Rodents are resourceful about finding a way in, and most houses offer openings nobody has ever noticed. Understanding how mice and rats actually get inside is the part that ends the problem, because trapping removes the animals present and changes nothing about the route.
That is the difference between a quiet month and a lasting fix, and it is what keeps a rodent control program from starting over every fall.
They need less space than you think
A mouse gets through about a quarter inch, roughly the width of a pencil. A rat needs about half an inch.
The reason is anatomy. The skull is the only rigid part of a rodent’s body, so once the head is through, the rest compresses and follows. Gaps that look far too small to matter are ordinary routes.
This is why walking a foundation looking for holes misses most entry points. The openings to look for are seams, gaps and worn edges rather than anything that reads as a hole.
If the skull fits, the body follows. For a mouse that means about a quarter inch.
The common entry points
Utility penetrations
Everywhere a pipe, wire or duct passes through the building envelope there is a potential gap: gas service, water line, electrical conduit, cable and internet, dryer vent, air conditioning line set. Builders leave working clearance around these, and whatever sealed them originally hardens and shrinks over time. In older Pittsburgh houses this is the single most productive place to look.
Foundation gaps and mortar loss
Concrete cracks as it settles, and stone or block foundations lose mortar steadily over decades. Both are common in the city neighborhoods and the older boroughs. A missing half inch of mortar between stones is a doorway.
Vents and screens
Attic, foundation and crawl space vents need intact covering. Standard window screen is not enough, since rodents chew through it. Hardware cloth is. Dryer vent flaps that no longer close are among the most common routes in, and they sit at exactly the height mice prefer.
Doors and garage doors
Weather stripping flattens, door sweeps wear through, and thresholds settle. The garage door bottom seal is the usual offender, because the concrete under it is rarely perfectly level and the rubber compresses over the years.
The pattern is predictable: rodents enter the garage first, feed on birdseed, grass seed or pet food stored there, then work through the interior door into the house. Sealing the house door without addressing the garage just relocates the problem.
A worn garage door seal is the first step, and the interior door is the second.
Roof line, soffit and chimney
Where the roof meets the walls there are usually small gaps, more so on older houses. Damaged soffit panels, lifted shingles and failed chimney flashing all give access, and mice climb well. A branch touching the roof or a vine running up a wall is a ladder.
Below grade
Basement windows and areaway doors that no longer seal, bulkhead doors, gaps around sump discharge lines and old coal chutes. Norway rats in particular work at ground level and below, so this is where rat entry concentrates.
Sewer and drain lines
Less common and worth knowing about. Norway rats travel sewer lines and can enter through a broken lateral, a dry floor drain or an unsealed cleanout. If rats are appearing in a basement with no sign of exterior entry, this is worth checking.
How to seal it properly
The material matters more than the effort, because rodents chew.
- Small gaps: copper mesh or steel wool packed in, then sealed over with caulk or expanding foam. Foam alone is chewed through in days
- Larger openings: hardware cloth, quarter inch mesh, cut oversize and fastened down
- Vents: hardware cloth behind the louver, not window screen
- Dryer vents: a proper flapped cover that actually closes
- Doors: new sweeps, and a rodent resistant threshold where the gap is persistent
- Sheet metal over chewed timber edges, particularly door bottoms and sill plates
Material matters. Anything a rodent can chew through is not a seal.
Finding the routes that are actually in use
A house has many possible openings and usually only a few in active use. Three things separate them.
Rub marks. Rodents travel the same routes repeatedly and leave a greasy dark smear from their fur along edges, pipe runs and the sides of openings. A rub mark at a gap means that gap is being used.
Droppings. They accumulate along runs rather than randomly, so a concentration near an opening is evidence rather than coincidence.
Gnaw marks. Fresh gnawing on wood is pale and sharp edged. Older damage is dark and weathered. The difference tells you whether an opening was enlarged last week or years ago.
A simple test for an uncertain gap: pack it loosely with steel wool or stuff it with paper and check in two days. Disturbed means active.
Grease marks and droppings show which openings are live rather than theoretical.
Order of operations
This is the part most people get backwards. Sealing a house that already has rodents inside traps them in, which means noise, smell and damage in wall cavities as they try to get out.
- Work out what is there, mice or rats, since the gap size differs
- Find the routes and note them without sealing yet
- Remove the animals inside
- Then seal, starting with the routes in active use
- Remove the attractants: stored seed and pet food, cardboard, clutter, water sources
Telling mice from rats covers the first step, and what attracts mice in winter covers the last.
What the outside of the house contributes
Sealing works better when the approach is less hospitable, because rodents reach the building before they enter it.
Vegetation touching the wall gives cover and a climbing route, so ivy, shrubs against the foundation and branches over the roof all matter. Firewood, lumber and stacked materials along a wall are harborage, and so is a compost pile or an unsecured trash can near a door.
Birdseed is the single most common exterior attractant in local yards, both at the feeder and in storage. Grass seed, pet food and garden produce left out do the same job.
Clearing a band of eighteen inches or so around the foundation, keeping it free of vegetation and stored material, makes the building edge visible and unattractive at the same time.
Timing
Exclusion work is most valuable in late summer and early fall, before the seasonal move indoors. Mice and rats live outdoors through the warm months and relocate when night temperatures drop, typically late September into November here. Sealing in August prevents a problem that sealing in December only contains.
Stewart Termite & Pest Control has handled Pittsburgh rodent problems since 1987, and every rodent job starts with an inspection that identifies the species, the activity and the likely entry points. Call 412-822-7610 or request an estimate online.
Frequently Asked Questions
How do mice get into a house?
Through gaps around utility penetrations, worn door and garage seals, unscreened or damaged vents, foundation cracks and mortar loss, and roof line gaps. They need about a quarter inch.
What size gap do rats need?
About half an inch. A house sealed against rats can still be open to mice, which need half that.
What should I use to seal rodent entry points?
Copper mesh or steel wool packed into small gaps and sealed over, and quarter inch hardware cloth for larger openings and vents. Expanding foam or window screen alone gets chewed through.
Should I seal the house first or remove the rodents first?
Remove them first. Sealing an occupied house traps rodents inside, which leads to noise, damage and odor in wall cavities.
Can rodents come up through drains?
Norway rats can travel sewer lines and enter through a broken lateral, a dry floor drain or an unsealed cleanout. It is worth checking when rats appear in a basement with no exterior entry evident.
When is the best time to seal a house?
Late summer and early fall, before rodents move indoors for the winter.