United States
Hong KongPlumbing drainage and venting
Horizontal drainage of small diameter commonly requires a quarter inch per foot of slope. Traps hold a water seal against sewer gas, and vents let air in so that seal is not siphoned away.
A general contractor coordinates plumbing rather than designing it, and three ideas cover most of what is asked.
Slope
A quarter inch per foot is a common minimum for horizontal drainage piping of small diameter.
Too little and solids do not carry. Too much and the water runs away from the solids, which is the counterintuitive half - steeper is not automatically better.
Traps
A trap holds a water seal that stops sewer gas entering the building. Every fixture has one.
A dry trap - from evaporation in an unused fixture, or from siphoning - is an open pipe to the sewer.
Water flowing down a stack creates suction that can pull the seal out of a trap. A vent admits air behind the flow so pressure equalizes and the seal stays put. Venting is not about smell escaping; it is about stopping the seal being removed.
Testing
Drain, waste and vent systems are commonly tested at 10 feet of water head, held for a period, before concealment.
Testing before concealment is the coordination point. A system closed up untested is a system that gets opened up again.
Cleanouts
Required at intervals and at changes of direction so the line can be cleared. Building over a cleanout is a defect that only shows up when somebody needs it. Vents protect the seal.
Common questions
What slope do drains need?
A quarter inch per foot is a common minimum for horizontal drainage of small diameter.
Is steeper better?
No. Too steep and water runs away from solids, leaving them behind.
What does a trap do?
Holds a water seal that stops sewer gas entering the building.
Why are vents needed?
To admit air so flowing water does not siphon the seal out of a trap.
How are drainage systems tested?
Commonly at 10 feet of water head, before concealment.