March 4, 2026
Most homeowners expect to find a big, obvious hole when they’re trying to figure out how rats got in. That’s almost never the case. More often, they’re slipping through small structural gaps you’d walk past without noticing, using those tiny openings to move in and out of the house night after night.
Most advice you find online often misses the key point: the size of the gap really depends on which type of rat you're dealing with. Norway rats usually come in from the ground up, sitting low and sneaky. Roof rats, on the other hand, love to drop in from above, just like little acrobats performing their tricks.
If you have roof rats and spend an afternoon crawling around your foundation, you'll likely find nothing because they entered from thirty feet above your head. In Marin and Sonoma, we frequently observe both species. Below is the map you can use to help determine which half of your house to address first.
Rats get into homes through gaps as small as 1/4 inch, roughly the width of a pencil. Norway rats (the burrowing species) come in at ground level; their entry points include foundation cracks, crawl space vents, gaps where pipes enter the wall, and under garage doors. On the other hand, roof rats (the climbing species) enter your home from above. They get in through soffit gaps, damaged roof edges, open attic vents, and utility lines running to the roofline. Knowing the species of rat you're dealing with is important because it helps you know which area of your home to check.
A rat's skull is the widest part of its body. Once it's head clears an opening, the rest just... follows. For an adult rat, that threshold is about 1/4 inch. Pencil-eraser wide. That's genuinely all it takes for a rat to be able to enter your home.
With that in mind, when you walk your perimeter looking for signs of rats, you are not searching for obvious holes or major damage. You are looking for the smallest gaps, anything wide enough for a pencil to slide through. That requires a very different type of inspection.
Homeowners often overlook very subtle entry points, such as the dried-out caulk ring around the gas line that was pulled away from the wall five years ago. They may also miss the corner where two siding runs meet, creating an opening. Other overlooked issues include the dryer vent housing that shifted a quarter-inch when bumped while moving furniture, and any gaps patched with spray foam. Rats can easily chew through spray foam. Despite most people using it as a tool for exclusion, it's just a placeholder because the rats will inevitably chew through it.
There are only a few materials that will actually keep rats out; these include solid concrete, properly fitted metal flashing, and hardware cloth with openings no wider than 1/4 inch, secured at every edge so there's nothing to grab and work on. When inspecting any type of gap, always ask yourself: Can a rat gnaw through whatever's filling it? If the answer is yes, treat the gap as open.
The Norway rat (Rattus norvegicus) stays low. It is a burrower, not a climber. For many homes in Marin County, signs of a Norway rat infestation include fresh holes in the dirt, greasy rub marks along the lower edge of the siding, and rodent droppings piled near the crawl area vents.
As concrete ages, it begins to crack. Settlement, wet-season soil pressure, and age all contribute to cracking, which tends to accumulate faster in older Marin homes near the water. Any crack big enough for a pencil to slip through can serve as a potential entry point. Norway rats might even enlarge it over time if they're determined enough.
The joint where the foundation meets the sill plate is the first place to look. It is the horizontal wood beam sitting directly on top of the concrete. It's a natural gap point, especially on homes with any settling. Also, it is worth checking anywhere the foundation's been patched before. Patches are also prone to cracking, typically at the edges where the gap is, rather than in the center.
One of the most common Norway rat entry points in older construction is the foundation vent. The mesh corrodes, the frame pulls away from the concrete, and what appears intact from the driveway often reveals a wide-open gap when viewed at ground level.
When searching for entry points, carefully examine all areas. Inspect all four corners where the frame touches the foundation. Three sides might appear perfect, but one corner could be bent outward by an inch and a half.
Every pipe entering your home from outside was passed through a hole just a bit larger than the pipe itself, such as gas lines, water mains, sewers, and electrical conduits. In many cases, the material used to fill the gap around the pipe has cracked, shrunk, or was never finished properly to begin with.
To verify that your gap is properly sealed, run your hand around each pipe close to where it penetrates the building. If you can feel a gap, even a thin one, so can a rat. The gas meter connection and the conduit running to the service panel on the side of the house are regular offenders.
When you're dealing with a rubber threshold seal that's been compressing for 15 years on an unevenly settled concrete floor, it can leave a gap. The gap may not be large enough to notice when walking through the garage every day, yet it is still enough for rats to get in.
It is important to check the vertical gap on either side of the door where the frame meets the floor. Homeowners often look under the door, but they neglect to look beside the frame.
The roof rat (Rattus rattus) doesn't burrow; it is an excellent climber. While a Norway rat is nosing along your foundation looking for a crack, a roof rat is running utility lines and scaling downspouts to get to the top of your house. The two species almost never use the same entry point.
In coastal California, specifically around Marin County, roof rats are the more common interior species. Mature trees, older homes with wood soffits, and ivy on every other fence help these rats get easy access to your roof. If you hear scratching noises above the ceiling at night, it's likely caused by a gap in the roof or upper exterior wall. The foundation is probably not affected.
Roof rats cannot climb onto your roof on their own; they use their environment to gain access. Tree branches that are a few feet from the eave are a common pathway, and homeowners often underestimate how close "close enough" really is. A clearance of just three feet between a branch and the roofline is easily enough for a roof rat to jump.
