How to Block a Fireplace for a Woodstove

Installing a wood stove directly into an existing masonry fireplace is one of the easiest venting configurations you can apply. But you must make some alterations to the chimney and fireplace for safe operation -- including removing the existing damper and installing a block-off plate in its place if using a freestanding wood stove. If you're installing a fireplace insert, the device usually has its own set of adjustable block-off plates. Before beginning though, the wood stove and its flue must fit properly in the fireplace. If using a prefabricated metal fireplace, check stove-manufacturer guidelines to ensure the stove is compatible with it.

Measuring tape
Sheet metal
Hammer
Wood block
Drill
Hole-cutting attachment
Masonry nails
Stove cement

Instructions

- 1
  Ensure the chimney is clean and equipped to vent your stove. If your chimney was not made for a wood stove, it must be lined or relined with a stainless steel, Underwriter's-Laboratory-approved liner to the top of the flue.
- 2
  Remove the fireplace damper by extracting the cotter pin holding it in place, and lifting the damper out.
- 3
  Measure the dimensions of the flue, starting from 2 inches above the fireplace opening, which is where the fireplace block will be installed. First, measure the front length across the opening. Starting from the same height, measure the back length of the opening. Finally, measure the central width of the opening from back to front.
- 4
  Adding an inch to each of these measurements, cut a 24- or 26-gauge piece of sheet metal to size. The extra inch on each side will serve as a flange for securing the plate.
- 5
  Shape the 1-inch flanges to 90-degree angles on all sides of the sheet metal, using a hammer and wood block to carefully bend them into shape. Create notches at all four corners.
- 6
  Slide the block-off plate into the chimney opening to see if it fits, and shape as necessary for a secure fit.
- 7
  Place the stove so the flue collar is positioned appropriately for venting up the chimney. Determine where the stove piping will pass through, which will guide you in cutting a hole in the plate. Mark the location where the stove piping will penetrate.
- 8
  Place a spare piece of pipe on the location you have marked, and trace around it. Use a hole-cutting drill attachment to cut out the circle.
- 9
  Place the block-off plate in the opening. Secure with masonry nails hammered through the flanges and into the mortar joints.
- 10
  Apply stove cement at the seam where the stove piping and block-off plate meet each other.

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