The six openings that let almost every mouse in
A mouse skull is the only rigid part that has to fit; the body compresses behind it. That is why the working rule is a gap of about a quarter inch, the width of a pencil or a dime’s edge. Walk a Detroit house with that measurement in mind and the list of suspects is short. Utility entries lead it: the holes drilled for gas lines, water pipes, cable, and air-conditioning lines are almost always cut wider than the pipe, and the soft fill around them weathers out.
Foundation features come next. The sill plate, the wooden beam sitting on top of the foundation wall, shrinks and rots over decades and opens a running gap at exactly the level mice travel. Coal chutes, standard in pre-war Detroit houses, have steel frames that rust and warp until the door no longer seals. Add block and stone foundation joints with soft mortar, porch steps that hide a void against the wall, and garage doors worn at the corners, and you have the full list an inspection works through.
| Entry point | Why it opens up | What closes it |
|---|---|---|
| Utility holes for pipes, wires, AC lines | Holes cut oversized; soft fill weathers and falls out | Copper mesh or steel wool packed tight, capped with sealant or mortar |
| Sill plate on the foundation | Wood shrinks and rots; a running gap opens at mouse level | Sealed along its length; rotted sections repaired, not just caulked |
| Coal chute frame and door | Steel rusts and warps in pre-war Detroit stock | Frame resealed or the chute closed off with masonry |
| Block and stone foundation joints | Old mortar softens and cracks | Repointed with fresh mortar |
| Garage door corners | The seal wears and the slab settles unevenly | New door seal and threshold fitted to the slab |
| Vents and chimney cleanout doors | Screens break; cleanout doors rust through | Hardware cloth fitted; door replaced or sealed shut |
Can mice climb to reach the higher openings?
Yes. Mice are strong climbers on rough surfaces, and brick, stone, and rough-sawn wood all give them grip. A mouse can run up a brick wall to a gap at the eaves, climb the inside of a downspout, or walk a utility wire to the point where it enters the house. That is why an attic problem does not always mean a roof problem: the mouse may have entered at the foundation and climbed the inside of a wall void to the attic instead.
Rats climb less eagerly but the Norway rat is still capable on rough surfaces and pipes; its usual route in Detroit is lower, through drains and foundation gaps. For both animals the practical answer is the same: the check has to run from the footing to the roofline, because sealing only the holes you can reach from the ground leaves the climbing routes open. Our exclusion page describes how that full pass is sequenced.
Cannot find the way they are getting in? Call for an inspection and get every entry point mapped and priced before sealing starts.
Call (313) 476-6003Finding your own entry points before you seal anything
Do the check at dusk, when mice are moving, and work from outside in. Look for the supporting evidence first: droppings concentrated at one foundation point, rub marks (dark greasy smears) where bodies brush a wall on a regular run, and gnawing at the edges of soft material around pipes. Inside the basement, a helper with a torch on one side of the foundation wall while you watch the other side will show light through gaps you would never spot in daylight.
Seal in the right order. If mice are active inside now, closing every hole at once can trap them in the walls, so the usual sequence is to clear the inside population while the sealing is planned, then close the points. Habits help the seal last: keep mulch, firewood, and stored clutter off the foundation line so the runs mice use to reach the gaps are exposed. The rest of the prevention routine is on our rodent prevention page.
- Check utility entries first; they are the most common and the easiest to miss
- Run a pencil along the sill plate line; anywhere it slides in is a door
- Look for rub marks and droppings to find which gaps are actually in use
- Use the torch test inside the basement to find light gaps in the foundation
- Seal with chew-proof material; foam alone is a snack, not a seal
Questions Detroit Homeowners Ask
Can a mouse really fit through a hole the size of a dime?
Yes, roughly. The skull sets the limit, and a young mouse can pass a gap of about 6 to 7 mm, a pencil width. Adult mice need a little more. The dime comparison is a fair working rule for homeowners: if a pencil slides into a gap, a mouse is worth assuming until the gap is sealed.
Do mice come in through toilets or drains?
Mice rarely do; that route belongs to rats. Norway rats can come up through a broken sewer line or a dry floor drain trap, which is one reason basement rat problems in Detroit often trace back to the drains. Mice overwhelmingly use gaps in the shell at foundation and wall level.
I sealed everything and still hear mice. What did I miss?
Usually one of three things: a gap hidden behind a porch or deck where it meets the wall, the top plate and eaves on a climbing route, or mice that were already inside and are now sealed in. If the noise continues past a couple of weeks, an entry point is still open, and a fresh inspection pass with the house quiet will usually find it.
Does sealing work on its own, without traps or bait?
Sealing stops new mice coming in, which is the part no trap can do. It does nothing about the mice already living inside, and a sealed-in population will die in the walls or chew a new way out. The working order is to clear the inside and close the shell together, which is why exclusion visits combine both.
More From the Blog
Work through the other Detroit rodent guides on the blog index, or go straight to the service page that matches your problem from the services list.