Smelly restrooms are often caused by floor drains that dry out. All drains to a sewer system have an “I” shaped trap which is usually filled with water. The trap provides a seal to keep out sewer gas. When the water evaporates, the seal is eliminated, allowing sewer gas (and smell) into your restroom. The solution is easy. By keeping water in the drain, you can eliminate the odor from releasing into the air. Simply poor water into the drain and the problem should remain at bay.