Footing thickness and projection
IRC R403.1.1 sets two rules for spread footings:
- Thickness (T): not less than 6 in (152 mm). Tables R403.1(1) to R403.1(3) use 6 in as the minimum concrete footing thickness.
- Projection (P): the footing must stick out beyond the face of the foundation wall at least 2 in (51 mm), and P shall not exceed the thickness of the footing.
Trap: reading only the 2 in minimum and assuming no upper limit. Second trap: capping P at 6 in; the ceiling is this footing's actual thickness. A 10 in thick footing may project anywhere from 2 in to 10 in on each side.
Footing width from the projection rule
Width = wall thickness + P on each side. For an 8 in wall centered on an 8 in thick footing:
- Minimum width: 8 + 2 × 2 = 12 in.
- Maximum width: 8 + 2 × 8 = 24 in.
Add the projection to both sides. Adding it to one side only gives 16 in, and capping each side at 6 in gives 20 in; both are wrong.
Slab-on-ground thickness
IRC R506.1: concrete slab-on-ground floors shall be a minimum 3½ in (89 mm) thick. Keep the numbers apart: 6 in is the footing thickness, 3½ in is the slab, 2 in is the footing projection.
Fill under the slab
IRC R506.2.1 limits fill depth, except where approved:
- 24 in (610 mm) of clean sand or gravel.
- 8 in (203 mm) of earth.
The most common error is swapping them. Earth gets the tight limit, 8 in; clean sand or gravel gets 24 in. A single 24 in limit for any fill is also wrong.
The limits are not absolute. The words "except where approved" mean the building official can approve deeper fill, so 14 in of compacted earth is allowed only with approval. Being free of vegetation and foreign material is a separate requirement and does not lift the 8 in limit.