How do mice get into a NYC apartment?

Through any gap 6mm or larger — roughly the width of a pencil. The most common entry points in NYC apartments are the gaps around water and drain pipes under the kitchen and bathroom sink, the hole where the gas pipe enters the wall behind the stove, gaps at the base of walls where the baseboard has separated from the floor, worn or missing door sweeps under exterior-facing doors, and pipe chases inside shared walls that run vertically through the building from basement to roof. In prewar buildings — the majority of NYC's residential stock — these penetrations were never sealed during original construction.

What is the best material for sealing mouse entry points?

Copper mesh (sold commercially as Xcluder) packed firmly into the gap, then sealed over with caulk. Mice cannot chew through copper mesh or steel wool — copper is the preferred choice because it does not rust and maintains integrity long-term. For larger structural openings (gaps under kick plates, exposed pipe chases), use galvanised hardware cloth at 1/4-inch mesh, fixed with screws or construction adhesive, then caulked around the perimeter. Do not use expanding foam or caulk alone — mice chew straight through cured foam. The mesh is the barrier; the caulk or foam is only the anchor.

When is the best time to mouse-proof a NYC apartment?

September, before the first sustained cold snap. Mice begin seeking warm shelter in mid-to-late autumn as temperatures drop — typically October in NYC — but the migration starts earlier. Inspecting and sealing in September gives you a sealed perimeter before mice begin actively looking for entry points. The second-best time is immediately after winter if you had any mouse activity during the prior season, using fresh droppings or grease rub marks as a map to find the entry points you missed.

Can mice enter through gaps in my apartment walls without going through the building exterior?

Yes. In multi-unit NYC buildings, mice most commonly enter individual apartments from shared internal spaces — not directly from outside. Pipe risers, electrical conduit, and wall voids connect every floor from the basement to the roof. A mouse that enters the building at ground level can travel vertically through wall cavities and enter apartments on upper floors through pipe penetration gaps. This is why apartments on high floors still get mice: the entry point for the building may be at basement level, but the entry point into your unit is the unsealed gap around the hot water pipe under your kitchen sink.

Does food storage actually prevent mice, or just reduce how often they visit?

Food storage reduces incentive for mice already inside your walls to enter your living space, but it does not prevent mice from entering a building or an apartment. A sealed apartment with accessible food is less safe than a sealed apartment with food locked away — but the sealing is what does the prevention. In practice: seal all entry points first, then secure food as a secondary layer. If you skip sealing and only secure food, mice already present in your wall voids will still enter looking for nesting material, water, and alternative food sources. Both layers are required.

My landlord's building has structural gaps. Is that my problem to fix?

No. Under NYC Housing Maintenance Code §27-2018, your landlord is legally responsible for maintaining the building free of conditions that allow rodent entry. Structural gaps in the building envelope, unsealed pipe penetrations in shared walls, and openings in the basement or foundation are your landlord's obligation to repair — not yours. If your apartment has active mouse activity and structural gaps are allowing entry, notify your landlord in writing and file a 311 complaint if they do not act within 72 hours. HPD can issue Class C (hazardous) violations requiring remediation within 30 days.

I sealed my apartment and still got mice. What did I miss?

The most commonly missed entry points are: the gap behind the stove where the gas pipe enters the wall (requires pulling the stove out to see it), the hole around the refrigerator water line if present, gaps at the back corners of under-sink cabinets where the cabinet meets the wall rather than at the pipes themselves, gaps inside closets at floor level particularly where closets share a wall with a building corridor or adjacent unit, and the gap between the floor and the base of the kickplate under kitchen cabinets. Use a torch at floor level in low-light conditions — gaps that are invisible in daylight show as light or airflow sources in the dark.

I see mouse droppings. Is it too late to prevent mice?

Droppings mean mice are already active inside your apartment — the prevention window is closed. At this point the correct response is exclusion plus active trapping: find and seal every entry point using fresh droppings and grease rub marks as a map, then set snap traps in pairs at every location where you found droppings. Sealing without trapping leaves mice already inside your walls with no exit; trapping without sealing catches some mice while replacement mice re-enter the same night. Both must happen together. See our guide on how to get rid of mice in a NYC apartment for the full trapping protocol.

