Marine damp is what makes the wood chewable
Elsewhere in California the story is that dry air keeps trim hard, so squirrels have to find a built gap. San Francisco inverts that. Average humidity near 75 percent, about 24 inches of rain a year, and long overcast stretches keep painted exterior woodwork damp for months at a time. What that does to old trim is gradual. Water sits in joints, paint film lifts at the edges, and the wood beneath goes punky. a squirrel testing the building finds an edge that gives, and a few sessions of gnawing turn a soft joint into an entry. This is why local entries so often appear in details rather than at obvious roof edges. The repair has to answer both problems. Replacing the soft wood and backing it with metal handles the animal; getting the drainage right handles the water that made it soft in the first place. A closure that excludes squirrels but traps moisture behind it simply moves the failure a few feet along the same piece of trim.
Bay windows, brackets, and light wells
The features that make these buildings worth looking at are also the features that make them easy to enter. Bay windows carry their own small roofs, with edges and returns that meet the wall at junctions rarely inspected. Decorative brackets and cornices are often hollow. Light wells create interior facing walls with framing, vents, and pipe penetrations that nobody sees from the street. Rear additions add another category. Where a later structure ties into the original roof at a different height, the flashing at that junction is frequently the oldest and least maintained part of the building. Because none of this is visible from the sidewalk, finding the entry usually means getting to roof level and working around the whole perimeter, including the light well faces. If a quote was produced from a ground level walk around, treat the opening count in it as a starting guess rather than a finding, and ask what parts of the building were actually reached.
The California species question in plain terms
Tree squirrels are game mammals in California, which puts homeowners in an unfamiliar position: what you may do depends on which animal you have. UC IPM sets it out clearly. Eastern fox squirrels damaging property may be controlled at any time. Gray squirrels are not covered by that, and controlling them requires a depredation permit from Fish and Wildlife. That makes identification the first task, not a detail to sort out later. A company that proposes trapping without saying which species it found has skipped the step that makes its own plan lawful. For most San Francisco buildings there is a simpler route. A one way door removes an animal from a structure without capturing it, so the permit distinction never arises, and it puts the emphasis where it belongs: on closing the opening properly. Given how much of the cost here sits in carpentry rather than trapping, that is usually the better shape for the job anyway.
A calendar with no winter
Guidance written for colder cities revolves around a cold season that drives animals indoors and a warm season that lets them out. Neither applies here. Summer highs near 67F and winter lows around 46F produce a year with no sharp edges, so squirrels using a roof space simply keep using it. What structures the year instead is breeding. UC IPM places California tree squirrel breeding in late winter or spring, with one or two litters a year of three to five young. Through that period an occupied space may contain animals that cannot leave on their own, and sealing has to wait for an inspection. That leaves late summer and autumn as the natural window for permanent repairs. Trim is at its driest, the litters are past, and the work can be done before the rains start softening joints again. If you are planning exterior painting or trim repair anyway, that is the moment to fold the exclusion work into it rather than treating them as separate projects.
Flats, neighbors, and who organizes the work
A great deal of San Francisco housing is small buildings divided into flats, and that changes the logistics more than the biology. The animals travel framing and roof structure that belongs to the building rather than to any one unit, so the noise a person hears in their bedroom often relates to an opening two floors up and around a corner. For tenants, the useful contribution is a written record: which room, which date, what hour, and whether it sounds like one heavy animal or several light ones. Roof and exterior repair is the owner's responsibility, and that detail speeds up the inspection. For owners in a small association, the thing to avoid is sequential patching. If each owner deals only with the opening nearest their unit, the building pays several times for what one coordinated pass would have solved. Get one inspection covering the whole exterior, agree the scope together, and have the work done in a single visit while access equipment is already in place.
Reading a quote for an old building
Start with what the quote says about the exterior. A document that names specific locations, a bay window return, a light well vent, a bracket cavity, tells you somebody looked. One that says all entry points will be sealed is not something you can hold anyone to. Then read the materials. Quarter inch hardware cloth fitted from behind, metal flashing, and replacement wood backed with mesh survive both teeth and weather. Sealant and foam do neither, and in this climate they fail quickly at a joint that stays damp. Third, ask how the repair handles water, because a trim joint that excludes a squirrel and holds moisture will rot out again. Fourth, ask about the species and what follows from it under California rules. Fifth, confirm whether the return visit to remove the one way door and finish the closure is included. Last, ask about cleanup and wiring. In older buildings with old electrical runs, that check is worth more than the price difference between two bids.