Properly constructed and maintained plumbing will prevent sewer smells from entering your pipes. What comes in must go out, and half of plumbing is about getting rid of waste. The plumbing system inside your home or any building consists of a network of pipes, vents and traps. These are the least visible and least glamorous elements of your home’s plumbing. But this system of branching pipes that carry off liquid and solid wastes is just as important as the incoming system. Each plumbing fixture (toilet, sink, shower, floor drain, etc.) is connected to this system of pipes that carry the wastewater to the public sewer. This system safely removes waste for treatment and provides a critical barrier that keeps sewer gases and bacteria from entering your home.
There are a variety of reasons that sewer smells may be entering a home
or business.

1. A dry trap.  If there is a trap in a drain line that is not often used, the water in the trap may evaporate, breaking the trap seal. The solution is easy: pour water regularly into the drain to fill the trap.
2. A damaged trap.  Obviously, if the trap is cracked and allows the water to run out, the trap seal is again lost.
3. A damaged drain line.  If the drain line is cracked or broken between the main sewer and the trap, there is nothing to impede the flow of gases through the crack or break in the line.  Also, a damaged drain line may allow sewage to drain into the basement or crawlspace under your house, and you may get a sewer odor from that accumulation
of sewer.
4. A damaged or plugged vent.  The vent system in your plumbing is designed to equalize the atmospheric pressure differences caused by wastewater flowing through the plumbing, and also to allow sewer gases to escape the plumbing system by venting them into the atmosphere above the house.  If this system has an untrapped opening inside your house, gases may escape through that opening.
Plumbing fixtures and their connecting systems that are correctly designed and installed are normally odorless. However, even the best plumbing may sometimes allow sewer gas into your home due to a simple problem that can be easily checked, and simply solved. If the problem persists, then it’s time to call in the professional – a plumber.