Rodent articles · Updated
How small a hole can a mouse or rat get through?
If a pencil fits in the gap, assume a young mouse can get through it. Rats need more room than mice, but a gap that looks far too small for the rat you saw can still be enough.
Smaller than you'd guess. A good rule is that if a pencil fits in a gap, a young mouse can get through it. Rats need a bit more room, but not much, and they can gnaw a tight gap a little wider.
That's why sealing a house takes a careful eye. The gaps rodents use are often ones you'd walk past without noticing.
Why rodents fit through such small gaps
A rodent's body is flexible. The skull is the widest part it can't squeeze, so if the head fits, the rest usually follows. Rodents also gnaw. A gap that's a little too small today can be chewed a little bigger tonight, especially in wood, plastic, foam or rubber.
Low gaps on a San Antonio house
Most houses here have brick or stone veneer, and the bottom row has weep holes, small open joints that let water drain from behind the brick. They're about the right size for a mouse. Don't fill them with mortar, because they need to drain. Fit them with weep hole screens or inserts instead.
- Weep holes along the bottom course of brick
- Garage door corners and a worn bottom seal
- Gaps under side and back doors
- Holes where the AC line set, condensate line and cable enter the wall
- Dryer vent hoods with a stuck flap
- Crawl space vents and skirting on pier-and-beam houses
High gaps where roof rats get in
Roof rats look for openings up high. The common ones in San Antonio are the joint where the roof deck meets the fascia, soffit corners and returns, gable vents with torn screen, and gaps around plumbing vent boots. On two-story houses, the seams where a lower roof tucks under an upper wall are a frequent problem. You'll rarely see these from the yard. A ladder and a flashlight, or a technician, find them.
Want a local technician to take a look? Same-day appointments are often available.
Call (210) 955-2948
What the city code says about openings
San Antonio's property maintenance code touches on this too. Section 304.5 says foundation walls must be kept free from open cracks and breaks and maintained so as to prevent the entry of rodents and other pests. Section 304.17 requires openable basement windows to have rodent shields or other approved protection, though few San Antonio homes have basements. Our tenant and landlord guide explains who's responsible for repairs in a rental.
A quick self-check around the house
Walk the outside of the house once with a flashlight and look low, then look high. You're hunting for small openings, not big holes:
- Bottom row of brick: count open weep holes
- Garage door: look for daylight at the corners and along the bottom seal
- Where pipes, wires and the AC line set enter the wall
- Dryer vent flap: does it close on its own?
- Crawl space skirting and vents on older homes
- From the yard, the roofline: soffit corners, gable vents and where roofs meet walls
Gaps that open up over time
A house that was tight when it was built doesn't stay that way. San Antonio's clay soil shrinks in drought and swells after rain, and that movement can open gaps around slabs, door frames and pipes. Summer heat cracks rubber door seals and sealant. Hail damage leads to roof replacements, and a rushed crew can leave openings at the drip edge and vents. Plumbers, cable installers and HVAC techs drill new holes and don't always seal them.
Recheck the outside of the house once a year, and after any roof, plumbing or HVAC work.
Why young rodents get in first
Young mice and rats are smaller than adults and wander more as they look for a place of their own. They're often the first ones to find a new gap, and once they're inside, adults follow the scent trail. That's one reason a gap that seems too small for the rats you've seen can still be the way in.
How to close small gaps
Use materials rodents can't chew. Copper or stainless mesh packed into a hole, then sealed with exterior sealant, works for pipe and line penetrations. Galvanized hardware cloth covers vents and larger openings. Sheet metal flashing closes roof joints. Foam alone doesn't hold, because rodents chew through it in a night. Read steel wool vs copper mesh for why copper is the better pick in our humidity.
Seal while trapping is going on, not before, so you don't lock rodents inside a wall or attic where they'll chew a new way out. The last opening gets closed once the traps stay empty. See rodent exclusion for how a provider handles it.
Ready to have the gaps found and sealed? Call and describe what you're seeing.
Call (210) 955-2948
Questions from callers
Can a mouse fit under my door?
If there's daylight under it, probably. A door sweep closes the gap, and a garage door needs its own bottom seal.
Are weep holes really a problem?
They're one of the most common ways mice get into San Antonio walls. Screens or inserts keep mice out and still let water drain.
How do I find gaps I can't see?
Look for droppings, rub marks and gnawing nearby. From inside a dark garage or attic, daylight showing through is a giveaway.
Can rats chew through concrete?
They can gnaw at weak, cracked or crumbling concrete and mortar, which is why cracks along foundations should be repaired, not just filled with foam.