12 x 16 Shed - Foundation Questions

   / 12 x 16 Shed - Foundation Questions #1  

Dougryan

Bronze Member
Joined
Apr 6, 2000
Messages
94
Location
Webster, NY
Tractor
Cub Cadet 3206
Hi,
I'm going to build a 12 x 16 shed this summer and would like some opinions on the foundation. What I would LIKE to do is pour a concrete pad... Unfortunately, the town does not allow a permanent foundation under a "shed".
So, what I'm GOING to do is build a pressure treated stick framed floor and support it (the ground is slightly uneven) with pre-cast concrete post bases and 4x4 posts.

One concern I have is that the ground under the shed may settle over the years and I may need to re-level it by jacking up parts and adding some shim material under the pre-cast bases.
Because of that, I really don't want to have to crawl under the shed. I'd like all of the supports to be around the perimeter.

Questions:

How many support posts should I use?
What pressure treated framing lumber is best to make the 12' span?
Is 2' on center the right spacing?

I have attached a quick drawing that shows the basic framing with my proposed post locations.

Feedback please...
Thank you,
Doug
 

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   / 12 x 16 Shed - Foundation Questions #2  
You could dig down below the frost line and put in footings for the post, or go with sonotubes to avoid having to relevel later. Other option would be to remove the top soil, compact the soil and use a compacted crushed rock base.
IAC, I think you need to get rid of the topsoil to minimize movement later on.
 
   / 12 x 16 Shed - Foundation Questions #3  
More digging, well a lot more digging, :laughing: but it would get you below the frost line.

Genest Precast: Frost Post

If you look for "frost posts" a local concrete products company in your area probably has them.
 
   / 12 x 16 Shed - Foundation Questions
  • Thread Starter
#4  
I think that digging and pouring concrete for footings or sonotubes etc would be considered a permanent foundation... Not allowed in my town.
There's not much topsoil, so clearing the sod and a few inches of dirt (to get to the clay) would be ok under the post bases.
 
   / 12 x 16 Shed - Foundation Questions #5  
The frost posts are precast, no pouring on site. If you would be allowed to dig a post hole and put a precast concrete disk in the bottom for a footing; you ought to be allowed to set a precast frost post.

Do you need a permit to dig fence post holes? Fence, shed--not a lot of difference in ground disturbance.
 
   / 12 x 16 Shed - Foundation Questions #6  
Does anyone in your area install screw piles? It's basically a pipe with a significantly larger helix welded onto it and you just screw them into the ground. I've also seen them in hardware stores where people use them to anchor boat docks. The oil industry has then built in large enough sizes to place buildings on. If you need them longer to get below the frost another piece of pipe is welded on.

If you do find some and use them you shouldn't have to worry about frost heaves if they are installed below the frost line.
 
   / 12 x 16 Shed - Foundation Questions #7  
Here is what I would do and it's just my opinion. You need to think carefully about what your actual soil conditions are, but frost heave is overrated.

I would make this a floating shed. Make a slab base out of ground contact treated 4x4 or 6x6 posts. Essentially a sled base. Then build a conventional stud framed building above it. Dig a shallow (12 inches or so) trench system where the base timbers will go, fill it with 3/8 to 1" fill rock and compact it level.

I'm sure a lot of people will say this is nuts, but I've seen a number of buildings (including houses) with little or no foundation in cold climates. As long as the area is well drained and the base is compacted under the building, they hold up pretty well. Keep in mind that there is another potential negative with this. You will have limited tornado resistance.
 
   / 12 x 16 Shed - Foundation Questions
  • Thread Starter
#8  
Yeah... permits are required for fences too. Sucks to own land and pay taxes on that land and still need to pay extra to do anything with that land.

I do like the idea of the floating shed. I have some dirt left from a pool excavation (needed a permit for that too) that I can use to build up a level pad. Get some gravel and I'm good to go.
Gotta do some measuring to see how un-level the area really is. Over the 16' length it may be off by about a foot +/- a couple of inches. I can deal with that.
Thanks.
I'll post some pics as I make progress.
Doug
 

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   / 12 x 16 Shed - Foundation Questions #9  
You can look at span tables for decks to get an idea what spans, spacing and sizes of joists are in general use. A 12' span and 24" would not break but it would be bouncy even with 2x12s. Myself, I would do it KennyG way with three parallel 16 ft 4x6s in ground contact on compacted trenches of crushed rock. This gives a 6ft span and use 2x6s or 2x8s for the joists 16" oc. Treated lumber is generally eastern yellow pine which is quite stiff.
 
   / 12 x 16 Shed - Foundation Questions #10  
I looked at your plan, you need to use 2*10 joists 2' O/C. The edge beam carrying roof and floor loads (30psf snow load) should be 3 ply 2*10. Skip the double joist in the middle and add blocking between the joists at mid-span. This will yield a strong floor that will have less deflection than span/360 (L/360) which could support ceramic tile without cracking. As for the foundation, all foundations that do not extend below the frost line are floating foundations. Settleage or frost heave could change the elevation of the footing, this can be minimized by digging out underneath the footing and installing clean crushed stone at least 12"deep and 2' square. Place landscape fabric on the inside of the hole to prevent soil contamination of the crushed stone. Set your deck blocks with 6*6 posts supporting the edge beam. Joist hangers connecting the joists to the beam and post to beam connectors will add considerable strength to the assembly. Don't skimp of the decking use 3/4" tongue and groove plywood at minimum. Use pressure treated lumber if the bottom of the floor is within 12" of the ground. Installing a 6 mil poly vapour barrier underneath the shed wouldn't hurt either! Sorry for being so long winded
 

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