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A friend of mine is building a 22×30 garage. He was asked by one of the contractors whether he wanted a single-pour or block foundation. Since the footer is only 18″ deep, is there an advantage to a block foundation? It seems like an unnecessary complication for a job of this type, but I could be mistaken. Does the single-pour method include the slab, or is that really a second pour? I assume the foundation wall is the width of the footer for its full height.
He asked me if I knew what the advantages and disadvantages were, but I couldn’t help. He needs to talk to his contractor some more. In the meantime, he got me curious, so I thought I’d ask here. Any comments?
Thanks,
Froed
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Froed - I'll give it a stab. The block may be cheaper depending on your area. For instance here in central NJ, it is invariably a lot more expensive to build a formed concrete basement or crawl space than to build with block.
In PA just 25 miles away, developers are frequently forming basement walls instead, so it must be less expensive due to availability of cheaper subs, forms, etc.
In your case, if you choose a single pour and the slab integral with the (short) stem wall you only have to form one side. There's no reason why the wall thickness needs to exceed 8" in these conditions (check your code) - it certainly doesn't have to be the width of the footing. Also, if you do a nice job forming the exposed side, you might not need any other finishing, whereas the block wall will need to be parged, painted, etc.
Block leaves more room for (fixable) mistakes prior to the slab pour, but, hey, no mistakes, right?
Is it a 3-car garage? If so, the 30 side might be a little tight, depending on the door width and vehicle size.
Just a few thoughts for a post that almost got away ...
*I would have to assume that frost protection isn't an issue where you live. If it was me I would pour slab and foundation wall all in one pour. Clean the area where the garage is going to be of grass ect. and excavate down as deep as you need to go to get to native soil. Compact in a good gravel base the size of the garage. Dig a trench around the whole perimeter of the garage over excavating on the exterior side of the garage down to the depth of 18".When finished find a laborer that will slope the ground on the inside of the trench back at a 45 degree angle. Form up the outside and brace the hell out of it. Put lots of rebar in and make sure that the slab rebar follows the 45 degree slope of the trench part. When you go to pour, fill the trench half full all around the outside with about a 4" slump. Then on the second time around fill it right to the top which should be height of the slab.
*Thanks for the input guys. I also talked to a local (southwest Ohio) inspector, and since it's a 2-car garage, either way would work. A single-pour (monolithic) needs 18" deep footers, 8" min width at the bottom tapering to 12" min at the top. Lots of mesh and rebar, especially where the slab joins up with the wall, due to the change in section thickness (4" slab and 12" wall at the top). His explanation indicated that a single-pour foundation would actually move somewhat, since it's not below the frostline. As a result, it won't last as long as a block foundation at 30" below grade, which better isolates the slab from heaving.Of course the block method is more expensive, and a single-pour is perfectly acceptable, but it sounds like the block would be the way to go, finances permitting.Thanks again for the info. I'll pass it along.Froed