Rodent Control in San Francisco, CA

Local Context

One Opening on the Block, Then Every Building on It

Roof rats (Rattus rattus) climb, and San Francisco hands them every route. Bay window brackets, downspouts, ivy on a retaining wall, a fire escape, a street tree touching a cornice — any of it puts an animal on a roof, where a half-inch gap at a rotted rafter tail or a corroded attic vent screen is enough. In attached flats the rat then moves sideways through the framing cavity along the party wall. That is why the noise is over your bedroom and the hole is on someone else's building.

Norway rats (Rattus norvegicus) stay low and burrow. The city's culverted creeks and shoreline give them continuous ground: the Islais Creek channel through the Bayview, Mission Creek at Mission Bay, the India Basin shoreline, and the storm-drain network feeding all of it. From there they move into garages and sub-areas at street level. On the Marina-style and Sunset houses with living space over a garage, they come in at the door sweep, the sewer lateral penetration or a broken foundation vent.

Age is the third factor. With a median build year of 1942 and thousands of buildings older than that, this city is full of galvanized vent screens that have rusted through, garage door sweeps flattened by decades of use, and wood-framed sub-area doors that no longer sit in their frames. House mice (Mus musculus) use all three at a quarter of the size. Trapping without closing those openings only clears space for the next animal out of the same population.