Rats & Mice inCrawl Spaces

The surest signs of rodents under the house are droppings, gnaw marks and tracks, and each one also tells you whether you are dealing with rats or mice. Measure before you guess.

Sign	Rat	Mouse
Droppings	Blunt; Norway rat 3/4 to 1 inch, roof rat about 1/2 inch	Pointed, 1/8 to 1/4 inch
Tooth marks	About 1/6 inch (4 mm) wide	Scratchy marks about 1/12 inch (2 mm)
Footprints	3/4 to 1 1/4 inches, sometimes with a wavy tail-drag line	Smaller, with no tail-drag line

The sizes come from Cornell University IPM, Penn State Extension and UC IPM. Our rodent droppings guide covers pellets in more detail.

A dirt-floored crawl space is one of the best places in a house to read tracks, because footprints and tail drags show plainly in loose soil. Look at the ground just inside the access door and along the base of the foundation wall, where rodents travel most.

Look, but do not disturb. The CDC warns against sweeping or vacuuming droppings, so leave them where they are until the space is cleaned properly.

Rodents use crawl spaces because they offer shelter, darkness and soil right next to the food and water inside a house. Cornell IPM puts it plainly: “Norway rats prefer to burrow in soil, but will take advantage of voids near a food source.” A crawl space is both at once.

Nest-building fits the space too. University of Nebraska-Lincoln Extension notes Norway rats usually build nests in below-ground burrows or at ground level, lined with shredded paper, cloth or other fibrous material. Under a floor, that material is often insulation.

Mice want the same shelter but need far less room to reach it, which is why they turn up under newer houses as often as older ones. U.S. Census Bureau estimates put 31.7% of Chesapeake’s housing units as built before 1980, but house age is a poor predictor here: a mouse needs only one torn vent screen or loose pipe sleeve.

Rat burrows around foundations are the clearest sign that the animal under your house is a Norway rat. UC IPM describes Norway rats as the burrowing species, while roof rats nest above ground. A hole in the soil hard against the foundation usually means rats have tunneled in rather than walked in.

Mice do not dig like this, so a foundation burrow changes the whole job. It means trapping alone will not finish it: the burrow and the nest at the end of it need treating too.

If you find holes along the foundation line, treat them as part of a rat problem, not a separate yard issue. Our rat control service covers burrow and nesting-site treatment along with trapping.

Mice usually enter a crawl space through foundation vents with damaged screens and through gaps around pipes, cables and ducts, not by digging. UC IPM reports house mice can squeeze under gaps 1/4 inch tall and through openings 3/8 inch wide. The CDC compares a 1/4-inch hole to the width of a pencil.

Check these spots from outside the house:

• Foundation vents. Look for rusted, torn or missing screens. The U.S. Department of Energy’s Building America Solution Center calls crawlspace vents “another common entry point for animals.”
• Plumbing and gas lines. Look where each pipe passes through the foundation; the hole cut for it is often larger than the pipe.
• HVAC lines and ducts. Check where refrigerant and condensate lines pass through the wall or foundation.
• Cable and electrical entries. These are often sealed with a dab of caulk that has since cracked.

Screen built to stop rats will not stop mice: half-inch openings let a mouse straight through. If the droppings under your house are rice-sized, our mouse control service is the starting point.

Damage under the floor comes from gnawing and nesting, and contamination comes from droppings and urine left on the soil, insulation and ducts. Both get worse the longer rodents stay.

Insulation takes the worst of it. University of Nebraska-Lincoln Extension notes that rat burrowing can do considerable damage to insulated structures, and insulation pulled from between the joists often ends up as nest material. Duct wrap and wiring sit within reach too.

Contamination carries a health risk. The CDC says people catch hantavirus “from contact with rodents like rats and mice, especially when exposed to their urine, droppings, and saliva,” with fatigue, fever and muscle aches among the early symptoms.

That is the reason not to crawl in and clear droppings by hand. Our rodent cleanup service handles soiled soil, insulation and nests once the rodents are gone.

Crawl space rodent removal works in a fixed order: find every sign and entry point, remove the animals, seal the openings, then come back to check. Closing the gaps first would shut rodents in under your floor.

In outline:

1. Inspection of the crawl space and the outside of the foundation, to confirm the species and find each entry point.
2. Removal with trapping and bait placement suited to rats or mice.
3. Sealing of vents, doors and gaps once the population is coming down.
4. A return visit to check every trap and station and confirm the activity has stopped.

What each stage involves, including how vents are screened without cutting off airflow and how pricing works, is set out on our crawl space rodent control page.

Keeping rodents out of a crawl space comes down to closing every gap larger than 1/4 inch and taking away the cover that draws them to the foundation. UC IPM sets that 1/4-inch threshold because it excludes both rats and mice.

Between visits, you can help:

• Walk the foundation every few months and check each vent screen.
• Keep the crawl space access cover closed and latched.
• Move firewood, stored lumber and debris away from the foundation wall.
• Keep shrubs pruned back so you can see the base of the house.
• Note any new holes in the soil along the foundation.

Some of the work is harder to do yourself. Screening vents with the right mesh, rebuilding a loose crawl door frame and sealing utility penetrations with hardened materials rather than foam are all part of our rodent proofing service.