Everyone knows a drain needs fall. Fewer people know that doubling it causes blockages, because the water runs away and leaves the solids sitting in the pipe.
Invert is the inside bottom of the pipe — the level that actually governs whether water flows, not the top of the pipe.
A drain does not simply let water fall downhill. It carries solids, and it carries them because the liquid moves them along the invert of the pipe.
Get the slope right and liquid and solids travel together. Make it too steep and they separate — the water accelerates away and the solids are left stranded on the pipe wall, where they dry and accumulate. The result is a blockage in a pipe that looks like it should drain beautifully.
This is why the standard grades are specified as figures to hit rather than minimums to exceed, and why a plumber will pull a run out and redo it for being too steep.
| Pipe | Slope | Ratio | Fall per 10 ft |
|---|---|---|---|
| Up to 2½″ | ¼″ per ft | 1 : 48 | 2½″ |
| 3″ to 6″ | ⅛″ per ft | 1 : 96 | 1¼″ |
| 8″ and larger | 1/16″ per ft | 1 : 192 | ⅝″ |
Bigger pipe uses less slope because it carries more volume at the same depth of flow, so it reaches a self-cleansing velocity with less help from gravity.
A quarter inch per foot is two inches over eight feet and four inches over sixteen. On a long run that is a real constraint — the drain can end up below where it needs to connect, or below the point it can discharge to.
Working out the fall before the trench is dug is considerably cheaper than discovering it afterwards.
Useful more often than you would expect: chlorinating a line, working out antifreeze for a seasonal shutdown, or estimating what is about to come out when you cut into a run.
The inside bottom of the pipe. It is what governs flow, and it is what elevations are set to. Measuring to the top of the pipe introduces the wall thickness as an error, which matters on a shallow grade.
At changes of direction, at the base of stacks, and at intervals along long runs — with the exact spacing set by code. They cost very little at installation and a great deal to add later.
Only where the code allows it. A buried joint that fails is expensive to reach, and shielded couplings exist specifically because unshielded ones deflect under backfill.