An 8-inch building sewer will run 300 feet from the building to its point of connection. Take these values as given: the minimum slope permitted for pipe 8 inches and larger is 1/16 inch per foot, and the survey shows 16 inches of available fall between the two inverts. Does the run work at the minimum slope?
- AYes; at the minimum slope the run needs only about 1.6 inches of fall.
This confuses inches with feet in the result. Three hundred feet at a sixteenth of an inch per foot is 18.75 inches, not 1.6.
- BYes; the run needs 18.75 inches of fall, and the 16 inches available is within the tolerance allowed on large sewers.
There is no tolerance that lets a sewer be built below the minimum slope. Falling short means redesigning the run.
- No; at the minimum slope the run needs 18.75 inches of fall, which is more than the 16 inches available.
- DNo; the run needs 75 inches of fall, because 1/4 inch per foot applies to every building sewer.
The quarter inch per foot minimum applies to small pipe. Larger diameters are permitted flatter, which is why 8 inch is in the 1/16 inch per foot row.
Why C is correct
Multiplying the minimum slope by the developed length gives the fall the run must have. Comparing that with the surveyed drop answers the question directly, and here the requirement exceeds what the site provides.
What this question is testing
Whether the candidate can convert a minimum slope and a length into required fall and compare it against site conditions in consistent units.
On the job
Long sewers on flat sites live or die on this arithmetic, and it should be done before anyone digs. When the numbers do not close, the options are a deeper tap if the authority allows it, a shorter alignment, or a lift station. Verify the manhole invert yourself rather than trusting a record drawing, because a few inches of error changes the answer.
Memory technique
Length times slope equals the drop you must have.
Exam tip
Fall in inches equals length in feet times the slope in inches per foot. Keep the units straight.
Where to look it up
Minimum slopes by pipe size are in IPC Section 704 and its slope table. UPC sets different minimum slopes, and flatter slopes on large pipe generally require specific approval, so confirm under the adopted code.