Rats in the walls, mice in the garage: find the way in and shut it

It usually starts at night. Something runs across the ceiling over the bed, or the drawer under the kitchen sink has droppings in it by morning. In a San Francisco house the way in is rarely a mystery once you look: daylight under a garage door at sidewalk level, a rusted screen on a foundation vent, a hole a cable installer left in the stucco, a lemon tree resting on the eaves. This guide walks the house the way a good rodent-proofing job starts, from the outside in.

Think of it as building repair rather than pest work. UC's IPM program calls exclusion the most successful and longest-lasting way to control rats around buildings, and the most permanent control for house mice. Traps remove the animals already inside. Only a closed gap keeps out the next ones.

#Roof rats, Norway rats or mice: who is in the house

San Francisco's Department of the Environment counts three rodents that matter here: the Norway rat, the roof rat and the house mouse (its IPM sheet). UC IPM describes roof rats as climbers that favor attics, wall voids, false ceilings and cabinets, and that prefer warm, coastal places. Norway rats burrow along foundations and, indoors, tend to stay in basements and on the ground floor. When both share a building, Norway rats take the lower floors and roof rats the upper ones.

Put that on the houses here. A Sunset tract house is one story over an integrated garage on a 25-foot lot, and much of the Richmond is tract housing with built-in garages too (SF Planning). Its ground floor is the garage: Norway rat territory. Victorians often stand on raised basements (SF Planning, Bulletin 18), so start there.

#How do mice and rats get in? Through gaps this small

You will read that a mouse needs a dime-sized hole and a rat a quarter-sized one. The official figures are smaller, and they are the ones to work to. UC IPM says house mice can squeeze under a gap 1/4 inch tall and through an opening 3/8 inch wide; the CDC puts it as a hole the width of a pencil. A rat, young ones especially, gets under a door with a 1/2-inch gap, and on a wood door it may gnaw the gap bigger.

#Walk the house from the sidewalk to the roof

Go out once in daylight and again at dusk with a flashlight, when rats travel fences and wires. Circle the house low first, then look up. On an attached row, check where your house meets the next one and tell the neighbors what you find. UC IPM notes that a roof rat may live in one yard and eat in another.

#Inside: wherever a pipe comes through

Builders cut pipe holes a little bigger than the pipe, and mice don't roam far: UC IPM says house mice seldom go more than 30 feet from their nest and food, so droppings in one cabinet mean the gap is close. The CDC's list covers every indoor spot; these matter most in an older house.

#What to close gaps with, and what they chew through

Most do-it-yourself rodent-proofing fails right here. The patch has to be harder than a rodent's teeth, and much of the sealant aisle at a hardware store is not.

• Galvanized hardware cloth, 1/4-inch openings. For vents and larger holes. The city's 2026 owner guidelines name 1/4-inch hardware cloth for openings in outside walls. UC IPM's vent spec uses 1/2-inch mesh, which stops rats but not a mouse that fits a 3/8-inch slot.
• Sheet metal. For gnawed door bottoms and corners, and to cover large holes; UC IPM suggests it on door and window edges that get chewed.
• Concrete or mortar. For holes in foundations and around pipes through masonry, both on the city's list.
• Copper mesh or steel wool, held with sealant. For small holes around pipes, the way the CDC does it. UC IPM calls steel wool a temporary plug that may rust; copper mesh, also on the city's list, won't.
• Not expanding foam. The city's 2026 guidelines say expandable foam sealer is not acceptable rodent proofing.
• Not chicken wire. UC IPM says never to use it; rats pass through the openings.
• Not weatherstripping alone. A foam or vinyl draft seal stops air, not teeth, and UC IPM warns that typical weatherproofing may not keep rodents out. A rodent-proof door bottom has metal in it.

A rodent-proof house has no opening bigger than a pencil, and every patch is harder than a rat's teeth.

#The weekend list, easiest first

#Seal, trap, clean: the order matters

If rodents are already inside, sealing and removal happen together. Close every gap but one and you herd them to it; close them all while a roof rat nests in the attic and it may chew a new way out. UC IPM notes that roof rats are strongly tied to their nest sites and come back if the nests stay. That timing is the part worth paying a pest professional for.

Leave trapping and baiting to that professional. San Francisco's own advice is blunt: snap traps can work if used right, and do not use poisons, because poisoned rodents die where you can't reach them. The city asks building owners to hire a company that practices Integrated Pest Management and holds a current state license; the Structural Pest Control Board confirms licenses at (916) 561-8704.

#San Francisco rules: owners, tenants and 311

Tenants carry part of it. The city expects them to keep units reasonably clean, store garbage and pet food properly and allow inspection and treatment (sf.gov). The health department and the Rent Board recommend reporting an infestation to the landlord in writing and keeping a copy (DPH rules). The city's 2026 owner guidelines give owners 48 hours to investigate a report; its vermin web page still says 72.