Cutting and Bending a 16-Inch Galvanized Flashing Rodent Guard for a 2-by-3-Inch Downspout: Center Opening, Radial Slit, and 36-Inch Mount Height

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Cut a 16-inch disc from 24-gauge galvanized sheet, snip a 2 by 3-inch opening in the center, slit it from edge to opening, then wrap and screw it to the downspout 36 inches above grade. The cutting is the whole job, and a straight-cut pair of aviation snips handles every line in it.

How wide and how high should a rodent guard on a downspout be?

Make the guard 12 to 18 inches across and mount it about 36 inches above the ground. A 16-inch disc of 24-gauge galvanized sheet works on a standard 2-by-3-inch downspout. Seal any gap wider than 1/4 inch around the pipe, and keep branches, fences, and rooflines more than 4 feet from the guard.

University of Nebraska Extension's rodent-proof construction guide says guards should be 12 to 18 inches wide and 36 inches above the ground, and that rats can jump 36 inches straight up and 48 inches sideways. The Building America Solution Center's pest-deterrent screens table lists galvanized sheet for pipe barriers at 24 gauge or heavier.

Aluminum flashing is softer than that. Use galvanized steel for this one.

Cut List for Common Downspouts

Downspout Opening to cut Disc diameter Overlap at slit Screws
2 by 3 in. rectangular 2 by 3 in. 16 in. 1 in. 4
3 by 4 in. rectangular 3 by 4 in. 18 in. 1 in. 4
3 in. round 3 in. circle 15 in. 3/4 in. 3
4 in. round 4 in. circle 16 in. 3/4 in. 3

For a round pipe, mark the opening with a compass and cut it as a series of short facets, then clean it up with tiny nibbling cuts.

Materials and Tools

  • Aviation snips for the disc, the opening, and the tabs
  • 24-gauge galvanized sheet, at least 18 by 18 inches
  • Four #8 by 1/2-inch stainless sheet-metal screws, plus two for the slit
  • Exterior polyurethane sealant, a mill file, and a 3/16-inch drill bit
  • Tape measure, marker, compass or a nail and string, work gloves, safety glasses
  • A stable ladder rated for your weight

Cutting and Mounting Steps

  1. Square the blank. Lay out a 17-inch square on the sheet and cut it with long, steady strokes. Open the snips fully and finish each stroke before re-biting so the edge stays straight.
  2. Mark the disc. Pivot a pencil on a nail 8 inches from the center of the blank and scribe the circle.
  3. Cut the disc. Trim the four corners off first, then snip short facets about 1 inch long. Keep the waste curling away on the left side of the blade.
  4. Open the center. Mark a 2 by 3-inch rectangle in the middle. Drill a 3/16-inch hole inside it, insert the snip tip, and cut to the lines.
  5. Cut the slit. Snip from the edge to one corner of the opening along a radius.
  6. Cut tabs. Make a 1/2-inch relief cut at the middle of each side of the opening and fold the tabs up 90 degrees with the flat face of a hammer on a 2x4.
  7. Deburr. File the cut edge until a gloved thumb slides along it. Zinc coating chips off at the cut, so touch up with a dab of paint if you have it.
  8. Wrap and lap. Spring the slit open, slide the disc around the pipe, and overlap the slit 1 inch to form a shallow dish. Run two screws through the lap.
  9. Fasten to the pipe. Drive one screw through each folded tab into the downspout.
  10. Seal. Run a bead of sealant on any gap wider than 1/4 inch, as the Nebraska guide advises.

The guard stops climbing on the pipe, not rats that reach the roof another way. If gaps exist where the pipe meets the foundation, the rodent-proof skirt for a garden shed on skids shows the footer approach. Match your mesh to the hardware cloth mesh size and gauge guide wherever you close a wall opening.

Cutting Notes for Galvanized Sheet

  • Cut with the sheet supported flat on a bench; hanging waste bends the edge
  • Cut on the waste side of the line, then dress the edge
  • Wear gloves, because the cut edge is razor sharp
  • Never work near overhead service lines

Edge finishing is the same routine used on mesh, covered in how to cut hardware cloth without sharp edges.

FAQ

Can I use aluminum flashing instead?

Thin aluminum bends under a rat's weight and is easier to gnaw. The 24-gauge galvanized spec is the safer choice for a pipe barrier.

Do I put the guard on both downspouts?

Put one on every pipe a rat can reach from the ground. A guard on a downspout beside a trellis or fence rail is wasted if the rail is within 4 feet.

Quick Guard Checklist

  • Disc 16 in., opening 2 by 3 in., slit cut, tabs folded
  • Edge filed smooth
  • Lap 1 in. screwed twice
  • Mounted 36 in. above grade
  • Gaps over 1/4 in. sealed

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