Rodent Control in San Francisco: Why Exclusion Does the Work

What to look for

Four rodents appear in California's species list: the roof rat, the Norway rat, the house mouse and the deer mouse. The two rats split a building between them. Roof rats are climbers. They arrive along fences, wires, tree limbs and downspouts, then enter at eave gaps, vent screens and roof plane junctions, and they nest above you in attics and soffits. Norway rats stay low, digging burrows against foundations, under stairs and beneath decks, and entering through the sort of ground-level gap you never look at. San Francisco's climate keeps both busy year-round. Summer highs average 67F and winter lows sit near 46F, so there is no freeze to knock populations back, and 75 percent average humidity keeps crawl spaces damp enough that water is never the limiting factor. Rats stay active all year in California, so checking periodically, rather than reacting once a season, is what keeps a building clear.

• 1Running or thumping in a ceiling after dark, moving along the same line each night.
• 2Half-inch droppings with blunt ends on a beam, a garage shelf or beside a water heater.
• 3Small pointed droppings clustered under a sink or behind a stove, which means mice rather than rats.
• 4A dark polished smear where an animal squeezes past a pipe, joist or foundation ledge.
• 5Gnawed vent screening, a chewed soffit corner, or a torn seal at the base of a garage door.
• 6Fresh burrow openings in soil against the foundation or under a deck.
• 7Fruit stripped from a backyard tree overnight, which points to roof rats rather than birds.

Treatment options and typical costs

Prices are typical published ranges. Ask what’s included before you compare quotes.

How California's rodenticide rules changed the job in San Francisco.

If a provider quotes you a monthly station service and nothing else, ask what the plan does to the building itself, because in this state that is where the result comes from. For years the standard first move on a rat call was a ring of bait stations. California's prohibition on most first and second generation anticoagulants, described by UC IPM, took that off the table for most situations. Brodifacoum, bromadiolone, difethialone, diphacinone, chlorophacinone and warfarin cannot be applied in most areas, and the remaining options, bromethalin, cholecalciferol and zinc phosphide, are not a drop-in replacement. The practical result is that a competent San Francisco plan looks like construction work with traps attached. Someone walks the roofline and the foundation, lists the openings, and closes them with metal, then traps out what is stuck inside and comes back to confirm the count fell to zero. That is slower and more physical than a bait route, and it is better. Bait never fixed the hole; it just intercepted animals on their way through. It also carried real risk to dogs, cats and the hawks and owls that catch a poisoned rat.

Roof rats overhead, Norway rats underneath.

A plan that only addresses one height leaves the other route open. The single most useful thing you can work out early is which animal you have, because the two rats in San Francisco use opposite halves of the building. Roof rats climb. Fences, cables, downspouts, ivy on a wall and any branch within a few feet of the eave are all ladders, and once at the top a chewed vent screen, a soffit gap or a junction between roof planes is enough. They nest above ceilings, which is why the first clue is often running noise overhead after dark. They also raid fruit trees, so stripped fruit in a back garden is a decent early warning. Norway rats are heavier and stay low. They dig burrows in soft ground against a foundation, under a deck or beside a garage slab, and they come in at grade, often under a door with more than a half-inch clearance. In a wet winter with 24 inches of rain arriving over a few months, saturated ground pushes them toward drier crawl spaces and basement garages.

Shared walls, flats and who is responsible.

Animals travel between flats through plumbing chases, shared basements, garage ceilings and the light wells that serve several units, so trapping in one apartment often just moves the traffic next door. If you rent, your best tool is documentation. Photograph droppings and gnaw damage with dates, put your request in writing, and ask specifically for exclusion at the building envelope rather than another round of traps inside your unit. Note where you hear activity and at what time, since that helps a crew find the runway. If you own or manage the building, the sequencing matters. Seal the shared envelope first, then trap unit by unit, then check the trash and recycling area, which may be an alley or a garage bay serving several buildings at once. With median home values around 1.3 million dollars, the sealing work is small relative to what chewed wiring and ruined insulation cost to put right across multiple units. Wherever San Francisco housing is stacked or attached, a rodent problem is rarely confined to the unit that reports it.

What to expect on the first inspection.

Then upward, which is the half people skip: eave and soffit gaps, attic and gable vents, dryer and bath exhaust terminations, roof plane junctions, and any tree limb, cable or vine offering a route to the top of the building. Inside, the attic and crawl space carry the evidence. Droppings, rub marks, flattened insulation and urine staining show where animals travel and roughly how long they have been there. You should end up with a list of specific openings and the material proposed for each, a trap plan with locations and a follow-up schedule, and a clear line between what is included and what is extra. A one-page price with no punch list is not an inspection result. A useful first visit spends most of its time on ladders and in tight spaces, not standing in your kitchen. Outside, the circuit should cover the foundation, hose bibs, gas and electric penetrations, garage and service door clearances, basement window frames, and any point where a deck, stair or addition meets the original wall.

A twelve-month rhythm instead of a season.

Most reinfestations begin at an opening that was fine last time you looked. Cities with hard winters get an autumn spike driven by cold. San Francisco's coastal pattern is year-round activity with a milder fall peak, and planning as though cold drives it is how buildings stay infested. With summer highs around 67F and winter lows near 46F, nothing forces a population crash, so the useful calendar is about weather rather than temperature. The rain window is the pressure point. As storms saturate the ground, burrowing rats look for drier shelter, and that is when garages, crawl spaces and basement storage start producing droppings. Doing your sealing before the first significant rain is worth more than doing it in January with water running past the foundation. The drier months are the working window. Metal work goes in more easily, roofs are safer to walk, and yard access is better for cutting limbs and pulling ivy off walls. Throughout, the maintenance habit that suits California's year-round rats is periodic checking. Twice a year, walk the perimeter and look up at the eave line, then look down at door clearances.

Simple ways to keep rodents away

• Fit door sweeps that close the gap completely, since a rat gets under a half-inch clearance.
• Screen attic, foundation and dryer vents with quarter-inch hardware cloth, not plastic louvers.
• Prune branches well back from the roof so roof rats lose their bridge.
• Strip ivy and heavy vines off exterior walls rather than trimming them.
• Pick backyard fruit as it ripens and clear windfall, which feeds rats through a mild winter.
• Keep compost and trash in bins with latching lids, especially where a shared alley serves several buildings.
• Dry out the light well or crawl space; 75 percent humidity already keeps those spaces damp.
• Walk the building twice a year rather than once, since the season never fully resets here.

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