How to Install Chimney Flashing

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Sheet-metal flashing -- or lack of it -- is commonly the problem when a chimney leaks. It is important to install flashing properly when you are adding a chimney or new roofing, or simply when you are trying to eliminate leaks. Chimney flashing should be redone any time you see that the metal is rusted through or is covered in roofing tar. Installing flashing yourself can save you hundreds of dollars in the end, but it requires some skills with tools and measurements. You also must be comfortable working on a sloped roof.

Things You'll Need

  • Ladder
  • Safety harness
  • Prebent flashing and saddle
  • Chisel
  • Crowbar
  • Ice and water barrier
  • Tin snips
  • Hammer
  • Roofing nails
  • Circular saw
  • Polyurethane caulk
  • Caulk
  • Plastic anchors
  • Set up a ladder, roof brackets and a safety harness before you begin working. Measure your chimney with a measuring tape to make sure your prebent flashing and saddle are the right size.

  • Remove the old flashing from around the chimney. Scrape as much roofing cement off the area as you can, using a chisel and crowbar.

  • Seal the area around the chimney with 12-inch-wide strips of self-sticking ice and water barrier. This material will stick to the roof's wood surface and seal around nails. Place the strips so they extend approximately 4 inches up the chimney. Overlap any adjacent strips to make sure water will be shed from the surface.

  • Install a 36-inch-wide strips of ice and water barrier strip along the length of your chimney in the back. Allow approximately 8 inches of the strip to run up the chimney.

  • Install the base flashing to the bottom front of your chimney. Position the flashing so it edges up to the chimney and lies across the roofing shingles surrounding it. Cut the edge of the flashing at a 45-degree angle with tin snips, so it can wrap around one side of the chimney. Secure the flashing to the roof by hammering in roofing nails.

  • Install 8-inch-square step flashing to the corners. Cut the flashing at a 45-degree angle to allow it to wrap around the front corner no less than 1 1/2 inch. Apply a small amount of caulk to the area where the chimney and roof meet. Secure each piece of flashing by hammering in two roofing nails.

  • Nail a roofing shingle over the first piece of step flashing. Repeat Step 6 without adjusting the flashing for the corner; simply move up the length of the chimney on all sides, alternating installing step-flashing and shingles in a way that would allow for water to shed over the top of the next shingle.

  • Install the chimney saddle to the back of the chimney by sliding it tightly against the chimney and nailing its top edge to the roof with roofing nails.

  • Install front-cap flashing by first sawing a 1-inch-deep groove into the chimney, using a circular saw; this will allow for the L-shaped cap flashing to fit into the groove. Measure the cap flashing 8 inches longer than the width of the chimney to wrap it around the corners and drive the flange of the cap flashing into the groove.

  • Secure the cap flashing by drilling holes through the metal and into the chimney. Press in expanding plastic anchors and drive nails into the anchors that will force them to expand, securing the flashing. Continue this step all the way around the chimney and finish by sealing the upper edge of all of the cap flashing pieces at the mortar joint with polyurethane caulk.

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