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xinull
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# Posted: 4 Sep 2020 03:35pm
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Lets get this out of the way immediatly, it's impossible to bring in heavy equipment where i'm building. Access is limited.
I'm going to be building a 16x28 cabin on skids, on cinder block pads. 5 pads on each side to support the walls and 3 larger pads in the middle to support the roof ridge beam at each end and in the middle. Those cinder blocks are going to be sitting on a 2'x2' patio block, resting on compacted gravel (i think i can get a small compacter there....i think).
I also plan on digging by hand 1 to 1.5 feet down, just to remove the soft part of the soil on top....and the holes will be filled and compacted with gravel i can get on the neighbors land.
I have a 2 question in my design
1. What's the best size gravel that i should use for under the pads, basically what's going to be better and prevent movement the most? 0-3/4" or 1/4-3/4"...or maybe something even thinner like crusher dust (tailings)
2. I've heard people placing 1" rigid foam under their pads to make a thermal insulation and help with the freezing. They say that 1" of rigid foam can prevent 1 foot of freezing i think...or something like that. So how does that work if I wanted to do that? do i put the foam under the cement pad but on top of the gravel? or do i put the foam deep at the bottom of my hole under the gravel? Should i sandwich the foam with 3/4" osb/plywood? I guess i'm not seeing how rigid foam is not going to just get crushed by the weight under gravel and/or the cinder blocks. Any advice and details of how this should work would be greatly appreciated 
All advise and comments are welcomed. And i know most of you will say just dig sonotubes instead and forget about the skids and pads. I just dont know if i have it in my back to do that by hand. I think frost is at almost 5 or 6 feet here. 
Thanks for your help
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NorthRick
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# Posted: 4 Sep 2020 03:58pm
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Angular gravel is best. It packs together well. I'd want something without fines (silt, clay) so that it drains well.
Unless you are trying to prevent permafrost from melting, I'd skip the foam. You'd be better off making sure the roof has a good overhang and that water drains away from the foundation.
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Brettny
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# Posted: 4 Sep 2020 04:36pm
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For the gravel anything with 0"-2" will be fine.
16x28 is pretty long and skinny. Have you played around with any interior layout programs? Why 16' wide when 20' lumber is usualy only cents more per foot?
If you use wall ties or have a loft you really dont need a load supporting roof beam and this would mean you dont really need extra big footings under it because the walls would be sharing the weight of the roof.
I have never heard of putting foam under a compacted gravel base but nor would I suggest it..
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xinull
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# Posted: 4 Sep 2020 09:37pm
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Brettny,
the reasoning behind 16x28 was because i wanted a multiple of 4 to save on waste. I guess i could have gone 16x24, i just felt that with 28 i'd get a little bit more real-estate inside
I didn't want to go 20' because i only wanted 2 skids on each side, and none in the middle. This should help me with any movement. Even though i'll be cantilevering 9" (because i'm using 2x10 floor joist i can cantilever up to 9.5" safely)...and my skids were going to be 4 plies of 2x, so 6" wide for the skids. That means that at 20', my span would be 17.5 feet. I was going to space my floor joist every 12 inches, and that's only because i wanted the floor to be firm, and i wanted my joist to line up with my wall studs, which were going to be 24" spacing. I'm no expert, but I'm not even sure if a 2x12 joist would be good enough to span 17.5 feet and keeping the floor stiff enough.
My walls will be 10' high, and I will have a loft set at 7.5 or 8', but only in one half of the cabin. The other half will be open. So that's why i need a ridge beam with support in the middle (according to my buddy who's a structural engineer)
In a way, I almost wish i could go 20'. it would make the cabin less "skinny" like you said. I've seen some pictures of another cabin at 16x28 and it doesn't look that bad, but 20 would seem better.
what do you think, could this be done at 20x28 with only 2 skids?
I'll stay away from foam It seemed more of a head-ache anyways.
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ICC
Member
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# Posted: 4 Sep 2020 11:11pm
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To help with joist spans use the AWC calculator.
Select the species, grade, size, joist spacing. Use the floor joist for type and deflection of L/360. L/360 is stiff enough to install ceramic tile. The normal code loads are LL=40 psi and DL=10psi. Your LL will probably not come close in a cabin as you probably won't have extra large refrigerators or water beds. DL=10 is fine unless you use a lot of very heavy materials like stone on walls or floor.
For example, spruce-pine-fir, 2x12, #2 at 16" OC, can span a max of 17' 10" with LL=40 and DL=10. 2x10 good to 15' 5".
FWIW, I have a friend with a 15'9" wide cabin built on two beams set in 9 inches from the walls who used 2x10 floor joists, 16" OC with a tile floor. No cracks anywhere after 11 years or so.
Regarding foam.... I would not use any in your application. I don't think it would do any good at all in your application. It works very well under concrete slabs. The XPS type found at big box stores is usually the type that is capable of supporting up to 15 PSI. 15 PSI is 2160 lbs per sq ft. With DOW the number in the name tells you its load bearing... Foamular 1500 is 15 PSI; foamular 2500 is 25 PSI.
I say don't bother because I assume this is a part time use cabin; not heated ful time. The patch of foam under each footing would not provide sufficient insualtion to make a difference, in your case. IMO.
I see nothing wrong with 28 feet length and 16 wide. The length of the cabin of the friend I mentioned is 30 feet. Just one room with a shower at the far end in a 3x7 foot walled room. He uses furniture to set the bed at the end off from the rest of the space. Easy to heat the whole space with a wood stove at the door end, away from the bed end.
