Hi: I am getting ready to build a 16 x 24 combination utility building/small woodshop. I prefer to build it on a slab rather than on piers (pylons/pillars). Building it on piers would require an elevated wood floor which would make an ideal home for a family of skunks below the thing. I live in the woods in central Illinois, and the only place I can build this shed is next to a large Tulip Popular tree which I do not want to lose. If I dig a footer around the perimeter of the slab it would cut the roots to the tree and kill the tree. However, test holes show me that I can pour a number of piers around the perimeter of the slab without disturbing the tree's roots. I am thinking that I could use these piers, possibly connecting them with a shallow, foot deep, trench into which I would add rebar and concrete. Then, I would rest the slab on the pillars/trench, add a row of cement blocks on top to elevate the shed out of the mud and termites, and frame up the shed on top the blocks. What do you think of this plan? Is it as stupid and far fetched as it sounds? Any advice would be appreciated.

 

If you are worried about your footers, I would build the building on a floating slab. Remove the top soil, level and consolidate the native soil, and place a geotextile fabric over the soil. If you have heavy soils, I would try and incorporate a weep to any excess water from this layer. The area should be at least 1' larger than you slab. Backfill this area to an elevation 6" above ground elevation in 6" lifts with a #2 crusher run gravel and compact each lift thoroughly. I would pour a 5½" slab with #4 rebar on a 16 grid that is placed on 2" chairs. I built a garage similar to this in WNY and had no problems with frost heave or root moving the slab.

 

Hmm-- So in other words, no footer at all is necessary. I take it your garage is framed so that it rests on the edge of your floating slab, and the edge is able to support the weight of the garage okay. Is that correct?

 

the piers will support the shed's floor,,, i'd dig them below frost line @ height you need for the slab,,, form & pour slab on top of leave-in-place plywood & nail on your toe plate - no need for blocks,,, but its yours, not mine

 

I'm not sure what you mean by "form & pour slab on top of leave-in-place plywood". Do you mean lay plywood to form a base under the slab and leave it there? I would think I should put sand under the slab instead, or else pour it directly on the prepared ground. I'm not arguing because I am sure you know more about it than I do. I'm just curious and confused.

 

I think he meant strips of plywood to act as a form to contain the concrete?

 

we often place floors on stay-in-place forms of galvanized metal - even bdge decks,,, i don't encourage that expense for you however a couple sheets of plywood w/lumber supports would do the trick,,, IF you want to remove the ply, lay down butcher paper inside the form & place away

 

stadry...I'm still not getting what you mean? Ply under the concrete, ply in contact with the ground? How is that good?

I'm getting that the OP wants to only use piers in the vicinity of the tree and pour the slab on top of them and the rest would be typical footers?

 

Yes, that's right Gunguy. My question is whether piers reaching down to the freeze line (about 2 feet here in Illinois I think) would be adequate to keep the edge of the slab from breaking off under the weight of the 16 x 24 foot building. I think I can put the piers about every four or five feet along the slab perimeter, and probably put them all around the slab perimeter instead of pouring a footer. I am thinking I will pour a 4 inch thick slab reinforced with wire. What do you think as Super Moderator, Gunguy? Is this a bad idea?

 

By the way-- a lot of people here just build their sheds on runners (saves on taxes). Some build their sheds on runners mounted on piers (as you might do a deck). I don't want to do either of those because I think it would eventually lead to floor rot, and because I don't want animals moving in under the shed, and because I don't want the shed setting high above the ground. But, since people build their sheds on runners, I don't see why my idea won't work. I guess I just need a little reassurance from the pros before I do something costly and stupid.

 

Oh, don't ask me for specifics on the work. Just being a Mod doesn't mean I'm an expert. I handle the admin and clarification stuff.

 

I'm having difficulty understanding how a CIP slab resting on poured piers won't kill the poplar tree. Elevating the slab enough to allow oxygen getting to the tree roots enough to avoid killing it will also be a haven for the skunks and other nasty critters that the OP doesn't want to have around. Unless there are mitigating circumstance requiring a solid concrete floor (like lots of flame-cutting, or welding of steel anticipated) in the work shop, I'd be tempted to elevate a conventional wood-framed structure, supported by concrete piers. The space underneath could be effectively screened off using heavy galvanized mesh to prevent any squatters from taking over.

 

I am not sure if the slab would kill the roots under it or not, but I can see how it might block water and oxygen from getting to them and would then kill the tree. I will have to call a tree-man to find out. If it turns out that the slab will cause the tree to die, then I will definitely go with the elevated wood floor. Thanks for the help to all who gave suggestions. Since I didn't hear anything to the contrary, I assume everyone thinks piers will be adequate footers for the pad.

 

ever try describing the color ' blue ' to someone over the phone ? probably the main reason pictures are a help

 

All we can to is offer our opinions. What he does is up to him!