The first cold nights of fall are when mice start looking for a new place to live. The University of California's pest management program says house mice frequently move into homes in autumn, once nighttime temperatures drop. Your house is warm, dry, and stocked with food. From where a mouse is standing, it's an easy decision.
Around here the attic problem is usually rats. Alameda County's vector control district says roof rats are found throughout the county, especially in the suburbs, and are the most common rodent found in attics. They climb, they like heights, and they don't need much of a gap.
Keeping both out comes down to two numbers and one material. Here they are, plus where to put it and what not to waste money on.
The Two Numbers: 1/4 Inch and 1/2 Inch
Mice need about 1/4 inch. The CDC says a mouse can fit through a hole the width of a pencil, about 1/4 inch. UC's house mouse guide says they squeeze under gaps 1/4 inch tall.
Rats need about 1/2 inch. UC's rat guide says rats of either species, young ones especially, can get under a door with only a 1/2 inch gap.
You'll see 1/2 inch hardware cloth recommended for rats in a lot of places, some of them good sources. It's heavy wire and it holds up to an adult rat. It does nothing about mice, which walk straight through a 1/2 inch opening, and it's right at the limit for a young rat. If you're screening a vent once and don't want to guess which rodent shows up, use 1/4 inch. That's what Alameda County Vector Control, the San Mateo County vector control district, and the National Park Service rodent exclusion manual all recommend. UC's rat guide says the same thing in one line: seal every hole larger than 1/4 inch to keep out both rats and mice.
The number on the roll isn't quite the hole, and here that works in your favor. Standard 1/4 inch hardware cloth is 4 mesh in 23 gauge wire, and because the wire takes up some of the space, the clear opening is about 0.224 inch. We explain why in mesh count vs. inches.
Where They Get In
Roof rats come in high. Norway rats and mice mostly come in low. UC notes that when both rats share a building, Norway rats stay on the ground floor and roof rats take the upper floors. Walk around the house twice, once looking at the roofline and once looking at the foundation.
- Attic, gable, eave, and soffit vents. Roof rats run along fence tops and overhead utility lines at night, and that's how they reach the roof. A vent with torn or missing screen is the front door.
- Foundation and crawlspace vents. Usually the oldest screen on the house, and often rusted through along the bottom edge, closest to the dirt.
- Pipes, wires, and lines. Anywhere plumbing, electrical, cable, gas, or AC lines pass through a wall. The CDC puts these on its short list of places to check.
- The gap between the foundation and the ground or siding. Easy to miss because you have to get down low to see it.
- Doors, including the garage. A 1/4 inch gap under a door is enough for a mouse. Garage door seals tend to get chewed at the corners, and the NPS manual suggests covering those corners with metal flashing.
- Chimneys. UC's rat guide says to cover every chimney with a spark arrester. Chimney caps have their own rules for opening size, so buy a proper cap rather than wrapping the flue in hardware cloth.
- Dryer vents. Don't screen these. California's Mechanical Code says screens shall not be installed at a dryer exhaust outlet. Lint builds up on a screen, and a clogged dryer vent is a fire hazard. Use a vent hood with a backdraft flap and make sure the flap actually closes.
What Stops Them and What Doesn't
Rodents chew. UC's rat guide says rats chew through plastic, foam, wood, and caulk, and Norway rats will gnaw soft metals like copper and lead. Whatever you use as the actual barrier needs to be steel, concrete, or something just as hard.
| Material | Works? | Why |
|---|---|---|
| 1/4" galvanized hardware cloth | Yes | The standard for vents and openings. Galvanized after weld lasts longer outdoors. |
| 1/4" stainless woven mesh | Yes, longest life | Heavier wire and no rust. Worth it for crawlspace vents and near the coast. |
| Galvanized sheet metal, 24 gauge or heavier | Yes | For solid patches, door kick plates, and wrapping around pipes. |
| Copper mesh or stainless wool, caulked over | Yes, for small gaps | Pack it in tight, then caulk over it. Rodents can push loose mesh aside. |
| Steel wool on its own | Temporary | Rusts, and rodents work their way around the edges. |
| Expanding foam or caulk on its own | No | Rats chew through both. Foam is fine as a cover over metal, never as the barrier. |
| Window screen, plastic, vinyl | No | Chewed through with no trouble. |
| Chicken wire | No | The openings are too big. UC's rat guide says never to use it. |
| 1/2" hardware cloth | Rats only | Stops adult rats. Mice walk right through it. |
Wire Gauge: When 23 Gauge Is Enough
Stock 1/4 inch hardware cloth is 23 gauge, about 0.026 inch wire. Screwed flat over a vent with framing on every side, that's the common choice, and it's cheap enough to do the whole house. There are two places where we'd go heavier.
