear by buying pre-engineered trusses for a gabled roof.

I would highly recommend PT joists/skids, especially if you build low enough to the ground that you can't get under it. I used PT joists/skids, and painted the underside of the ply to protect it from moisture/mold.

For your skids, 4x8 would be much stronger than a 6x6. Rather than buy glulam, you could just make your own skids out of PT 2x6s or 2x8s.

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