Along one 15-foot length of continuous footing trench, the trench bottom drops a total of 24 inches from one end to the other, and the crew plans to pour it as a single sloping ribbon of concrete. How should the inspector judge that trench?
- AIt is acceptable, because the bottom of a footing may be allowed to slope as steeply as one unit vertical in five units horizontal without any step.
One in five is twice the slope the code allows on the bottom of a footing. Recalling that there is a limit but doubling the number lets a run of footing be poured on a grade that will want to slide under load.
- BIt is acceptable, because only the top surface of a footing is required to be level, and the bottom may simply follow the ground.
The top surface has to be level and so does the bottom, within a maximum slope. Stopping at the top surface misses the reason for the rule, which is that a sloping bearing surface pushes the footing downhill under load.
- It is not acceptable, because the bottom slopes about one in seven and a half, steeper than the one-in-ten limit, so this run has to be stepped.
- DIt is not acceptable, because a footing on a sloping site has to be poured dead level in one continuous piece with no steps at all in it.
Stepped footings are exactly what the code prescribes on a sloping site. Requiring one continuous level pour would mean excavating the entire run down to the lowest elevation, which this section is written to avoid.
Why C is correct
Convert both dimensions to the same unit: 15 feet is 180 inches, and 24 inches of drop over 180 gives a slope of one in seven and a half. That is steeper than the one-in-ten maximum for the bottom of a footing, so the run must be broken into level steps. Stepping also lets the top surface stay level, which the code requires everywhere.
What this question is testing
Converting a fall and a run into a slope ratio and comparing it to a code limit, plus knowing that the top and the bottom of a footing are governed by two different requirements. It also tests knowing that stepping is the accepted remedy rather than a defect in itself.
On the job
Hillside footings are where excavation crews take the most liberties, because benching a trench into steps costs time and every step needs its own forms. The failure mode is real: concrete poured on a steep grade segregates, the top surface finishes out of level, and the sill plate then bears unevenly. On a walkaround, sight down the trench with a level or laser rather than eyeballing it, and record the fall over a measured length so the correction notice carries a defensible number.
Memory technique
Top level, bottom no steeper than one in ten, and step it when the hill wins.
Exam tip
Put the run and the rise into the same units before forming the ratio; mixing feet and inches is the classic slip here.
Where to look it up
IRC Chapter 4, the paragraph on footing slope and stepped footings, near the minimum depth provisions.