Plan to make the cabin a few inches narrower than the 16 feet if you will be using T&G subfloor sheets. You lose 1/2" at every T&G joint. Planning for that will make the floor sheathing go nicer. The exterior wall sheathing might need slight trimming but that is better than having the floor sheathing come up too narrow. Don't forget the rim joists eat up 1-1/2" per side when you plan subfloor sheathing layout.
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ICC
Member
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# Posted: 4 Sep 2020 11:16pm
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You can go to a 20 foot floor joists width with 12"OC spacing, but those 20 foot 2x12's are somethimes hard to find and may cost more per foot of length than 16 footers. Unless you also run a center beam full length to support the middle all the way. Then the joists can be much smaller and 16"OC easy. 2x6 would just make it.
Play with the calculator I linked to.
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rpe
Member
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# Posted: 5 Sep 2020 07:45am
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There is an 8x12 storage shed on our cottage property that the previous owners put on short sonotube piers (floating, not pinned to bedrock). It was falling down when we purchased, as frost had shoved the piers over.
To fix, I jacked up the building, stuck 6x6 PT beams underneath. To support the beam, I dug out at the corners, pulled out the remains of the sonotube posts, and filled the area with gravel. A concrete pad block was placed and leveled at each corner, and set the building down. It's been fine since. It does move a bit in the winter, but seems to return to level in spring when the frost leaves the ground.
I'm not experienced with that sort of setup for a building your size, but this site has a wealth of information if your search keywords are well chosen.
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Brettny
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# Posted: 5 Sep 2020 08:04am
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ICC posts are spot on.
Do some more information finding and figuring. Since adding a 3rd girder would allow you to use shorter thinner floor joists it may just be a wash money wise. Remember you only have to make a good foundation once. You have to live with to skinny of a cabin for ever.
Same goes for driveways (and many other things). Some times the easiest location to build is right by the road but the best location would require a longer driveway.
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Brettny
Member
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# Posted: 5 Sep 2020 08:11am
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Also one of my pet peeves is noisy floors especially in cabins with a sleeping loft. For floor joists we are doing 8in lumber for 8' span, 10" for 10' span and so on.
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Aklogcabin
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# Posted: 5 Sep 2020 01:16pm
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Welcome. On the floor joists. Our cabin is. 16’x24’ , 10’ high walls. Pics on our ak log cabin dream. I used BCIs. Manufactured joists. I’m remote so lighter and one person can handle the weight. And there straight. Wood move. Always.
I believe that NorthRick is correct on the gravel. Fractured edges like 3/4†crushes rock. No clay but a few fines for binder is good. Water it as you compact. Blue board is used frequently in Ak where permafrost is an issue. Sandwiched between aww plywood. I used cedar , treated utility poles for pilings. They are working good.
I like to have the wood stove so the chimney is near the ridge so snow doesn’t take it off. I don’t like squeaky bouncy floors. And didn’t want a center beam in the middle of the cabin. Photo shows how I used beams to support floor and tie building together. I have 6†logs so I put a 8†wide log at 8’ so t&g 2-6 nailed at ends . We have a full upstairs. Love it. Last night in bed we opened the door n enjoyed the evening air. And the sounds. Actually a sandhill crane being asked to supper by the local lynx or wolf.
Your concept for construction seem good to me. Lots of great photos around. If I remember correctly there was a very helpful fellow. Gary O maybe. Who has built many cabins on skids. Some old 243 guy to. Offer nothing but positive. And others who have figured it out when there’s no book.
Good luck. Always stay positive. For all those heading out this time of year to enjoy the outdoors. Remember take one make one.
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xinull
Member
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# Posted: 17 Sep 2020 08:49pm
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Thanks for the good advice.
I've decided to make it 20' instead of 16'. Brettny, you're absolutly correct. I'd rather make it a little bit bigger right now than having to fix it later. I'll just need to workout a middle girder.
I'll probably stay with 2x10 just so i cantilever my 9" safely. It's just a preference for me, i want my piers to be under the cabin, not extruding out. So this way it's not raining directly on the piers, they are sorta protected underneath.
For the subfloor, should i be using plywood or OSB?
i guess the same question goes for the sheating of the walls. I hear a lot of people saying that OSB now is almost as good as plywood. Most house builds use it now. And wood is so expensive these days. i'm wondering if does make a difference at all, being a cabin.
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xinull
Member
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# Posted: 17 Sep 2020 08:50pm
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Making it 20' I feel will also make it more stable on the piers
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Brettny
Member
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# Posted: 18 Sep 2020 08:14am
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Yes the cost difference between a 16' bs 20' is pretty minimal and you really dont need 20' lumber.
Last time I built with OSB( 2 summer's ago) we used OSB for sheething. It's real crappy for holding nails, so bad that I nailed my vinyl siding to the studs. A nail gun will also shoot a nail right through OSB and out the other side. Plywood this dosnt happen.
For the sub floor it's really all about how much weather it will see prior to being dryed in. We used OSB 3/4in tongue and grove sub floor on my back porch, it had no exposure. If it was going to see exposure I would use plytanium dry ply. I believe HD has it now for $51 a sheet. It use to be $31 in jan this month.
Plywood prices are why we are waiting to build our cabin. I would be putting in sono tubes right now if the prices weren't so high. Now I'm just doing more dirt/site work and witing out the covid over charge prices.
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