Rats and mice are limited to the size hole they can get through by the skull. They cannot compress their skulls. In the case of a rat, generally that means a hole the size of a quarter [25-cent piece] or a crack or seam around a door or window that is as narrow as the skull is thick. My wife has a house in Boston, and there is a real rat problem there. She has been fighting the problem for over a year. Fingers crossed, looks like the problem is contained, but I am not putting any cash on the line for betting it's over.
A mouse or rat can squeeze beneath or through a crack [garage door one example at the bottom of the door] that is about the thickness of the animal's skull. We saw this happen one evening, and it is amazing to see.
I always thought it was an urban myth that rats can come up through a toilet. However, this also happened in the Boston house. The evidence was finally found while a local handyman was at the house tearing out some old cabinets in the basement to try to find a point of ingress from the outside for the rats. He had spent countless hours sealing up holes, etc. over many months. He took a break to go upstairs to the second floor of the house to the bathroom. As he walked into the room, a rat ran out past him. The bathroom floor had wet rat tracks, as did the rim of the toilet. The rat was soaking wet. Later that day, with the old cabinets in the basement all torn out from the wall, he found an abandoned ssewer clean-out with the cap gone. The feces around that spot showed the rats had been using the clean-out as a way to get into the sewer line, then they crawled up the line for two stories, through the water-filled trap in the toilet, and into the room. He plugged the clean-out, and that was apparently the last way they were getting inside the house living quarters. Earlier that day, he'd found the final ingress point which was a hole to the outside for the basement sump pump hose to run. The space around the pipe was about a quarter inch in size and the insulation around it had been chewed and pushed away. He sealed that, and for the first time in years, no rats inside [yet] or for the past five months.
All this meaning that it is extremely difficult to stop the rats coming into a house, even more insane for a garage or old, leaky basement. We had put out poison and traps, checking them a couple times a day for over a year. Problem with poison is that the rats would eat it, then run away to their places of hiding, such as inside walls, die, and begin to smell. That is another bad story, but for now we're getting a bit of a break [hope, hope].
Good luck.
