Frost Depth, Basements and Where Pests Get In

Why do Twin Cities homes have basements at all?

Because foundations in a cold climate must go below the frost line. If footings sat in ground that freezes, the swelling soil could lift and crack the building. Digging that deep leaves a space, and finishing it as a basement costs less than filling it. The exact depth is a code figure to confirm locally.

Frost heave is the reason. Water in the soil expands when it freezes, and the ground can push upward with great force. A footing placed in soil that freezes would move with the seasons. Minnesota building rules therefore require footings below the depth to which the ground is expected to freeze, and the required depth depends on where in the state you are. Secondary sources point to Minnesota Rules chapter 1303 as the place the zones are set, but we did not see the official text, so we print no depth. Confirm it with your city's building department before any footing or deck work.

The result is visible all over Minneapolis and Saint Paul. Much of the housing dates from the early to mid twentieth century, and most older homes here have basements. That is hedged on purpose: we have no sourced citywide percentage. But the pattern is clear enough to compare with warm-climate cities, where slab foundations are common and the ground-level entry points are different.

For pest purposes, the basement is a feature that cuts both ways. It gives a technician an inside view of the foundation and the rim joist, which a slab house does not. It also means a house has a long run of wall below grade, where gaps, damp and cold interact.

Where are the weak points in a cold-climate foundation?

The sill plate and rim joist, window wells and basement windows, utility penetrations for gas, water, cable and electric, dryer and exhaust vents, old coal-chute or flue openings, cracks in masonry, and the seam where the garage meets the house. The table lists each, what it looks like and what usually closes it.

The table is organized by location. It does not claim numbers for how often each one is the route in, because no source ranks them. It shows where a technician looks first, and why.

What does freeze-thaw do to a foundation over the years?

Repeated freezing and thawing works water into small cracks, where it expands and opens them wider, and it crumbles old mortar. Over decades, a stone or block foundation that was tight when built can develop gaps. Mice need only about a quarter inch, so small cracks matter long before they threaten the structure.

A Minnesota winter is a long cycle of cold, with thaws in between. Moisture gets into mortar joints and hairline cracks. When it freezes, it takes up more room and pushes the crack wider. When it thaws, the water goes deeper. We do not give numbers for how many cycles a year the Twin Cities sees, because sources disagree on the winter details. The mechanism is what matters.

In an older house with stone or brick foundation walls, the lime mortar between stones weathers fastest. Pieces fall out and leave pockets. In block, cracks often follow the mortar joints. In poured concrete, cracks form at corners and over windows. Each one is a potential route.

Part of the fix is drainage. Downspouts that dump water at the foundation, soil that slopes toward the wall and window wells that fill with water all speed the damage. A technician cannot repair a drainage problem, but can point it out, and a mason or a landscaper can. Our page on basement and foundation sealing explains the part that is pest work.

How do pests use these routes?

Mice take the smallest gaps at the sill, utilities and garage seam; rats can widen loose openings; insects such as boxelder bugs get under siding and into wall voids; and spiders and others follow insects into window wells. Which one uses which route depends on the pest and the season.

Mice are the clearest case. A gap of about a quarter inch is enough, and a mouse will use the same route repeatedly. They enter at the base of the wall in autumn, as nights cool, and settle along the warm foundation near the furnace and water heater. See our guide on when mice come in.

Rats are larger and tend to dig. Burrows along a foundation or beside a slab give them a route to loose masonry and sewer connections. We do not claim a Twin Cities rat count; a technician judges burrow signs on site, and rat control describes what is checked.

Insects mostly come in higher up. University of Minnesota Extension says boxelder bugs go for buildings with southern exposure, and many get under shingles and into wall voids and attics. That is above the foundation, but the same principle applies: any gap that stays open invites them. The window well is a middle case, a pit beside the glass where leaves, insects and small animals collect.

What can a homeowner check, and what needs a professional?

A homeowner can walk the foundation in late summer with a flashlight, mark gaps, clean window wells and fix downspouts and grading. A professional is useful for masonry repair, high roof edges, hidden gaps behind finished walls and anywhere the work is unsafe. Do it before the ground freezes.

Timing matters in this climate. Once the ground is frozen, exterior masonry and sealant work is harder, and many materials need to cure above freezing. The best window is late summer to early autumn, before the first hard frosts. If you miss it, interior work and trapping can continue in winter, and the exterior work can wait for a thaw.

If you hire help, ask what each repair uses. Steel wool, mesh and masonry are the materials mice cannot chew. Foam and caulk alone are not enough. The guidance on this comes from extension services in other states and from University of Minnesota Extension's school pest guidance, which stresses door sweeps and removing food, water and shelter. It is general advice, not a Minnesota code.

• From outside: look for gaps at the base of the siding, cracks in the foundation, loose bricks and missing vent covers
• From inside: shine a light along the top of the foundation wall and the rim joist and look for daylight, gnawing and grease marks
• Window wells: clear leaves, check the screens and frames, and confirm that water drains away
• Drainage: confirm downspouts discharge away from the wall and that soil slopes away from the house
• Garage: check the door seal and the door into the house

What do newer houses change, and what do they not?

Newer houses are usually tighter, with poured walls and better sealing, but they still have utility entries, garage seams, vents, window wells and walkout thresholds. Age lowers the number of gaps but does not remove them. The same inspection applies; it is just usually faster.

A house from the 1990s or later often has poured concrete walls, sealed sill plates and a finished lower level. That helps. The weak points shift toward the places where builders connect parts: the garage door and the door into the house, the sliding door of a walkout, the penetration for the gas meter and the vents.

Finished basements add a different issue. Drywall hides the top of the foundation wall and the rim joist, so a gap can run behind a finished surface for years. Signs are a smell, a sound or a gnawed wire rather than droppings on a floor. A technician may use a camera or open a small section to look.

Whether the house is from 1920 or 2020, the principle is the same, and so is the schedule. Walk it in late summer, close what you find, and call if you still hear movement in winter.

What about porches, decks, crawl spaces and outbuildings?

They are part of the same envelope. A porch built on posts or a short wall, a deck attached to the house and a detached garage each have their own gaps, and animals use them to reach the main house. A foundation inspection that stops at the basement wall can miss the real route in.

Older houses often have additions, a back porch, an enclosed entry or a sunroom, built on different foundations from the main house. The joint between old and new is a seam that moves with the seasons. In a freeze-thaw climate, additions settle at different rates, and the seam can open.

Decks are another place. A deck ledger bolted to the house leaves a gap along the rim joist, and the space under a low deck is a sheltered spot for mice, chipmunks and sometimes larger animals. Wood that stays damp under a deck also draws carpenter ants, as described on our carpenter ant control page.

Detached garages and sheds follow a simpler rule: if they are not sealed, they are shelter. Mice and field mice use them in autumn and move on to the house if there is a route. Keep stored items off the floor, keep seed in metal containers and close the gaps at the base of the door.

Walk the whole property, not only the house. The technician does, and it helps if you have done it first.