How to Prevent Mice in a NYC Apartment | Pest Control New York City

Why Entry Prevention Is the Only True Mice Prevention

Most mice control advice for NYC apartments focuses on trapping — snap traps, glue boards, bait stations. These tools are useful once mice are inside, but they are not prevention. Trapping removes mice after they enter. Prevention stops them entering.

The biology is unforgiving: a single female mouse produces 50 to 75 pups per year across multiple litters of 6 to 8 young, with offspring reaching sexual maturity in six weeks. An apartment with accessible entry points and an active population in the building will refill faster than any trap can clear it.

The only intervention that breaks this cycle is removing the entry points. Mice enter through any gap 6mm or larger — roughly the diameter of a pencil. In NYC’s prewar building stock, which makes up the majority of the city’s residential apartments, pipe penetrations, baseboard gaps, and structural voids were never properly sealed. Prevention means finding and closing every one of these gaps before mice use them.

Step 1: Inspect Every Likely Entry Point

Inspection comes before materials and before sealing. You cannot seal what you have not found.

Do this inspection at floor level with a torch — gaps that are invisible from standing height show clearly when you are at skirting level and shining light across the wall surface. Conduct the inspection in low-light conditions if possible; a gap that communicates with a building void or an exterior space will often show as a point of light or as moving air.

Under the kitchen sink Open the cabinet doors and examine the entire back wall. The hot water supply pipe, cold water supply pipe, and drainpipe each penetrate the wall. In most NYC apartments these penetrations have gaps ranging from 10mm to 50mm around the pipe — far beyond the 6mm threshold. Push a piece of paper into any gap you find to mark it.

Behind the stove Pull the stove forward until the gas supply pipe at the back is visible. The hole where this pipe enters the wall is one of the most commonly missed entry points in NYC apartments precisely because it requires moving the stove to see it. Check also the gap between the stove and adjacent cabinetry at floor level.

Bathroom plumbing penetrations Under the bathroom vanity: supply pipes and the drainpipe. Around the toilet supply line at the base of the wall. At the base of the bathtub surround where it meets the floor.

Baseboard separation Walk the perimeter of every room looking for gaps where the baseboard has separated from the floor, the wall, or at internal corners. Even a 10mm gap at baseboard level is a usable mouse entry.

Under exterior-facing doors Crouch down and look for daylight under your front door and any door that faces a building corridor, stairwell, or exterior. If daylight is visible, the gap is large enough.

Behind the refrigerator Pull it forward. The wall behind older NYC refrigerators often has an open pipe chase or an unfinished penetration from the building’s plumbing rough-in.

Inside closets at floor level Check the back corners of closets that share a wall with another apartment, a building corridor, or a utility riser. These corners are frequently unfinished and communicate with building wall voids.

Step 2: Seal With the Correct Materials

The single most important thing to know about sealing mouse entry points: expanding foam alone does not work. Mice chew straight through cured expanding foam. The foam offers no resistance; it takes less than a minute to gnaw through.

The correct method uses a physical barrier — copper mesh or steel wool — with sealant applied over it to anchor the mesh in place.

Copper mesh (Xcluder) or steel wool Pack the mesh firmly and completely into the gap before applying any sealant. Mice cannot chew through copper mesh. The commercial product Xcluder is tightly woven copper-and-stainless fibre and is the industry standard for exclusion work. Steel wool also works but rusts over time, particularly in humid spaces like under-sink cabinets — copper is the longer-lasting choice. Fill the gap completely; if it is large, use multiple compressed handfuls.

Sealant over the mesh Apply caulk or low-expansion foam over the packed mesh to hold it in position and create a finished surface. The sealant anchors the mesh; the mesh is what stops the mice.

Hardware cloth for large openings For gaps wider than roughly 50mm — large pipe chases, gaps under kick plates, structural voids — use galvanised hardware cloth at 1/4-inch mesh. Fix it with screws or construction adhesive, then caulk around the perimeter. Hardware cloth is available at any hardware store in NYC.

Door sweep If daylight shows under your front door, a metal-reinforced rubber door sweep is a straightforward fix available at any hardware store for $10 to $20. Standard rubber-only sweeps compress and allow entry; metal-reinforced versions maintain the seal.

Do not use caulk alone for any gap larger than 2mm. Mice can re-open dried caulk.

Step 3: Secure Food Storage

Food storage does not prevent mice from entering an apartment, but it removes the primary reason a mouse in a building’s shared wall void has to enter your specific unit.

