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Aptos, CA, United States.

As you've probably noticed from my prior post, I'm in the process of building a foundation for a garden shed. My specific county/region does not have strict restrictions/requirements on shed foundations or concrete use. Good news: I'm on my own. Bad news: I'm on my own. At this point, I'm pretty sure I want to go with a concrete pier foundation.

The floor measurements are as follows:

flooring

I was originally planning to set three rows of 8 inch piers (three piers per row) with a pressure-treated 4x4 lining each row... but now I'm wondering if that's not enough or too many...

My questions are:

  • How many concrete piers would you suggest I use in order to support a floor of this size and an empty distributed building weight of 1600 lbs (without people standing on it).

  • Do piers always need to be at the very end? Or can there be a small overhang of wood? Ideally I'd like to have the piers away from the edges of the perimeter so they're not as visible.

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2 Answers 2

As with any advice you get here, keep in mind that you must stay within the bounds of IRC building codes adopted by your local municipality. Obtain a building permit and a local inspector will give you guidelines on minimum requirements for posts, beam spans, joist spans, etc...

Your concrete piers must be a set depth to go below the frost line in your area. You must set the posts on top of concrete with a post base to protect from rot.

When in doubt always make it stronger! Increase to 12" piers with 6x6 posts. Consider using 2x10 or 2x12 beams doubled or tripled on each row. Make sure that each beam span is no longer than 7' if you are worried. When you submit your plan for the permit then the inspector will point out any flaws or deficiencies that you need to address before you start.

NOTE: Any specific sizes I use are merely examples of how one can increase the strength of a structure. Nobody can say for sure without more specifics about the shed design.

Do piers always need to be at the very end? Or can there be a small overhang of wood? Ideally I'd like to have the piers away from the edges of the perimeter so they're not as visible.

When the beam extends past the last post this is called a cantilever. IRC building codes specify the maximum allowable length for live load for different types of lumber, and different sizes.

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does this apply if the shed floor is 2x2s for resting on a slab? or if the floor is 2x12s needing only point support at the four corners? frost line for a location that does not freeze? 'small overhang' = cantilevered? –  mike Aug 6 '13 at 17:08
    
If you were to support serious weight in this shed and you wanted to support the structure on rows of posts on concrete pillars, then you would run beams across the posts, then floor joists across the beams perpendicular and the floor would sit on top of the joists. The joists would not only support the weight of things on the floor but also the weight of the walls which carry the weight of the roof. A slab is different because the walls carry the weight of the roof, and the walls bear down on the slab itself. The floor over a slab will only have to support what is on the floor. –  maple_shaft Aug 6 '13 at 17:39
    
Then i think i've made my point. It's impossible to advise without knowing the structural design of the shed. –  mike Aug 6 '13 at 19:00
    
@mike That is a good point, but it is not impossible to advise just impossible to authoritatively answer. The OP asked about general advice on supporting a small structure on concrete pillars. We can advise about the way that this could be done, but the specifics in my answer are just examples. Perhaps I should state that in my answer. –  maple_shaft Aug 7 '13 at 1:50
    
So... would it be worth me elaborating more on the design, structure, and measurements? I'd be happy to if that would be helpful in clarifying. –  Mike B Aug 7 '13 at 20:08
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Impossible to say. The shed was designed to be supported in a particular manner. It has to be supported that way. If you're not inclined to that, though, an intermediate structure can be built underneath the shed that will support the shed in that manner, and which itself is designed to be supported in some other way, such as an array of 9 footings. Kind of an adapter between two load patterns.

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