What Can Mice and Rats Chew Through? The Sourced Chart for Long Island Homeowners (12 Materials They Beat, 6 They Can't)

Mice and rats can chew through wood, plastic, rubber, vinyl, fiberglass, window screens, spray foam, caulk, lead, thin aluminum sheeting, electrical wiring, and low-quality concrete or concrete block. If a material is softer than steel and has an edge they can get their teeth on, treat it as something a rodent can open.

The most useful single source on this is "Rodent-Proof Construction and Exclusion Methods" by Baker, Bodman, and Timm, published by the University of Nebraska in 1994 and still widely used in the pest control trade. It says rats and mice can "gnaw through a wide variety of materials, including lead and aluminum sheeting, window screens, wood, rubber, vinyl, fiberglass, plastic, and low-quality concrete or concrete block." UC IPM adds foam and caulk to the list for rats, and says rats damage electrical wires and gnaw soft metals such as copper and lead.

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Two more worth knowing. The National Park Service notes that mice and rats can chew through plaster. And Baker, Bodman and Timm specifically call out neoprene seals and spray-in-place foam as not rodent-proof, which covers most of what gets sold as a quick fix at the hardware store.

Mice and rats cannot chew through steel hardware cloth of the right gauge, galvanized sheet metal 24 gauge or heavier, reinforced concrete at least 2 inches thick, brick with fully mortared joints, heavy aluminum flashing, or cement mortar once it has fully cured. The thickness matters as much as the material, which is why the chart below gives the numbers.

Every spec in that chart is from Baker, Bodman and Timm. Notice what is missing: anything that comes in a spray can or a squeeze tube. Those products have a job, which is holding the real barrier in place and stopping drafts. They are not the barrier.

Mice and rats chew constantly because their front teeth never stop growing. The National Park Service puts it plainly: the incisors "grow continuously throughout the mouse or rat's lifetime and are sharpened by gnawing." Chewing is maintenance for them, and a gap in your house is just a convenient place to do it.

There is one weakness in those teeth, and good exclusion work is built around it. Baker, Bodman and Timm point out that the incisors curve slightly inward, which "makes it difficult for them to gnaw into a flat, hard surface." Give a rodent an edge, a corner, or a small starter hole and it will widen it. Give it a flat sheet of metal with no edge to grab and it has nothing to work with. That is why the same source says rodents usually gnaw in at corners and at joints in building materials, and why a patch that leaves a lip fails while a patch that sits flush holds.

Yes. Mice and rats chew through cured spray foam easily, and three separate sources say so. Baker, Bodman and Timm list spray-in-place foam as not rodent-proof, UC IPM says insulating foam is unsuitable for plugging holes used by mice, and UC IPM's rat guidance says rats can chew through plastic, foam, wood, and caulk.

Foam still shows up in the right way to do the job. The CDC's advice for small holes is to fill them with steel wool, then put caulk around the steel wool to keep it in place, or use spray foam for that same holding job. The metal stops the teeth. The foam or caulk only keeps the metal from being pulled out. We see a lot of Long Island basements with a neat bead of foam around every pipe and a mouse problem anyway, and the fix is almost never more foam.

Not when it is packed tight, but both are plugs, not permanent repairs. The CDC recommends steel wool for small holes. Baker, Bodman and Timm call steel wool, copper gauze, or screen wire packed tightly into an opening "a good temporary plug," and UC IPM uses the same word, temporary, for stainless steel scouring pads.

The reason for the word temporary is practical. Loose mesh can be pulled out, and Baker, Bodman and Timm warn that if steel wool is used, rust stains are likely. Outdoors or anywhere damp, we reach for copper mesh because it does not rust, pack it tight, and then lock it in with caulk or mortar. For anything bigger than a pipe gap, UC IPM's instruction is the one to follow: seal cracks in foundations and openings around water pipes, vents, and utility cables with metal or concrete.

Rats can chew through low-quality concrete, concrete block, and concrete that has not fully cured, but not through sound, cured concrete or mortared brick. Baker, Bodman and Timm set the line at 2 inches of reinforced concrete, 3 3/4 inches of unreinforced concrete, or 3 3/4 inches of brick with mortared joints.

Timing matters on a fresh repair. The same source notes that rodents "can claw and gnaw at concrete and Portland cement until it is fully cured," so a patch that goes in while rats are still active at that hole can be opened before it hardens. And rats do not always need to chew at all. They can burrow straight down at least 36 inches, which is how they get under a shallow slab or a set of stairs without touching the concrete.

A mouse needs an opening just over 1/4 inch, about the width of a pencil, and a rat needs just over 1/2 inch. The CDC gives the pencil comparison for mice, and UC IPM says a rat can squeeze beneath a door with only a 1/2 inch gap. Anything larger than 1/4 inch on the outside of a house should be sealed.

Gap size is only half of it, because of what they can do to reach the gap. These numbers are all from Baker, Bodman and Timm.

So the opening that matters is not always at ground level. A cable line to the roof, a downspout, or a tree limb over the gutter is a road. Baker, Bodman and Timm recommend that the gap under an exterior door not exceed 1/4 inch, which is a quick thing to check with a pencil tonight. If you want the full list of entry spots, see where mice are getting in, and for the plumbing route see whether rats can come up through the toilet.

After thirty five plus years in the field on Long Island, these are the six places we find a gnawable material doing a job that needs metal or masonry. Work through them in this order.

1. The garage door. The rubber bottom seal and the wood at the two lower corners of the frame are both on the can-chew chart. Look for daylight at the corners and gnaw marks on the seal. The lasting fix is metal edging on the lower door frame and a seal that closes the gap completely.
2. Pipe and cable penetrations. Oil fill and vent pipes, the gas line, the AC line set, the cable and electric service. If the fill is foam or caulk alone, it is a plug waiting to be opened. Pack with copper mesh, then seal over it with mortar or caulk.
3. Dryer and bathroom exhaust vents. Plastic louvered hoods are plastic. Cover the opening with 1/4 inch hardware cloth where the vent type allows it, and keep dryer vents clear of lint.
4. Basement windows and the bulkhead door. A screen by itself is on the can-chew chart. Check the wood frame at the bottom corners and the gap where the bulkhead door meets the foundation.
5. The sill line. Where the wood framing sits on the foundation, look for gaps larger than 1/4 inch, especially at corners and where additions meet the original house. Seal with metal or cement, per UC IPM.
6. The roofline. Soffit corners, where the fascia meets the roof, and gaps around plumbing vents. Mice and rats climb wires and rough walls, so a high gap is still a gap.

For the full seasonal checklist, including what to do about food and nesting material, read our Long Island mouse-proofing checklist. For how to tell which animal you have, see the house mouse profile and the Norway rat profile.

Call a professional when you have sealed the obvious gaps and still have activity, when the openings are at the roofline, or when you are dealing with rats. UC IPM calls exclusion "the most successful and permanent form of house mouse control" and the most successful and longest-lasting strategy for rats, so it is worth getting every opening found the first time.

One rule before anyone seals anything: deal with the animals that are already inside first, or seal and trap at the same time. Closing the last hole on an active house traps rodents indoors. If you want a licensed applicator to find every opening on the house and tell you what each one needs, that is what our rodent exclusion service on Long Island is for. If they are already inside, start with our mice exterminator or rat exterminator pages.