Mice can smell food through cardboard, thin plastic bags, and foil packaging. Dry goods left in their original packaging are accessible to mice. The standard cardboard cereal box, plastic bag of pasta, or paper sack of flour are not barriers.

Transfer these to hard-sided sealed containers:

Dried grains, pasta, rice, flour, oats, lentils
Bread and crackers
Pet food — never leave out overnight; decant into a sealed container
Snacks in cardboard or thin plastic packaging
Coffee and tea if in cardboard or paper packaging

Glass jars with screw lids, hard plastic containers with locking lids, and steel tins are all mouse-proof. Under-sink cabinet space in NYC apartments is a primary mouse travel corridor — keep it completely clear of food.

Rubbish: Use a metal or hard-plastic lidded bin inside the apartment. Do not leave open bags of rubbish on the floor overnight.

Step 4: Reduce Clutter and Nesting Material

Cardboard boxes and loose paper provide nesting material for mice. A storage area filled with cardboard moving boxes is attractive to mice for two reasons: the boxes provide both nesting material and shelter.

Practical steps:

Replace cardboard storage boxes in basements, closets, and under-bed storage with plastic bins with lids
Do not accumulate cardboard in corners or along walls
Keep storage areas tidy and accessible for inspection — a cluttered storage area is harder to inspect for signs of activity

This is a secondary measure. It reduces habitability for mice that are already inside. It does not replace sealing.

Step 5: Inspect Seasonally — Autumn Is Critical

A sealed apartment is not permanently sealed. Building movement, temperature cycling, and general wear open new gaps over time. Seasonal inspection — particularly in autumn — catches new entry points before mice do.

September inspection (annual) Before the first cold snap, walk the perimeter of your apartment repeating the Step 1 inspection. Mice begin actively seeking warm shelter from late September through October in NYC. A September inspection gives you the chance to seal any new gaps before the autumn migration is underway.

Post-winter inspection (annual) After winter, check any gaps you sealed the previous autumn. Mice that were active in the building during winter sometimes gnaw at previously sealed gaps. Any gap with mesh that has been disturbed — where the caulk shows teeth marks or the mesh is displaced — needs to be re-packed and re-sealed.

Monthly perimeter check A five-minute walk around the apartment at floor level with a torch, once a month, is enough to catch early signs: fresh droppings (dark, shiny) along baseboards or under sinks, grease rub marks on baseboards, or gnaw marks on food packaging or wiring.

Step 6: Know Your Landlord’s Obligations

If you rent your apartment, the structural work required to mouse-proof the building is your landlord’s legal responsibility, not yours.

Under NYC Housing Maintenance Code §27-2018, your landlord is required to maintain your building free of conditions that allow rodent entry — including unsealed pipe penetrations in shared walls, gaps in the foundation, and structural openings in the building envelope. Mice in a rental constitute a Class C (hazardous) housing violation when an active infestation is present.

If your building has structural gaps allowing mouse entry:

Notify your landlord in writing — a text message or email with a timestamp is sufficient. State clearly that the apartment has conditions allowing rodent entry and request repair and extermination.
Document any signs of mouse activity with photos and timestamps.
File a 311 complaint if your landlord does not respond within 72 hours. HPD will inspect and can issue violations requiring remediation within 30 days for a Class C violation.

You are legally entitled to housing that is free of rodent entry conditions. You should not be paying for exclusion materials or extermination services in a rental — that cost belongs with your landlord.

Step 7: Recognise When Prevention Has Failed

The moment you see fresh mouse droppings — dark, shiny, roughly the size and shape of a grain of rice — the prevention window is closed. Mice are already active in your apartment.

At this point, switch to active exclusion and trapping. The correct response is:

Use the fresh droppings and any grease rub marks on baseboards as a map to find active entry points. Fresh mouse activity concentrates near entry points.
Seal every identified entry point using the materials described in Step 2.
Set snap traps in pairs flush against the wall at every location where you found droppings. Bait with a pea-sized amount of peanut butter.
Check traps daily and remove dead mice within 24 hours.
Secure all food in hard-sided containers.

Sealing without trapping leaves mice already inside with no exit. Trapping without sealing catches individual mice while replacement mice re-enter the same night. Both are required together once droppings are present.

For the full trapping protocol, see our guide on how to get rid of mice in a NYC apartment.

How to Prevent Mice in a NYC Apartment