Rodent proofing for attics and crawl spaces

Roofline or foundation?

Which rodent you have, and how to tell they've moved in, is on our rats in the attic page. For this job the useful split is simple: roof rats climb, so they mean the roofline, the vents and anything that touches the house above head height; Norway rats burrow and stay low, so they mean the foundation, the crawl space and the garage. Mice can turn up anywhere, through gaps you'd barely notice.

Where they get in on a Bay Area house

Most entry points sit in a handful of places:

• Gaps at the eaves and roof edges, especially where a roofline meets a wall or a second-story roof meets the first.
• Vent screens. Eave, gable, roof and foundation vents with screens that are torn, rusted or missing. UC's guidance specifically calls for repairing or replacing damaged screens on foundation vents, roof and attic vents and under the eaves.
• Pipe and wire penetrations. Where the AC line set, cable, gas line or plumbing goes through a wall.
• Vent caps. Dryer, bath fan and kitchen vents without a working flap or screen.
• Crawl space access doors and gaps under them.
• Garage door corners and weatherstripping that's been chewed.

UC's rule of thumb is that rats can squeeze through a gap as small as half an inch, and that anything over a quarter inch should be sealed. That means a gap you'd barely notice is an open door.

How we do it

1. Walk the whole house. At the inspection we go along the roofline, around the foundation, into the attic and, where there is one, the crawl space. Every entry point we find goes on the quote, with where it is.
2. Close them with metal. Galvanized hardware cloth over vents and larger openings, sheet metal where a gap needs a solid patch, and metal mesh packed into irregular gaps, finished with sealant where the weather needs it. Damaged vent screens get replaced.
3. Do the vents once. If eave or gable vents need new screens anyway, it can make sense to swap them for ember-resistant vents at the same time. Same openings, one job.
4. Clean up what they left. Droppings and nesting are cleared through attic cleaning and sanitizing; soiled insulation comes out through insulation removal.
5. Then insulate. With the entry points closed and the attic clean, new insulation goes in without becoming the next nest.

What we don't do is trap, bait or exterminate. That's structural pest control, regulated separately by the state's Structural Pest Control Board. If the animals are still active inside, the order matters: they come out first through a pest control company, then the house gets closed up, then the cleanup. Sealing a live rat into your attic helps nobody.

What the quote lists

Rodent proofing is fiddly, scattered work, and a quote that just says "seal entry points" gives you nothing to check against. A quote worth signing lists:

• Every entry point by location, such as "north eave above the kitchen" or "AC line set, east wall", so you can walk the house afterward and see each one.
• What closes it: hardware cloth, sheet metal, packed metal mesh or a replacement vent screen, and the sealant that finishes it.
• Vents swapped or screened, by location and type. Where screens are being replaced anyway, an ember-resistant vent closes the same opening against embers as well as rats, so the quote should say which vents get swapped and which are only re-screened.
• Foundation and crawl space openings, if the house sits on a raised foundation: vent screens, the access door and gaps around pipes.
• What isn't included, such as trapping, cleanup or work a roofer or pest control company has to do first, so nothing falls between trades.

If an entry point sits under roofing that has to be lifted, that's a roofer's work, and the quote should say so plainly.

Don't forget the crawl space

Attics get the attention, but in houses on a raised foundation the crawl space is often where Norway rats and mice live first. Torn foundation vent screens, a loose access door and gaps around plumbing that runs through the foundation all lead under the floor. Once they're there, the underfloor insulation tends to get pulled down for nesting, and ducts that run under the house get chewed. That's why the inspection covers the crawl space when there is one, and why closing the foundation openings is usually part of the same quote. If the insulation under the floor is damaged, our crawl space and underfloor insulation page explains what replacing it involves.

What helps from the outside

A few things you can do yourself keep the work holding up, all from UC's pest notes:

• Cut back tree limbs within 6 feet of the roof. Roof rats use them as bridges.
• Pull climbing plants like ivy off the walls.
• Leave space between shrubs and the house.
• Pick up fallen fruit, and keep pet food and birdseed indoors or in metal bins.

Safety: droppings and nests

If you find droppings in the attic, don't sweep or vacuum them, says the California Department of Public Health; are rodent droppings dangerous? has its guidance, quoted with the source. If you're not going to clean them up that way, leave them alone and close the hatch.