Spread footing size
2018 IRC R403.1.1 requires concrete spread footings to be not less than 6 in. (152 mm) thick. The trap: 3-1/2 in. is the minimum slab-on-ground thickness, not footing thickness.
Projections past the wall must be at least 2 in. on each side and no more than the footing thickness.
Footing width arithmetic
Width = wall thickness + projection on each side. For an 8 in. wall centered on a 6 in. thick footing:
- Narrowest: 8 + 2 + 2 = 12 in.
- Widest: 8 + 6 + 6 = 20 in., each projection capped at the 6 in. thickness.
Traps: 10 in. and 14 in. project on one side only; "no upper limit" ignores the thickness cap.
Fill under a slab
2018 IRC R506.2.1 limits fill under a concrete slab-on-ground, except where approved, to:
- 24 in. (610 mm) of clean sand or gravel; or
- 8 in. (203 mm) of earth.
The fill must be free of vegetation and foreign material and compacted. The trap: swapping the limits (24 in. of earth, 8 in. of sand or gravel).
Bearing: the Georgia amendment
The Georgia DCA amendment to 2018 IRC R502.6, effective January 1, 2020, sets end bearing for joists, beams and girders:
- at least 1-1/2 in. (38 mm) on wood or metal;
- at least 3 in. (76 mm) on masonry or concrete.
So a floor joist on a concrete foundation wall needs 3 in. Using 1-1/2 in. on concrete is the common error; 2 in. is the sill plate minimum, not a bearing length.
Bearing on masonry or concrete is either direct, or over a sill plate of at least 2 in. (51 mm) nominal thickness. A 1 in. plate is too thin, and a plate is not always required. Approved joist hangers are an alternative, not the only method, and the 1x4 ribbon strip is for joist ends nailed to an adjacent stud, not set into concrete.