# 12x16 Foot Shed Foundation?



## More Power! (Aug 2, 2011)

Hi There,

The poorly-constructed 12'x12' shed on our property is too far gone to save. We're going to replace it with a new 12'x16' shed. One of the things that destroyed the old shed is the people that built it simply placed it on the bare ground (near as we can tell). Besides other things: Animals keep digging under it and tunnelling, so the floor's all twisty and saggy.

Site is S.E. Michigan. Soil ranges from clay (mostly) to sandy loam (very little).

We don't wish to go to the expense of a 4" concrete pad. I was thinking 6" of crushed stone to a level of a couple inches above grade. Then there was the concern of the stone migrating, over time, and the animal digging problem, so a landscaper suggested two courses of creosote-treated railroad ties around the perimeter.

To keep everything in place I figured on drilling the bottom course of ties and driving 3' of rebar into the ground, maybe twice on each short side and thrice on each long. The same number of lag bolts to fasten the two courses together. Angle brackets in the corners and a pair of front-to-back and side-to-side cables. Then the crushed stone inside that. Maybe drill some horizontal holes above grade just in case we ever got _that_ much rain all in one go?

What do y'all think?


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## cleveman (Dec 17, 2011)

You seem to be prepared to go to extremes in order to avoid pouring a simple slab.


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## joecaption (Nov 30, 2011)

Where do you think used railroad ties come from?
There being replaced with new ones because the old ones have reached the end of there life span.
With a slab there's no need for the cost of floor joist, rim joist, joist hangers, some form of footings, and plywood for the floor.
Other then that there should be footings below the frost line, and piers to build it on at least 6" above grade.


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## More Power! (Aug 2, 2011)

cleveman said:


> You seem to be prepared to go to extremes in order to avoid pouring a simple slab.


I'll get it quoted tomorrow, but I'm _told_ a 4" concrete slab, 14'x12', would be "expensive." My labour suggests they'll need about eight yards of crushed stone at $20/yd.



joecaption said:


> Where do you think used railroad ties come from?
> There being replaced with new ones because the old ones have reached the end of there life span.


They won't be directly structurally critical. They're only to keep the crushed stone from "migrating" away from the structure.



joecaption said:


> With a slab there's no need for the cost of floor joist, rim joist, joist hangers, some form of footings, and plywood for the floor.


The shed is a pre-built kit that's assembled on-site. It comes with all that.



joecaption said:


> Other then that there should be footings below the frost line, and piers to build it on at least 6" above grade.


I initially thought about concrete piers, but was thinking, for a 16'x12' span, that might mean a _lot_ of piers... 12 of them, at least? Maybe 16? The shed comes with 4 4x4s running the 12' span. There's going to be a heavy commercial-grade mower, plus more, in that shed.

Thanks for the feedback!

Jim


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## steveinNEPA (Jun 13, 2014)

Go with a built up bed of stone OR pour the slab yourself. Rent the mixer, buy the concrete etc etc. 

Or you COULD set it on PT 6x6 posts. But they will eventually rot away being in contact with the ground. 

Just my 2 cents

edit** Dont use old re-purposed railroad ties to hold the stone bed... BUY NEW PT 6x6s... The old ties, will rot faster...


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## Curlie (Nov 22, 2013)

*Treated lumber?? not all equal*

If you use treated lumber, find a reputable lumber yard that knows their stuff. That may not be easy, but different chemical treatments are available at different treat rates, depending upon the intended usage of the lumber. The old CCA treated lumber is still available and at concentrations suitable for ground contact. The newer ACQ formula is available in 2 or 3 concentrations, again depending on intended use. There is also marine grade treated lumber for building boat docks and those have the highest chemical concentrations (and cost the most). Do not put something down that you will have to replace in 4-5 years. 
If the shed is pre-built, it is on a skid. Buy some of the round tubes, dig to frost level, pour in concrete with a threaded rod that can bolt down the skid. Use skirting to keep the critters from under it. The builder of the shed should be able to tell you how many piers you need to pour. and hey Jack, remember the greek Pi, well use Pi r squared times the height to figure concrete. Just remember to keep your units all the same/ Half of the diameter is the radius so 5 squared for a ten inch form (25) times Pi (3.14) times a height of 36 inches is about 1.6 cu ft, or about 3 80 lb bags. If you have it done for you (I thought this is DIY) you could pour 17 such footings from a yard of concrete.  Just think of the post hole digger as your trip to the gym!


