Footing Size?

[techinstructor]

I am designing my own home and need a suggestion as to the proper size and depth of the footing. The house will have a full basement 8'8" below grade with 12'x12" CMU walls -- 30'x48'. The main floor exterior wall will be 8" CMU with 9'4" walls. There will be two more rows of block added to get some more roof height due to 3' overhangs. The gables will be on the two shorter 30' walls and will extend 14'8" above the first floor. Total wall height on the 48' sides will be 22'8" and the total wall height to the top of the gables will be 36'. The walls will be poured solid with concrete and have steel added where recommended by the engineer.

There will be a monolithic 6" slab with perimeter footing that is 2' x 2'. Is this anywhere near adequate for the height of the walls? A 2' x 2' footing will be under two load-bearing walls in the basement as well. Should be footing be wider and/or not as deep? Please advise.

Because of the height of the walls I have to have my plans approved by an engineer, but I would like to at least have a ball park idea of what would work best. We like in the Piedmont of NC and have clay soil. Most of the rock in the area is slate.

Thanks.

[Richard A Hetzel]

The footings should not be monolithic with the floor slab. Unless very carefully designed and constructed, that is guaranteed cracks. If you have an engineer, he or she should be recommending a footing design. We can't do it here. We have no idea of the loads which will arrive at the footings. Your soil composition should be taken into account also, and we don't know that.

The rule of thumb for footings is that they should be twice the width of the wall that bears on them, and their thickness should be equal to the wall thickness, but that is only a rule of thumb. By the way, blocks are 12x16 or 12x18, not 12x12, and they are always 8 inches in height. Your engineer should recommend the exact block to use. Lightweight blocks may not be adequate, and they might have to be 75% solid load-bearing blocks.

[techinstructor]

The footings should not be monolithic with the floor slab. Unless very carefully designed and constructed, that is guaranteed cracks.

What are the problems that will cause a monolithic slab to crack?

[Richard A Hetzel]

The transition from one thickness to another has to be very gentle. If it suddenly transforms from a slab thickness to a wall thickness, a crack at that point is almost guaranteed. A load on the thicker part can also cause it to crack away from the thinner part, unless the transition is very carefully reinforced to control that cracking. All concrete cracks. The trick is to keep the cracks microscopic in size and not gaping chasms. That is done by careful design of the reinforcing, and great care in change of cross-sections.