A mouse can fit through a hole about a quarter inch across, roughly the width of a pencil, according to the CDC. Extension wildlife guidance says rats can get through openings larger than about half an inch. Those are small gaps, and most houses have more of them than their owners think. Finding and closing them with chew-proof material is the long-term fix for rodents.
Here is where those gaps hide in El Paso homes and which materials hold up.
Why such small gaps work
Rodents also gnaw. A gap slightly too small can be chewed wider if the edge is wood, plastic, foam or soft sealant. That is why material choice matters as much as finding the gap.
Where the gaps are in El Paso homes
The CDC lists places to check inside and out. These are the ones that come up most here.
- Inside, under and behind kitchen cabinets, and around pipes leading to the water heater and furnace
- Floor air vents and dryer vents
- The roof: rafters, gables, eaves and soffits, plus the openings around a rooftop evaporative cooler and its lines
- Attic vents
- The line where the foundation meets the ground, and stucco cracks at the slab
- Garage door bottoms and sides, and exterior doors. Extension guidance says door gaps should be no more than a quarter inch.
A good test: if you can slide a pencil into it, a mouse can probably get through it.
Steel wool, copper mesh or hardware cloth?
Each material has a job.
- Steel wool: the CDC recommends it for small holes, held in place with caulk or spray foam. Extension guidance notes it rusts and can stain, so it is better indoors or in dry, hidden spots.
- Copper mesh: works like steel wool but does not rust the same way, which makes it a common choice for outdoor gaps. Pack it tight and lock it in with sealant or patching compound.
- Hardware cloth: for vents and larger openings. Extension guidance specifies 24-gauge with quarter-inch mesh for mice and 19-gauge with half-inch mesh for rats.
- Sheet metal: 24-gauge or heavier for wall or pipe barriers, and 22-gauge or heavier for kick plates and door edging, per extension guidance.
- Cement or mortar: for gaps in block, stucco and at the slab. Extension guidance gives minimum thicknesses for concrete barriers.
Foam and caulk alone are not enough. Rodents chew through them. Use them to hold mesh in place, not as the barrier.
How to seal a pipe gap
Most homeowners can handle the gaps under sinks.
- Clean out any droppings first, using the CDC wet method.
- Pack the gap tightly with steel wool or copper mesh, pushing it in until it is firm.
- Cover it with caulk or a thin layer of foam to hold it in place.
- For bigger openings, cut a piece of hardware cloth or a metal plate to cover the hole and fasten it.
- Check the spot after a few days for new chewing.
A one-hour gap hunt
You can find most gaps yourself with a flashlight, a pencil and an hour. Work from the inside out.
- Empty the cabinet under each sink and look where pipes enter the wall.
- Pull the stove and fridge out if you can, and look at the wall and floor behind them.
- Check the water heater closet and the laundry area around pipes and the dryer vent.
- In the garage, close the door and look for daylight at the bottom and sides.
- Outside, walk the house at ground level and look at every pipe, wire and vent where it enters the wall, plus cracks at the slab.
- From a ladder or the ground, look at roof edges, vents and the cooler opening.
Mark every gap you find with tape and a number. That list is what a technician needs, and it makes a rodent inspection faster.
Check your work
After sealing, look at each spot again a few days later. Fresh gnaw marks, pushed-out mesh or new droppings nearby mean a rodent is still trying that route, and the repair needs heavier material.
Gaps you should not seal yourself
Roof work, cooler openings and anything near gas lines or electrical boxes are better left to a professional. Sealing rodents inside a wall without trapping first can also leave them stuck and dying there. A rodent exclusion job handles the order: trap, then seal, then follow up. If you are dealing with scratching in the walls, see our mice in the walls article.
Frequently asked questions
Can a mouse fit through a hole the size of a dime?
A dime is wider than the quarter-inch gap the CDC gives for mice, so yes.
Is steel wool or copper mesh better?
Both work for small gaps. Copper resists rust better outdoors. Lock either one in with sealant.
Will expanding foam keep mice out?
Not alone. Mice chew through foam. Use it to hold steel wool or mesh in place.
What size hardware cloth keeps out mice?
Extension guidance specifies 24-gauge hardware cloth with quarter-inch mesh for mice.
Can rats chew through hardware cloth?
Heavier hardware cloth holds up well. Extension guidance specifies 19-gauge with half-inch mesh for rats.
Sources
- CDC: How to seal up to prevent rodents
- Wildlife Damage Management, eXtension: Rodent exclusion methods
- CDC: How to clean up after rodents