Crawlspace and foundation vents. The California Residential Code (Section R408.2) caps the covering on underfloor vents at 1/4 inch and lists hardware cloth of 0.035 inch wire or heavier among the allowed materials. Standard 23 gauge is lighter than that. If the vent is going to be inspected, or you just want to do it once, 4 mesh stainless in .035 or .047 wire clears both numbers. Your building department has the final word on what it accepts.
Anywhere rats are already chewing. The NPS manual recommends 1/4 inch hardware cloth in 16 to 19 gauge for rodent work. Heavier wire is harder to chew and harder to pry out of shape.
| Where | What we'd use | Wire | Clear opening |
|---|---|---|---|
| Attic, gable, and eave vents, general gaps | 4 Mesh 23 ga galvanized hardware cloth | 0.026" | 0.224" |
| Crawlspace and foundation vents | 4 Mesh 304 stainless, .035" wire | 0.035" | 0.215" |
| Spots where rats are actively chewing | 4 Mesh 304 stainless, .047" wire | 0.047" | 0.203" |
| Near the coast or salt air | 4 Mesh 316 stainless, .063" wire | 0.063" | 0.187" |
If you're in a mapped wildfire zone, vents are held to a different standard, and since January 2026 a new or replacement vent there generally has to be a listed vent rather than a screen you add yourself. We cover that in ember-resistant vent mesh in California. For 304 vs. 316, the short version is that 304 covers most of the Bay Area and 316 earns its price near salt water.
How to Install It So It Stays Put
- Cut it bigger than the opening and fasten it to solid framing on every side. Rodents start gnawing where they can get a grip on an edge. A tight, flat screen with no loose corner gives them nothing to start on.
- Use real fasteners. Screws with washers, or a wood strip screwed over the edge, hold a lot better than a few staples.
- Small gaps around pipes and lines. Pack copper mesh or stainless wool in tight, then caulk over it so it can't be pulled out.
- Holes 3 inches and up. Back them with 1/4 inch hardware cloth first, then patch over it with mortar or patching compound.
- Decks, sheds, and slabs where rats dig. The NPS manual describes an L-shaped strip of heavy 1/4 inch hardware cloth, fastened to the wall a few inches above the ground, bent outward along the ground, and covered with soil and rock. A rat digging at the base of the wall hits mesh instead of dirt.
One Rule Before You Seal Anything
Sealing the house with mice already inside doesn't solve anything. It just locks them in with you. If you're hearing scratching in the walls or the attic, or finding droppings, deal with the ones inside first with traps or a pest control company, then put the mesh up.
Be careful with the cleanup. Rodent urine and droppings can spread disease, including hantavirus, and the CDC says not to sweep or vacuum them because that puts the virus in the air. Follow its cleanup steps instead.
Frequently Asked Questions
Will 1/2 inch hardware cloth keep rats out?
Adult rats, generally yes. Young rats can get through a 1/2 inch gap, and mice go right through. If you want one material that handles both, use 1/4 inch.
Can mice chew through hardware cloth?
Steel hardware cloth is what extension services recommend for this job. The things rodents chew through easily are plastic, foam, wood, caulk, and soft metals like aluminum, copper, and lead. Keep the mesh flat and tight, use heavier wire where rats are working at it, and replace it once it rusts through.
Is steel wool enough to plug a hole?
Only for a while. UC calls steel wool a good temporary plug that can rust over time. For anything permanent, pack copper mesh or stainless wool into the gap and caulk over it, or cover the hole with hardware cloth or sheet metal.
Should I put mesh over my dryer vent?
No. California's Mechanical Code says screens shall not be installed at a dryer exhaust outlet, because lint clogs them. Use a vent hood with a backdraft flap that closes on its own.
When should I do this?
Now, before the cold nights set in. Mice move indoors in the fall, and it's a lot easier to keep them out than to get them out. Roof rats are active all year, so for rats there's no bad time to start.
We cut hardware cloth and stainless mesh to size, so you're not buying a 100 foot roll to cover six vents. Send us the vent sizes and how many you've got, and we'll quote it. If you want to see the 1/4 inch before you order, we'll mail you a free sample. Family-owned in Hayward, California since 1938, and a real person answers the phone at (510) 887-8787.
This article is general information about wire mesh, not pest control or building code advice. For an active infestation, talk to a licensed pest control operator or your county vector control district. For code questions, your local building department has the final word.