Ahead of the Rain: Why Wet Weather Sends Rodents Looking for Higher, Drier Ground

For most of the year, a rat living outdoors in Southern California has it pretty good. There is ivy on the back wall, fruit on the neighbor's tree, a burrow under the shed, and a dry season that lasts the better part of eight months. Then the rain comes. Burrows fill. Ivy stays soaked for days. And somewhere above your ceiling is a dry, insulated, perfectly sheltered attic that has been there the whole time, with a gap at the eave just wide enough to fit through.

The rodents around Southern California homes split roughly into two groups by where they nest, and rain affects them differently.

Norway rats are burrowers. They dig along foundations, under slabs and sheds, beneath woodpiles and into garden beds and slopes, and they line the burrow with whatever soft material is handy. House mice nest close to the ground too, in dense ground cover, under debris and in the back corners of garages. Rain is a direct threat to both. A burrow is a hole in the soil, and when the soil saturates, the hole fills. The animals that live there move up and out, and the nearest dry, enclosed space is often a garage, a crawl space, or a void beneath a porch or patio.

Roof rats, the dominant rat in many Los Angeles neighborhoods, are climbers. They nest above ground in palm skirts, ivy-covered walls and fences, bougainvillea, dense trees and, given the chance, attics. Rain doesn't flood them out the way it floods a burrow, but it soaks nests that stay wet for days in dense foliage, and cold, wet nights make a dry, insulated attic very appealing. Roof rats already travel along fences, utility lines, branches and rooflines, so the move from the ivy to the attic is a short one.

From a rodent's point of view, the inside of a house during a storm is close to ideal. Attics are dry, warm and quiet, and the insulation makes excellent nesting material. Wall voids are protected highways between the attic, the kitchen and the subarea. Garages usually have a gap or two at ground level, plenty of clutter to hide in, and often a bag of pet food within reach. Raised-foundation homes add a crawl space, which stays dry under the house when everything outside is soaked, and a single damaged vent screen is all it takes to get in.

Fall already pushes rodents toward houses. As nights cool, outdoor food gets scarcer and shelter matters more, which is the pattern our fall rodent season article covers. Rain stacks a second reason on top of the first.

This winter is forecast to bring a super El Niño, and with it a better-than-usual chance of a wet season. If the forecast holds, the shift indoors that normally builds gradually through fall and winter could show up earlier, or more noticeably, than it does in a dry year. That's a general expectation, not a prediction for your street, and the rain hasn't started yet, which is exactly why now is the time to act: before the first storm, not after the scratching starts.

Rodent-proofing is mostly a slow walk around the house with a flashlight and a critical eye. Rodent-proofing guidance commonly uses a dime as the benchmark gap for a mouse and a quarter for a young rat, so think small.

The garage is the front door for a lot of rodent problems, and it rarely gets the attention the attic does. The corners of the roll-up door seal wear out first, often leaving a gap you can see daylight through. Side doors without a sweep, the gap where the slab meets the stem wall, and the holes where water lines, gas lines and the water heater vent pass through all add up. Inside, garages tend to offer everything a rodent wants: clutter to hide in, cardboard to shred, a bag of pet food or grass seed on the floor, and very little foot traffic after dark.

Plenty of rodents live in a garage for a long time without ever coming into the house. The trouble starts when the garage gets crowded, or when they find the gap around a pipe behind the washer, the space under the door to the kitchen, or the opening where the water heater closet meets the wall. Seal the garage and you close one of the most common routes into the living space.

Many older homes on the Westside and in parts of the South Bay sit on raised foundations, with a crawl space underneath ventilated by screened openings around the perimeter. Those screens have often been there for decades. They rust, get pushed in by a garden hose or a curious dog, or get pulled loose when someone runs a new cable. In a wet winter, the crawl space is the dry room directly beneath the house, and a single torn screen or a loose access door is all a rat needs. Walk the perimeter and look at every vent; it is one of the most useful ten-minute checks on an older house.

This is a common mistake, and an understandable one. Someone hears scratching, goes up the ladder, finds the gap, and seals it that same afternoon. If the rat was outside at the time, that works. If it was inside, it is now trapped in the attic or the wall.

A trapped rodent doesn't give up. It chews a new way out, and that new way is often into the living space rather than back outside. Or it dies in a wall void or ceiling cavity, out of reach, and the odor and the flies that follow can last for weeks.

So before you seal anything, check for signs that rodents are already inside: droppings in the attic, the garage or under sinks, gnawed wood or wiring, greasy rub marks along beams and pipes, shredded insulation, or scratching and running in the ceiling after dark. Our droppings identification guide will help you tell rats from mice. If you find signs, deal with what is inside first.

If any of that sounds familiar, our article on rats in the attic goes through the signs in more detail, and why the attic is usually where the problem starts.

If you do find droppings or nesting material, don't sweep or vacuum them up dry. Public health agencies advise against it, because it stirs dust into the air. The usual guidance is to ventilate the space, wear gloves, wet the droppings and nesting material thoroughly with a disinfectant first, then wipe them up and bag them. Your county public health department has specific instructions, and for a large or heavily soiled area it is worth getting help rather than doing it yourself.

Rodent work often takes more than one visit, since trapping only works if the traps are checked and adjusted along the way. Our rodent control page explains what to expect in more detail.