The idea of rats entering a home or building via a toilet is the stuff of urban legend. Unfortunately, the myths are true. Rats frequently make their way into sewer pipes via cracks and vents. The rodents can swim half a mile in open water, travel against heavy currents and tread water for three days, making the journey from sewer pipe to toilet bowl an easy trek. Blocking rats from paddling about in your toilet involves more than fixing cracks and covering drains. Long-term maintenance and deterrence is required to assure that rat populations do not escalate.
Locate the ingress through which rats are entering sewer pipes using a smoke-producing leak detector. This device will send smoke down sewer lines, indicating cracks and breaks and will likely require the cooperation of town, city or county inspectors. Replace cracked and broken pipes entirely or wrap the pipe with 1/4-inch hardware cloth. Cover the hardware cloth with concrete patching material.
Cover sewer vents with 1/2-inch hardware cloth or 1/2-inch welded wire and make sure it cannot be easily pulled away by a rat. Inspect any drain covers for damage or cracks. Replace drain covers with grate openings of more than 1/2 inch with a new, 1/4-inch metal drain grate, 1/4-inch hardware cloth or 1/4-inch welded wire. Fix any holes or cracks around the drain with concrete patching material.
Install a one-way toilet flap, also known as a "rat guard," if main sewer lines and laterals are known to be rat infested. Install one-way flap valves on the ends of open drain pipes and gutters. Install the one-way valves so that the flap is pushed outward by the force of draining water.
Place either lethal or non-lethal traps in homes or buildings until rats are no longer caught. Set 12 to 24 rat traps in the average home and 50 to 100 traps in barns and larger structures. Check traps daily, dispose of rats that have been captured and re-set the traps. Do not use standard mouse traps, as they are too small to capture a rat. Set traps in pairs and place them against walls every 5 to 10 feet.