Best rat-proofing materials

What a rat can get through

A rat's incisors grow continuously and it gnaws to wear them down, which means gnawing is not a means to an end for a rat — it is maintenance. Given a reason, it will chew:

• Expanding foam. Cured polyurethane is soft. A rat goes through it in a night, and often takes the foam away as nesting material.
• Plastic. Vent covers, drain pipe, flexible duct, plastic air bricks.
• Wood. Softwood trim, plywood soffits, the bottom edge of a door.
• Thin aluminum. Soffit panel and flashing of the lightest gauge.
• Grout and poor mortar. Slowly, but it happens.

What it cannot get through is steel in a form it cannot grip: a woven mesh too fine to get a tooth into, or a wire grid too stiff to bend. That is the whole principle of rat proofing. Everything else is cosmetic.

And the gap you are looking for is smaller than people expect. A rat gets through roughly an inch and a half — the width of a soda can — and a young one through considerably less. A mouse needs a quarter of an inch, so if you are proofing to a rat standard and the site also has mice, proof to the mouse standard and you have covered both. The mouse version of this page is best steel wool for mice.

The four materials, and what each is for

Stainless fill fabric is the general-purpose answer. It is a dense stainless wool that you tear off by hand and pack into a void: around a pipe collar, into the gap at the top of a block wall, behind a loose piece of trim. It does not rust, which matters because ordinary steel wool rusts into orange crumbs in a wet gap within a couple of seasons and then the hole is open again. Buy this if you are buying one thing.

Quarter-inch galvanized hardware cloth is the material for anything you cannot pack — openings with a shape. A crawl-space vent, a gable vent, the gap along the top of a foundation wall, a soffit opening, the underside of a shed. You cut it with tin snips, you overlap the opening by a couple of inches on every side, and you fix it with screws and washers or staple it into sound timber. Quarter-inch is the mesh to buy: half-inch excludes rats but admits mice, and insect mesh excludes nothing structurally.

Copper mesh is knitted copper rope on a roll. It behaves like the fill fabric but comes in a long continuous run, which makes it the right thing for a long gap — a weep line, a run of soffit, a series of pipe penetrations — and for anywhere permanently damp, because copper does not rust. It is softer than hardware cloth, so it packs rather than spans.

Pest-rated expanding foam is the finisher. Its jobs are sealing air, keeping the metal in place, and making the repair look tidy. Both foams in our picks are sold for pest blocking and the labels are honest about what that means. Used over mesh it is excellent. Used alone it is a waste of an afternoon, and the one mistake this page exists to prevent.

The order of work

1. Find the holes properly. Outside, at dusk, with a flashlight held low and angled along the wall so shadows show gaps. Inside, look for rub marks — rats leave a greasy dark smear on a well-used route — plus droppings, gnawed edges and a distinct ammoniac smell. Check every pipe and cable entry, the rim joist, under the doors, air bricks, the dryer vent, the gap where the roof meets the wall, and the garage door seal. Rats in the attic and rats in the walls list the usual sites by location.

2. Trap first, seal second — with one exception. If rats are living inside, sealing every hole before you have trapped them leaves animals shut in with no exit, which produces a smell problem and some very determined gnawing. Trap until the catches stop, then seal. The exception is a hole you can see is the entry while the colony is outside: close that one at once.

3. Pack or span with metal. Small voids get packed with stainless fill fabric or copper mesh, pushed in firmly with a screwdriver so the whole depth of the gap is filled rather than plugged at the mouth. Shaped openings get hardware cloth, overlapped and screwed.

4. Foam over the metal. A bead to lock the mesh and seal the air gap, cut back flush when cured.

5. Fix the materials problem, not just the hole. If a rat chewed through a plastic air brick, replace it with a metal one. If it got in under a wooden door, fit a metal kick plate. Sealing the hole in a chewable material gets you the same hole next year.

Doors, vents and drains

Three sites that defeat ordinary proofing and have specific answers.

Under a door. The gap under a garage or outbuilding door is the most common rat entry on a property. A metal threshold strip or kick plate, or a brush seal with a metal spine, lasts. Rubber alone does not.

Vents. Never block a vent solid — you will cause condensation or a combustion-air problem. Screen it with quarter-inch hardware cloth on the outside face, overlapped and screwed, which lets the air through and stops the animal.

Drains and sumps. Rats use sewers and come up through broken drain lines, open floor drains and uncapped vent stacks. A drain cover with no grille, a dry trap or a cracked clay line is a route that proofing the walls will never fix. If you are catching rats indoors with no visible exterior entry, get the drains surveyed.

What proofing cannot do

It cannot make the property unattractive. A rat on a property with food and cover will keep testing the shell, and the shell only has to fail in one place. Proofing goes with housekeeping: bins with lids that latch, no pet food left out, fruit picked up, wood stacks off the ground, vegetation cut back from the walls. What attracts rats and rats in the yard cover the exterior side.

And it is not a one-afternoon job you never revisit. Walk the perimeter twice a year with the flashlight. New holes appear — frost moves mortar, a contractor drills a cable entry and leaves it open, a soffit panel lifts in a gale. Twenty minutes of looking is cheaper than the whole job again.