Whatever materials you use to seal out rodents, make sure the final patch is smooth. An uneven surface will trigger a rodent to gnaw. Rodents use their whiskers to feel gaps where their bodies might fit. Their whiskers let them know where to try to fit. From there they gnaw and dig to make the gap big enough. If a rat or mouse’s skull can fit through a hole or gap, the rest of the body can squeeze through—1/4" for young mice; 1/2" for rats.

Common points of entry include where pipes/wires go through walls and under doors.

• For the gap under doors, install door sweeps that touch the ground and go the entire length of the bottom of the door. If properly installed, the sweep will brush the ground, but not hold the door open (a fire hazard). Another test is to stand inside the door with the lights off and see if any light shines through gaps in the bottom. Pay attention to the corners.
• For utility penetrations, small gaps can be filled with some kind of foam, spackle, or caulking. Larger gaps will need reinforcement. Cover the hole with wood or a metal plate (escutcheon plates around pipes) or stuff the hole with wire mesh and cover that with a hardening compound. Expandable foam will work even though rodents can easily chew through it. If the repair is done well, the surface will be smooth so rodents won’t be triggered to start chewing. Use low expansion foam—it’s easier to be accurate and a lot less messy.

For more details, see the rodents page.