How to Get Rid of Snakes Under a Concrete Slab

A snake under a slab is there for rodents and shelter, and it got in through a gap you probably cannot see. The sequence matters more than the materials — and sealing before the snake leaves is the one unrecoverable mistake.

Before anything: which snake

Photograph it from a distance if you get the chance. This determines whether the rest of this is a Saturday job or a phone call.

The work involves reaching into and sealing gaps you cannot see into, which is an acceptable risk with a garter or rat snake and an unacceptable one otherwise. If there is any possibility of a venomous species — and across much of the US there is — call a licensed operator or your state wildlife agency.

Why it chose your slab

Two reasons, in order of importance:

1. Prey. A slab edge with gaps almost always has mice or voles under it. The snake followed them in. This is the root cause, and the one most people skip
2. Shelter. The space under concrete is dark, thermally stable, predator-proof and often slightly humid — close to ideal

Which means the rodent work is not a side task. Remove the food and the space stops being worth occupying, so it does not get re-colonised after you seal it.

The sequence, and why order matters

1. Clear the perimeter. Move log piles, stacked timber, firewood, dense ground cover, leaf litter and stored materials away from the slab edge. A slab surrounded by short mown grass or a gravel strip is much less attractive.

2. Address the rodents. Secure bird seed and pet food, clear fallen fruit, and deal with any active mouse population.

3. Fit a one-way funnel. Seal every gap except the main one, and at that one fit a cone of hardware cloth narrowing outward. A snake can push out through the narrow end and cannot find its way back in.

4. Wait. At least a week of warm weather. Snakes move on their own schedule, and in cool conditions they may barely move at all.

5. Verify. Smooth a band of fine sand or flour across the funnel exit and read it over several warm days. No fresh track means the space is likely empty. Watching at dusk helps too.

6. Seal properly. Quarter-inch galvanised hardware cloth, buried six inches into the soil and turned outward in an L so nothing digs under. Mortar or expanding foam for small cracks.

Gap size, and why the obvious seal is not enough

A quarter-inch gap is enough for a young snake. An adult passes through openings far narrower than its body appears to need, because it has no rigid shoulder girdle to clear.

This is why quarter-inch mesh is the standard, and why sealing one large obvious gap while leaving four small ones accomplishes nothing. Work the whole perimeter with a torch, low down, at dusk when shadows pick out openings.

Plastic mesh and steel wool are both inadequate here — rodents chew through plastic and steel wool rusts away within a season.

Timing: avoid autumn if you can

Snakes may brumate under a slab through winter, in a state of near-dormancy where they are far too sluggish to use an exclusion funnel.

If you find the problem in autumn, do the rodent and cover work now and leave the sealing until spring. A funnel fitted in October is likely to be ignored, and sealing in October risks exactly the outcome described above.

What does not work

Mothballs and sulphur. No reliable evidence of repelling snakes, and scattering mothballs outdoors is an illegal pesticide use in the US because the label does not permit it. Naphthalene is toxic to pets, children and soil life, and in a confined space under concrete the fumes achieve nothing.

Commercial granular snake repellents rest on the same premise and the same absence of evidence.

Glue traps. Inhumane, frequently catch non-target animals, and leave you with a live snake stuck to a board — a worse problem than the one you started with.

The things that work are food, cover and holes. There is no product shortcut.