How to Rat-Proof a House in Southern California: The Exclusion Checklist
Trapping without sealing just makes room for the next family. This is the roof-to-foundation checklist we walk on an exclusion job in the San Gabriel Valley.
Rats fit through a half-inch gap, mice through a quarter inch. Seal roof returns, vents, tile edges and utility penetrations with quarter-inch hardware cloth, sheet metal or copper mesh backed by mortar; fit door and garage seals; trim trees off the roof; pick up fruit and store pet food in metal. Seal all but one entry, trap inside, then close the last one. Foam and caulk alone get chewed through.
Local help: Roof rats in an Arcadia, Pasadena or Temple City attic? ExterMetro does the exclusion work with a one-year warranty, from our Arcadia base. Call or text (626) 409-1584 or get a free quote.
Rats do not need an open door. A roof rat fits through a gap the width of your thumb, a mouse through one the width of a pencil, and both can climb stucco, wire and citrus trees. Trapping without sealing just makes room for the next family. This is the checklist we walk on an exclusion job in the San Gabriel Valley, from the roof down.
The gap sizes that matter
| Rodent | Minimum gap | Everyday example |
|---|---|---|
| House mouse | About 1/4 inch | A gap you can slide a pencil into |
| Roof rat and Norway rat | About 1/2 inch | A gap you can fit your thumb into |
The roof line (roof rats live up here)
- Roof returns and the gaps where fascia meets the roof deck; birds and rats both use them.
- Attic and gable vents: screened with 1/4-inch hardware cloth, not window screen.
- Eave and soffit vents, especially where an addition meets the original roof.
- Tile roof edges: open ends of barrel or S-tile need bird-stop or mortar.
- Plumbing vent stacks and the flashing around them.
- Tree limbs, palm fronds and ivy touching the roof: trim them back several feet. Power and cable lines are highways you cannot trim, so the entry points at the end of them matter more.
The walls
- Gaps around AC line sets, gas lines, cable and electrical penetrations. Stuff with copper mesh, then seal with mortar or a sealant. Spray foam alone gets chewed through.
- Weep screed gaps at the base of stucco where the wall meets the foundation.
- Dryer, bathroom and kitchen exhaust vents with missing or broken flaps.
- Doors: sweeps on exterior doors, a tight seal on the garage door bottom and side jambs. The garage is the most common ground-level entry we find.
The foundation and crawl space
- Foundation vents screened with 1/4-inch hardware cloth, frames intact.
- Crawl space access door fitting tight, with no gap at the bottom.
- Cracks in the foundation wider than a half inch, and gaps where decks or steps attach to the house.
Around the property
- Fallen citrus and fruit picked up; ripe fruit harvested. Roof rats are named for the roof but fed by the orchard.
- Pet food and bird seed stored in sealed metal containers, bowls brought in at night.
- Woodpiles and stored material lifted off the ground and away from walls.
- Dense ground cover and ivy cut back from the house.
Materials that hold
1/4-inch galvanized hardware cloth, sheet metal, copper mesh backed with mortar or cement, and steel or aluminum flashing. Foam, caulk and plastic are fillers, not barriers; use them behind metal, not instead of it.
The order of work
- Inspect and map every entry, inside and out, before sealing anything.
- Seal all but one main entry, set traps inside, then close the last one so nothing gets sealed in.
- Trap and remove for one to two weeks; check for new droppings.
- Fix the food and harborage around the property so the pressure drops.
That sequence is our rodent exclusion service, and the exclusion work carries a one-year warranty. Poison outside without sealing is not on the list: it leaves the entries open and the rats die inside walls.
Sources
Common Questions
Quick Answers.
How small a gap can a rat get through?
About half an inch, the width of a thumb. A mouse fits through a quarter inch, about the width of a pencil. Anything that size or larger on the roof line, walls or foundation is an entry until it is sealed with metal.
What material should I use to seal rat holes?
Quarter-inch galvanized hardware cloth, sheet metal, or copper mesh packed into the gap and backed with mortar or a hard sealant. Spray foam, caulk and plastic screen get chewed through; use them only behind metal.
Why do roof rats live in the attic?
Roof rats are climbers. They travel along fences, power lines and tree limbs, enter at roof returns, vents and tile edges, and nest in the attic close to fruit trees and bird feeders. Sealing the roof line and trimming vegetation off the roof are the fixes; ground-level bait does little for them.
Does rodent exclusion come with a warranty?
Yes. ExterMetro's rodent exclusion work carries a one-year warranty, with the terms in writing before the work starts. Trapping and follow-up checks are part of the job, because sealed entries only help once the rats already inside are removed.
Should I seal all the holes at once?
No. Seal everything but one main entry first, set traps inside, then close the last one. Sealing everything on day one traps rats inside the walls, where they die and smell.
About the Author
Joshua A. Westad is the owner and licensed operator of ExterMetro Termite and Pest Control in Arcadia, CA. He holds California SPCB Company Registration #8828 (Branch 2 & 3), is a licensed WDO inspector, and has worked San Gabriel Valley homes and businesses for over twelve years — hands-on in the field on every kind of job the valley throws at him.
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