How to Get Rid of Mice Under a Shed, and Keep Them Out

A shed sitting on bearers is a dry, insulated, predator-free void with a heated ceiling. The mice are not being opportunistic; it is better habitat than most of your garden.

The order, and why it matters

Step one: take away the food and the cover

Nothing else on this page survives a bird feeder twenty feet away.

• Birdseed into a metal lidded bin, and sweep the spill under the feeder
• Pet food and chicken feed likewise. Not in the original paper sack
• Compost in a closed bin, no cooked food or meat
• Fallen fruit picked up
• Clear the shed surroundings — log stacks, bags of compost, tarpaulins, pots, the pile of offcuts along the back wall. Mice travel under cover and will not cross open ground willingly
• Cut back the vegetation against the shed walls to leave a clear band

Step two: reduce the population

• Snap traps in protected bait boxes, set along the shed walls and against the runs where you see droppings. Boxes keep birds, pets and children out
• Bait with peanut butter or chocolate spread, a smear rather than a lump
• Set them perpendicular to the wall, trigger end toward it
• Many traps, not three. A dozen traps for a week clears a population that three traps take a month to dent, and a slow cull breeds around you
• Check daily. Wear gloves to clear and reset
• Keep going for a week after the last catch

Step three: skirt the void so it cannot be re-colonised

This is the part that ends the problem, and the material matters more than the technique.

1. Use quarter-inch hardware cloth. A mouse passes through a quarter-inch gap — about a pencil’s width — so half-inch mesh, which is what most garden wire is, excludes nothing
2. Dig a trench around the shed, six to twelve inches deep
3. Fix the mesh to the shed frame at the top with battens and exterior screws. Not staples alone
4. Angle the bottom six to twelve inches outward, away from the shed, into an L, and backfill over it with gravel. A rodent digging at the base meets a horizontal apron
5. No gaps at corners, posts or the door threshold. Every failed barrier failed at a corner
6. Backfill with gravel, not loose soil

Clearing up afterwards

• Dampen first with a disinfectant spray and leave it ten minutes
• Never dry sweep or vacuum droppings and nest material
• Bag it, double-bagged
• Gloves and an N95 respirator for anything in an enclosed space
• In an open, breezy void under a shed the risk is much lower than in a closed crawlspace or loft, but the method costs nothing

Dry rodent droppings and urine can carry hantavirus, leptospirosis and salmonella, and the transmission route for the first is dust raised by sweeping. That is the whole reason for dampening.

If the shed floor is already chewed

• Check the floor from inside with a bright light — gnawed joists, droppings along the edges, insulation pulled into a nest
• Line the inside of the floor with hardware cloth if the void cannot be fully skirted
• Store seed, bulbs and feed off the floor in metal, inside the shed as well as out
• Consider lifting the shed onto a slab or piers with an open, visible gap, which is a bigger job and the one thing that removes the habitat rather than fencing it

What does not work

• Peppermint oil, dryer sheets, cotton balls. A few days, outdoors less
• Ultrasonic repellers. Habituation within days, and the sound does not travel round obstacles
• Mothballs. No effect, and illegal used this way
• A cat. Helps at the margin, and will not clear a nesting colony under a floor it cannot reach
• Sealing the gap with foam. Chewed out by the weekend