What size hole can a rat fit through?

A rat can squeeze through a gap about the size of a quarter — pest and extension sources put the minimum at roughly a half-inch opening, because a rat's skull and ribs compress. Mice need even less, about a quarter-inch (pencil-width). The practical rule for rat-proofing NYC homes: seal anything a quarter could pass through, and remember rats gnaw small gaps wider, so patch even the ones that look too small.

What is the best material to keep rats out?

Stainless steel or copper mesh packed tightly into the gap and locked with caulk or mortar is best for small openings, because rats can't gnaw through metal. For larger holes and all vents, use 1/4-inch galvanized hardware cloth, and use cement or mortar for masonry gaps. Avoid plain expanding foam, plastic, or wood alone — rats chew straight through them.

How do rats get into NYC apartments and brownstones?

Rats enter NYC buildings through foundation and sill-plate cracks, gaps around basement windows, openings where pipes and utility lines penetrate the wall, deteriorated mortar in brick foundations, uncovered vents and weep holes, and the gap under basement or service doors. In attached buildings they also travel between units through shared wall voids, plumbing risers and connected basements, which is why one-unit fixes often fail.

Can I rat-proof my home myself or do I need a professional?

You can seal most accessible entry points yourself — door sweeps, vents, visible foundation cracks and pipe gaps — with hardware cloth, copper mesh and mortar. A professional is worth it for shared basements, party walls, subterranean burrows and the hard-to-spot penetrations DIY misses, and for buildings where rats are already established. Our rodent-control team finds and seals the entry points homeowners can't reach and coordinates building-wide.

Do rats burrow under foundations in NYC?

Yes. Norway rats — NYC's dominant species — dig burrows typically 12 to 18 inches deep, and up to several feet, along building foundations, under sidewalks, and beside shrubs and trash areas. These burrows connect to building interiors through cracks and utility gaps, so rat-proofing has to address the ground line and basement, not just the walls above it.

Will rat-proofing get rid of rats I already have?

Rat-proofing keeps new rats out, but it won't remove an active infestation on its own — and sealing rats inside can create odor and damage problems. The right order is to knock down the existing population with trapping and tamper-resistant baiting first (or alongside), then seal the entry points so the cleared building stays rat-free. See our guide on how to get rid of rats in NYC for the removal side.

How bad is NYC's rat problem?

The NYC Health Department receives roughly 40,000 rat-activity complaints through 311 each year, and the city runs designated Rat Mitigation Zones in Bed-Stuy/Bushwick, Harlem, the East Village/Chinatown and the Grand Concourse. Complaints have fallen recently — partly credited to the rule requiring 1–9 unit buildings to containerize trash in sealed bins — but rat pressure remains relentless, which is why exclusion is essential.

How to Rat-Proof Your NYC Home: Seal Every Entry Point

The short answer

To rat-proof your NYC home, seal every gap a quarter could fit through. A rat only needs an opening about a half-inch wide to push its compressible body inside, and mice need just a quarter-inch — so the rule is simple: if a quarter fits, a rat fits. Pack small openings with stainless steel or copper mesh backed by caulk or mortar, cover larger holes and vents with 1/4-inch hardware cloth, mortar masonry gaps, and fit metal door sweeps on every exterior and basement door. In New York, exclusion — keeping them out — is the only rat control that holds, because the city’s food supply and connected underground highways mean any building you clear without sealing fills back up within weeks.

This is the “keep them out” pillar. If you also have rats right now, pair this with our guide on how to get rid of rats in NYC — and read on for exactly where rats get in and how to seal each point properly.

Why exclusion matters more in NYC than anywhere

New York has one of the densest rat populations on earth. The NYC Department of Health fields roughly 40,000 rat-activity complaints through 311 every year (NYC DOHMH), and runs designated Rat Mitigation Zones in Bed-Stuy/Bushwick, Harlem, the East Village/Chinatown and the Grand Concourse, where pressure is heaviest. Aging infrastructure, restaurant-dense blocks, shared basements and constant construction give rats endless food and travel routes between buildings.

There is genuine good news: 311 rat complaints have fallen over the past year, a decline the city credits in part to the rule requiring buildings with 1 to 9 residential units to put trash in sealed, lidded bins (NYC DSNY) — proof that cutting off food works. But the pressure never disappears. That’s why baiting alone fails here: you can poison every rat in a building, and the connected sewers, party walls and neighbouring basements will repopulate it. Sealing the how-they-get-in is the part that lasts. It’s the foundation of our rodent-control service, and it’s something a careful homeowner can do a lot of.

