Rodent exclusion is the work of physically closing the openings rats and mice use to get into a building. In Los Angeles that usually means roof and attic vents, gaps at the eaves and roof edges, pipe and utility penetrations, crawl-space vents, foundation cracks, garage door edges and door thresholds. UC IPM's standard is to seal every hole larger than 1/4 inch with metal or concrete, because rats chew through plastic, foam, wood and caulk. Rodents already inside are trapped before or during the sealing.
How small an opening can rats and mice get through?
Smaller than most people expect. UC IPM's Pest Notes: Rats says all holes larger than 1/4 inch should be sealed to exclude both rats and mice, and that young rats can squeeze under a door with a 1/2-inch gap and may gnaw the wood to widen it. The house mouse can slip under a gap 1/4 inch tall and through an opening 3/8 inch wide.
That is why exclusion is measured in fractions of an inch rather than in obvious holes. A torn vent screen, a gap around an A/C line or a worn garage door seal is enough. The LA County rodent-borne disease recommendations (admin.publichealth.lacounty.gov) use the same 1/4-inch benchmark.
Which openings does rodent exclusion seal?
A full exclusion job covers the whole building envelope, from the roof down to the slab. These are the points a technician checks on every house:
- Roof and attic vents, gable vents and under-eave screens that are torn, rusted or pulled loose.
- Rooftop plumbing vents more than 2 inches across, which UC IPM recommends screening.
- Chimneys without a spark arrester or tight cap.
- Gaps where roof edges, fascia and soffits meet the walls.
- Holes around gas, water, electrical, cable and A/C lines where they pass through stucco or siding.
- Crawl-space vents, crawl-space access doors and cracks in the foundation.
- Garage door bottoms and side edges, exterior door thresholds, and the gap between double doors.
- Dryer and bath vent hoods with missing or broken flaps.
Older housing usually has more of these gaps. County planning profiles show how much of the San Gabriel Valley is decades old: in the combined East Pasadena–East San Gabriel area (planning.lacounty.gov), 83.6% of 9,517 housing units were built before 1979, and in South San Gabriel (planning.lacounty.gov) about two-thirds of homes predate 1979. Years of weather loosen vent screens, eave boards and door seals.
What materials keep rodents out for good?
Metal and concrete. UC IPM and LA County agree that openings should be closed with materials rodents cannot chew, and they name them specifically:
- Galvanized mesh or hardware cloth. For vent grills, UC IPM specifies 18- or 16-gauge galvanized mesh with 1/2-inch openings to stop rats. Where mice are the problem, the mesh openings need to be 1/4 inch or smaller.
- Sheet metal for larger gaps, gnawed edges and the bottoms of doors and window frames, where UC IPM says a metal covering may be needed to stop gnawing.
- Concrete or mortar for foundation cracks and utility openings in masonry.
- Door sweeps and threshold seals sized to close the gap completely. UC IPM notes that standard weatherproofing may not be enough.
- Coarse steel wool packed into small pipe gaps, but only under a permanent seal. UC IPM calls it a temporary plug that can rust.
What does not work: lightweight chicken wire, which rats pass through, and plastic screening, rubber, vinyl, wood, caulk or expanding foam used on their own. Foam can fill space behind a metal patch, but it is never the barrier.
How does the rodent exclusion process work?
PesPro's rodent exclusion service follows the same order on every property, because sealing in the wrong order can trap rodents inside:
- Inspection inside and out. The roofline, eaves, vents, foundation, utility lines and garage are checked, along with the attic and crawl space for droppings, runways and nests. Each opening is photographed and listed in a written scope.
- Reducing the rodents already inside. Traps go in first or at the same time as the sealing, so animals are not shut inside to die in a wall.
- Sealing with metal and concrete, starting where activity is heaviest and working around the whole building.
- Outside changes. Limbs that touch the roof, ivy on walls and fences, and stored lumber against the house are flagged so rats lose their routes and cover.
- Monitoring and cleanup. Traps stay in the attic and are checked afterward, and soiled insulation or nesting material can be removed as a separate sanitation job.
Should rodents be trapped before the house is sealed?
Yes, or at the same time. A rodent sealed inside will gnaw at the new patch or die in a wall or ceiling void, leaving odor and flies. LA County recommends snap traps indoors in most cases for that reason. UC IPM adds that traps can be left in an attic where rats have been a problem, which is a simple way to confirm the sealing worked. The trade-offs between trapping, baiting and sealing are laid out in rodent exclusion vs. baiting.
Can you seal a house against rodents yourself?
Small, ground-level gaps are a reasonable weekend job: a door sweep, hardware cloth over a crawl-space vent, mortar in a foundation crack. The harder part is finding every opening. Roof rats use the roofline, so the gaps that matter most are often at the eaves, vents and roof edges, which means ladder work and an attic check. Sealing most of the openings but missing one usually just moves the rats to that one.
Roof rats also have strong nest-site loyalty, according to UC IPM, and will return to old nests if they are left in place. Removing nesting material from the attic is part of finishing the job. If your home sits near canyon brush or mature trees, why rodents enter hillside homes covers the outside pressure that makes complete sealing more important.
Frequently asked questions
Is spray foam enough to seal rodent holes?
No. UC IPM lists expanding foam with plastic, rubber, vinyl and wood as materials mice can gnaw through, and notes rats chew caulk as well. Foam can fill space behind a metal patch, but the barrier itself should be galvanized mesh, sheet metal, or concrete and mortar for masonry.
How long does rodent exclusion take?
It depends on how many openings the inspection finds, the roof style, whether there is a crawl space and how easy the attic is to reach. A compact single-story house may be sealed in one visit, while a large or hillside home can take longer. You get the scope in writing before any sealing starts.
Is rodent exclusion the same as rodent control?
No. At PesPro, rodent control is the recurring program of inspection, trapping, bait stations and monitoring visits. Exclusion is the structural sealing, and it can be added to a control program or ordered on its own. Our LA County rodent control guide explains how to tell which rodent you have.
Will exclusion keep rodents out forever?
Metal and concrete seals hold up well, but houses change. Door sweeps wear, remodels cut new holes, and storms damage vent screens. Keeping limbs off the roof, thinning ivy and checking vents and door seals once a year keeps a sealed house sealed. If activity returns, the first step is finding the new opening.
If you want a clear answer about what is getting in and how to stop it, PesPro can inspect the property and put the plan in writing. Pricing is quoted after the inspection, and the quote is free. We are licensed in California (#PR-6378) and QualityPro certified, our technicians speak English and Spanish, and the office is in El Monte. Request your quote here or call (323) 472-5329.