Utility lines extending from the street to the house provide rats with access to your roof. Other typical entry points include dense ivy on exterior walls, stacked firewood against the house, and outdoor AC units with a clear path to the eave. While blocking access routes might not completely stop entry points, it often significantly reduces traffic. Sometimes, this decrease in activity encourages rats to seek easier targets, making it a helpful step in managing the problem.
Soffits, the panels on the underside of the roof overhang, take a beating from moisture and age. Wooden soffits, in particular, degrade over time. A rotted corner, a panel pulled away from its neighbor, and a joint that's opened a quarter-inch are all standard entry points for roof rats.
Walk your perimeter while looking up at soffits and the roof edge to identify entry points. Carefully inspect the roof, as quick glances may overlook important entry points. You're looking for gaps between panels, spots where the soffit meets a wall and the joint has separated, and any area that looks darker or water-damaged.
Every attic vent poses a risk if the screen's gone bad. Make sure to inspect the fixed louver vents, gable vents, and turbine vents; check the screening for rust holes; check the frame contact; and check the surrounding fascia for cracks. Even vents that look intact upon initial inspection can have a corner that's been quietly failing for years.
What makes attic vent entry particularly frustrating is that once a roof rat gets through, it moves into the insulation and sets up before you have the chance to notice anything. By the time you're hearing it, the rat has already been there for a while.
Everywhere two roof planes meet, at attached additions, or where HVAC equipment passes through, the aging process looks different from the surrounding materials. Flashings may corrode, and caulking can crack due to summer heat. If your home has older additions or HVAC systems that haven't been updated in years, these areas are often common entry points.
Finding entry points can show you how the rats entered, but it doesn't help locate those already inside your home.
Once inside, rats move through wall voids and ceiling cavities like they own the place because at that point, they kind of do. They can cover a lot of ground quickly. A Norway rat coming through a crawl space vent can be heard in a completely different wall cavity within a few days.
This is where people waste the most time. The wall where you hear the noise indicates that something nearby is attracting rats: pet food, water, or warmth. It doesn't tell you where they got in or where they're sleeping. Chasing the sounds rarely solves anything.
Our article on the 5 early signs of a rodent infestation covers what evidence of rodent infestation looks like and how you can locate it before you call in a professional.
Here's the thing about finding an entry point: sealing it immediately is often the worst decision you can make.
Never seal an entry point before trapping the rodents. Doing so risks trapping a rat inside a wall, leaving it with no escape. The rat may chew through a new, unknown opening or die within the wall cavity. If it dies, it will create a persistent odor and lead to additional rodent problems.
For results that last, remove first and seal second.
Before anything gets sealed, you need reasonable confidence that the structure is clear. On a property with multiple gaps and established travel routes through the walls, it is very hard to confirm on your own. A professional inspection maps the rodent activity patterns inside the house, not just the gaps on the exterior, which is a different and more useful picture.
Best practice is to complete full removal before beginning any structural sealing. Sealing entry points while rats remain inside often leads to re-infestation within months.
Hardware cloth (steel mesh) with 1/4-inch openings, secured at every edge, works well for securing vent covers and larger openings. Metal flashing is ideal for cracks in the roofline and soffit joints. Steel wool packed tightly into pipe penetrations is a decent temporary fix, but it needs concrete or mortar behind it to last. Hydraulic cement is typically used to properly mend foundation cracks.
People often reach for spray foam as a cost-effective DIY method for sealing cracks and small holes, but it is not an effective means of prevention. Over time, spray foam compresses, and rats tend to chew through it. If a gap's been filled with foam, treat it as open until it's properly resealed.
Can you identify every entry point? And can you confirm the structure is clear before sealing anything?
If the answer to both is yes, and you are dealing with a single-story home, straightforward gaps, and no signs of activity inside the walls, a capable homeowner may be able to handle it.
If either answer is uncertain, an inspection is the more cost-effective starting point. Leaving even a single entry point open allows the infestation to persist, as the remaining opening still provides access to the entire rat population.
At North Bay Rat and Rodent in Marin County, we inspect the entire structure from foundation to roofline before starting any removal or exclusion work, which is the only reliable way to account for both species on the same property. See how our rodent exclusion service handles permanent sealing with materials that actually hold.
The rat's species determines where you look. Norway rats are a ground-level problem, while roof rats are a roofline problem. Inspect the wrong zone, and you'll walk away thinking your house is safe when it isn't.
The entry gap they are targeting is likely much narrower than you imagine. People tend to look for obvious openings, but the real threshold is about a pencil-width, roughly 1/4 inch. Many gaps seem insignificant until you actually check them.
You must only seal entry points after the rats have been trapped, not before. Getting that sequence wrong is the most common mistake homeowners make, and it consistently makes the rat problem worse before it gets better.
If you've found signs of rats on your property and want to know exactly where they're getting in, a comprehensive inspection maps out every entry point across the full structure of your home before any trapping or exclusion work begins. When you choose a professional rodent control company like North Bay Rat and Rodent, you can be confident that nothing will be overlooked, and there's no need for guesswork. If you are seeing signs of a rodent infestation and want professional help, call North Bay Rat and Rodent today or fill out this form to schedule an inspection.
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