Find every entry point: inspect low to high

Rats are climbers and burrowers, so a real inspection runs from below grade to the roofline. Norway rats — NYC’s dominant species — dig burrows typically 12 to 18 inches deep, and up to several feet, along foundations, under sidewalks and beside trash areas (NY State Dept. of Health), then push inside through the nearest crack. Work the building in this order:

Basement and foundation

Foundation cracks and the sill plate — the seam where concrete meets the wood framing above it.
Basement and cellar windows — gaps around the frame, broken panes, missing well covers.
Floor drains and unused pipe stubs in the basement floor.
Where the building meets the sidewalk — look for burrow holes about two inches wide at the ground line.

Pipes, vents and utility penetrations

Every spot a water line, gas line, electrical conduit, cable or HVAC line enters the wall is a potential highway — rats can even gnaw through soft PVC and squeeze around loose escutcheon plates. Check dryer and bathroom exhaust vents, weep holes in brick, and crawl-space openings.

Doors and the roofline

Under every exterior and basement door — the most common single entry point of all.
Roof vents, soffits and trim — rats climb brick, stucco and downpipes to reach gaps up high.

In a brownstone or pre-war building, pay special attention to deteriorated mortar joints and the deep gaps around old plumbing risers — these are the classic NYC rat routes that DIY checklists from other cities miss.

Seal it right: materials that actually stop rats

The cardinal rule: rats gnaw through anything soft. Wood, drywall, plastic, screen and expanding foam alone are not rat-proof. You need metal or masonry at the barrier. Here’s what to use where, drawn from CDC and university extension guidance:

Small gaps (up to ~2 inches)

Pack the opening tightly with stainless steel mesh or copper mesh, then lock it in place with caulk or spray foam. The CDC specifically recommends filling small holes with steel wool and sealing around it with caulk. Plain steel wool works but rusts and stains — copper mesh or a purpose-made stainless fill fabric is the longer-lasting choice for exposed spots.

Larger holes and all vents (over ~1 inch)

Use 1/4-inch (19-gauge) galvanized hardware cloth. It blocks rats while keeping airflow, so it’s ideal for vents, weep holes and crawl-space openings. For big holes, back the hole with hardware cloth first, then patch over it with mortar or cement so rats can’t re-gnaw the patch.

Masonry gaps in brick and foundations

Repoint open mortar joints and patch foundation cracks with cement or fast-setting mortar, embedding hardware cloth or copper mesh in any larger void. This is the highest-value work in older NYC housing stock.

Doors

Fit brush or metal door sweeps so the under-door clearance is under 1/4 inch, and add metal kick plates to wooden doors a rat could chew. Do the basement, cellar and rear service doors first — they’re where the rats are.

Cut the food and harbourage

Exclusion holds only if you remove what’s drawing rats in. Even a perfectly sealed building will be tested relentlessly if there’s a buffet outside the door. So:

Containerize trash in hard, lidded bins (now required for 1–9 unit buildings in NYC).
Clear basement and yard clutter that gives rats cover and nesting material.
Fix leaks — rats need water, and a dripping basement pipe is an invitation.
Keep stored items off the floor so you can spot droppings, grease rub-marks and fresh gnawing along walls and runways.

A health reason to take this seriously

Rat-proofing isn’t only about the unsettling sight of a rat in the basement. Rat urine spreads leptospirosis, a bacterial infection that can cause kidney failure, liver damage and, rarely, death. NYC saw a record 24 human cases in 2023 (NYC DOHMH), up sharply from an average of about three per year in earlier decades — the city links infections to contact with environments contaminated by rat urine. Keeping rats out of your basement, storage and living space is a public-health measure, not just a comfort one.

When to call a professional

You can seal most accessible entry points yourself. But rats are relentless probers, and several jobs are genuinely beyond DIY:

Shared basements, party walls and plumbing risers in attached buildings, where rats travel between units.
Subterranean burrows tunneling under foundations and sidewalks.
An active infestation — sealing rats inside creates odor and damage problems, so the population has to be knocked down first.

The right sequence is removal then exclusion: trap and bait out the existing rats, then seal so the cleared building stays clear. Our rodent-control program is built around exactly this — a building-envelope inspection that finds the gaps homeowners miss, professional sealing with rodent-proof materials, and coordination across shared spaces so the problem doesn’t simply move next door. If you’re weighing DIY against hiring out, our rat exterminator cost guide for NYC breaks down what professional rat-proofing and removal actually runs.

Seal the quarter-sized holes, cut the food, and you’ve done the single most effective thing there is to keep rats out of a New York home — for good, not just until the next wave.

Sources: U.S. Centers for Disease Control and Prevention (CDC); NYC Department of Health and Mental Hygiene (DOHMH); NYC Department of Sanitation (DSNY); New York State Department of Health; university extension rodent-proof construction guidance.