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## More Power! (Aug 2, 2011)

Curlie said:


> If the shed is pre-built, it is on a skid. Buy some of the round tubes, dig to frost level,


Yeah... that's 42" in S.E. Michigan. That's quite a bit of digging--in clay.

But maybe no worse than 16'x12' to 6".



Curlie said:


> The builder of the shed should be able to tell you how many piers you need to pour.


Very well. I'll give 'em a call.

The mower, alone, weighs nearly 1000 lbs.



Curlie said:


> [for a depth of] 36 inches is about 1.6 cu ft, or about 3 80 lb bags. If you have it done for you (I thought this is DIY) you could pour 17 such footings from a yard of concrete.


Hmmm... That would be do-able, but...



Curlie said:


> Just think of the post hole digger as your trip to the gym!


For 17 footings, 42" deep, in clay? Yeah, I'd say so. I'm thinkin' rent a powered digger, for that 

Thanks for the math, Curlie 

Jim


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## More Power! (Aug 2, 2011)

Hmmm... Can't edit posts...

The shed builder said the piers should be every 3'. They'll add a 4th 4x4 runner at no charge. That'd be 24 piers (6/runner).

If my math is right: With a yard of concrete I can fill 35 6" diameter piers 46" long (42" in the ground + 4" above grade).

Jim


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## More Power! (Aug 2, 2011)

So... email from my wife just a few minutes ago...



> Just talked to [elided] Lumber.
> Asked about their sheds, how they compare with...
> ...
> Asked about recommended foundation. Guess what the answer was??
> ...


Hmmm...

Jim


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## tony.g (Apr 15, 2012)

Couple of inches of stone should do fine IMO.


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## More Power! (Aug 2, 2011)

tony.g said:


> Couple of inches of stone should do fine IMO.


lol!

FYI: The 12'x12' shed the new one's replacing has been there probably 30 years--just plunked down on the bare ground.

Jim


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## ChantryOntario (Apr 22, 2013)

In my experience, the old creosote-treated ties only have about a ten year lifespan then they are completely rotten. I have a couple flower boxes built out of them that came with my house and they are totalled, whereas the PT stuff that was installed at the same time is almost good as new. 

I don't know what the lifespan was for them embedded in the crushed stone track base, but at one foot above grade in the weather they did not do so well.


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## More Power! (Aug 2, 2011)

ChantryOntario said:


> In my experience, the old creosote-treated ties only have about a ten year lifespan then they are completely rotten.


Thanks for the comments, CO. I think that question may be moot, anyway, as it seems creosoted RR ties are at least difficult to find, if they can even be had, anymore. At least here in MI.

My wife expanded on the conversation she had with a big local lumber yard yesterday. Apparently they claimed "nobody" bothers with pilings or concrete pads for sheds the size of what we're talking about. They also claim retention for the crushed stone or gravel is unnecessary, as well.

Jim


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## charliek2 (May 10, 2011)

I've done a couple and I just level the ground, put down two rolls of chain link and then 4" of gravel. You get ventilation, up off the ground, the groundhogs are discouraged by the chain link and the gravel, and you'll get 20 years out of it at least if it is built on a skid of pressure treated 4 X 4's.


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## Canarywood1 (May 5, 2012)

More Power! said:


> I'll get it quoted tomorrow, but I'm _told_ a 4" concrete slab, 14'x12', would be "expensive." My labour suggests they'll need about eight yards of crushed stone at $20/yd.
> 
> 
> They won't be directly structurally critical. They're only to keep the crushed stone from "migrating" away from the structure.
> ...


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## More Power! (Aug 2, 2011)

*What We Finally Did*

So here's what we finally did:










That's ten used railroad ties, 1-1/4 gallons of turpentine/linseed oil mix ("booster shot" for the creosote treatment), eight corner brackets and four straps, 72 ea. 5/16" x 3" lag bolts, twenty ea. 1/2" x 18" pieces of rebar, approximately six square yards of crushed stone--filling the foundation to 6-8" of depth, four guys, one woman and a boatload of power- and hand-tools 

The gravel walkway was an unplanned bonus, when we found ourselves with an extra yard or so of crushed stone (7-1/2 yards, in total, were purchased).

I strongly suspect that foundation will outlive both my wife and I.

Jim


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## TexasWilson80 (Apr 7, 2021)

More Power! said:


> *What We Finally Did*
> 
> So here's what we finally did:
> 
> ...


Not sure if you still look at this site, but how is that foundation holding up? I'm building the same size shed and live in Texas where frost isn't really a thing and that looks pretty stable to me.


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