What's up with a drain when it's suddenly stinky? Most likely, it's one of two things: crud inside the pipe (which may be related to a clog) and sewer gas (which is always present in part of your drain system but isn't supposed to get into the house). Usually a smelly drain problem is easy to solve.
Of clogs and crud
A really gunked up drain can smell enough to carry into a room, and there are a couple of good clues that this is the source of your problem. A slow drain is one indication. A partially clogged drain doesn't get rid of waste effectively, leading to more crud buildup. Ultimately, this may lead to a total clog, but it can take a long time. Meanwhile, the buildup just gets worse. The solution is to clean the drain thoroughly with a snake (see below).
The other way to tell if your problem is crud is the smell. A dirty drain smells bad, but there's nothing like the gaseous pungency of sewage. The latter indicates a different drain problem (discussed in the next section).
A few drains that are most prone to crud-related smells and their solutions:
Bathroom sink: The drain stopper assembly catches hair — then everything else — several inches below the drain opening. Remove the stopper and clean this section of pipe periodically to eliminate odors.
Tub/shower drain: Hair clogs lead to soap buildup and badly gunked pipes; clean thoroughly with a snake.
Kitchen sink: Food disposers are the culprits more often than drains. Freshen inside the disposer with Borax or a commercial disposer cleaner, and clean the gunk from the underside of the rubber baffle around the drain opening. You have to do this by hand, and it's not pretty, but it works.
If your drain smells like sewage, that's probably exactly what it is, or at least sewer gas. Every drain in your house has (or should have) a trap — a U-shaped piece of pipe that holds a small amount of water at all times. The water serves as a plug to keep sewer gas from rising up the drain. (Toilets have their own internal version of a trap.) This simple system works beautifully unless one of the following happens:
• The original plumber or remodeler failed to install a trap, in which case you'd probably smell gas all the time.
• The drain isn't properly vented, and a suction effect in the system siphons the trap dry. The same thing can happen if the vent is blocked by tree leaves or something else.
• The fixture served by the drain is too close to other fixtures, such as a toilet drain that's too close to a sink drain. The force of the toilet flush can siphon the sink trap dry.
If you suspect a dry trap, run water in the fixture slowly for a few minutes, then shut it off and do a smell test. This should fill the trap and stop the sewer smell. This can help diagnose the problem but not solve it. However, most venting problems can be remedied by installing an air admittance valve, a pipeless air vent that can go almost anywhere along a drain line. Talk to your plumber.
You could turn your smelly drain into a chemical spill site by using drain cleaners with no effect on the clog whatsoever. That's why plumbers clean drains with snakes or pressurized water, not chemicals. They also tend to snake a drain repeatedly even after the blockage is cleared. This is to remove the buildup of crud along the walls of the drain pipe, which has almost undoubtedly been exacerbated by the clog. So the next time you think a clog is causing a smelly drain, use a snake, not drain cleaner.