All of the drains in your house lead to the sewer, and if it weren't for the P-traps sewer smells might be an everyday nuisance in your living space. Traps are curved pipes holding water that seals the drain. Sewer smells can signify leaking waste pipes, but they may also mean that there is poor venting and water is being sucked out of the traps.
A plumbing P-trap is a length of pipe shaped like an inverted "P" connected to every household drain and built into every toilet. It was perfected by 19th century plumbers and is required by the modern plumbing code. Since the inlet and outlet of a P-trap are at higher levels than the bottom of the "P," water collects there and forms a seal that keeps gases inside the pipes. If the trap is installed poorly or cracked, water may leak out, but a trap also may empty because of poor venting. Sewer smells may be the result in either case.
Early P-traps frequently emptied during normal use until an unknown plumber got the idea to add vents to the waste lines. The purpose of vents isn't to provide an escape path for sewer gases, but to allow air into the waste lines to replace the vacuum created by water rushing through the pipe. Without venting, this vacuum sucks the water out of the P-traps and they allow gases to escape. Modern waste systems are required to have appropriate venting, usually in the form of 2-inch pipes that extend vertically through the roof of the house.
Because the vent outlets are on the roof, they can collect debris from overhanging trees, other kinds of dirt and even an occasional small animal that may crawl inside and die. The result is that the vent pipes become blocked. A gurgling sound coming from a nearby shower or sink drain when you flush a toilet is a sign that the vents are blocked. The sound is produced as air is sucked into the drains by the vacuum inside the pipes. Slow draining is another sign of poor venting, because the vacuum holds the water back.
Other Causes of Sewer Gases
Cracked waste pipes and broken seals also can allow sewer gases to escape. You often can locate cracks in sewer pipes by looking for water around the pipes or dripping on the floor, but it may not be visible if the pipes are buried in the wall. If you suspect a rupture in a drain line that you can't pinpoint, have a plumber pressure-test the line. Sewer gas smells in the immediate vicinity of a toilet often mean the the wax ring seal has broken and probably will be accompanied by water around the toilet base. Correct this problem by resetting the toilet with a new wax ring.