Code Applied by the app plumb.drain.slope
Slope of horizontal drainage pipe
Horizontal waste pipe falls at least 1/4 inch per foot at 2-1/2 inch and smaller, 1/8 inch per foot at 3 to 6 inch, and 1/16 inch per foot at 8 inch and larger, except that pipe upstream of a grease interceptor falls at least 1/4 inch per foot at any size.
| When | The app uses |
|---|---|
| 2-1/2 in and smaller | 1/4 in/ft |
| 3 in to 6 in | 1/8 in/ft |
| 8 in and larger | 1/16 in/ft |
| upstream of a grease interceptor, any size | 1/4 in/ft |
- Source
- IPC · 704.1, Table 704.1 · read the public text
- Editions checked
- 2018 · 2021
- Used by
- Courses
- Amendments
No state or local amendment to this rule is on file. Not on file means the model code as written — check your jurisdiction.
A model code the authority having jurisdiction adopts.
Waste has only gravity. A horizontal drain must fall steadily toward the sewer or the solids settle out and the pipe stops, so the code sets a minimum slope by pipe size. The course's 4 inch sanitary drain falls 1/8 inch per foot: over 29 feet the far end sits 3-5/8 inches higher than the wall. Grease is the exception: waste on its way to a grease interceptor falls at least 1/4 inch per foot whatever its size, so the course's 3 inch grease line falls twice as steeply as the sanitary line beside it.
On a bid slope is depth. Under a slab every foot of horizontal waste is a foot of trench, and the trench gets deeper the farther the drain runs from the sewer; that is why an engineer runs drains the short way out. A grease line at 1/4 inch per foot gets deep twice as fast.
What the app does with it
Nothing yet: the app does not apply this rule. It is on a course card (the plumbing course, Chapter 4, "Which fixture must never drain through the interceptor?"). Bid Check has a manual row, Slope set on every waste run, that you tick when you have read the slope on the set; it checks nothing itself, and the app does not work out trench depth from a run's length.
Verify against your edition
The grease interceptor exception is in 704.1 itself, with a footnote on the table pointing to it, in both the 2018 and the 2021 text. The course card says 1/8 inch per foot for 3 inch and larger: right for the course's sanitary pipe and conservative above 6 inch, but not for pipe upstream of the interceptor. The Uniform Plumbing Code starts from 1/4 inch per foot and allows less only where the AHJ approves it (its Chapter 7; not opened for this sign-off). A jurisdiction may set its own minimum. Settled 2026-09-27 by Claude on the owner's delegation, from the dossier's research and the model code text; not checked against a printed book or a local amendment.
Cited by section, not reprinted: the code text belongs to its publisher. Company practice is not a rule here — it lives with your pricing.