How To Install Aluminum Flashing Around An Edge Of Your Chimney To Prevent Water Intrusion Into Your Roof

Any point-of-entry into the roof of your home is an opportunity for water to penetrate and cause damage in the attic. Chimneys are no exception to this rule, and proper care must be taken to prevent a destructive leak from forming around the edges of your chimney. That is why the installation of metal flashing is critical to preventing water intrusion. Below is a list of tools and materials needed, as well as a guide to installing metal flashing around your chimney:

What you will need

  • Roll of aluminum flashing

  • Metal scribe

  • Straight edge

  • Measuring tape

  • Tin snips

  • Stiff bristled brush

  • Leather work gloves

  • Circular saw with diamond masonry blade

  • Hand seamer

  • Lead wool

  • Chisel

  • Hammer

  • Fast mortar mix

  • Five-gallon bucket

  • Trowel

  • Garden hose

  • Nylon mallet

Step-by-step procedure

1. Cut a shallow groove into the chimney - To begin, you will need to cut a groove along the side of the chimney where you desire to install flashing. Using a circular saw with a diamond masonry blade, set a cutting depth of ½-inch on the saw. Be sure not to penetrate too deeply, or you risk weakening the structural connection between brick layers. Next, select the second mortar layer from the point where the roof intersects the chimney, and carefully cut a groove into this mortar with the saw. Avoid cutting into the brick, and work slowly to make the cut as straight as possible. Remove excess mortar powder and other debris using a stiff bristled brush.

2. Measure, mark and cut the flashing to size - Once you have cut the flashing, use a measuring tape to determine the distance between the top of the shingles where they intersect the chimney and the groove you just cut. Then, measure the distance between the chimney/roof intersection and the bottom of the first row of shingles that lies below the chimney. Add both of these distances together, then add an additional inch to the total to determine the width of the flashing that needs to be cut.

For example, if the groove you cut is 6 inches above the shingles, and the distance between the point where the bottom of the first row of shingles and roof intersect is 8 inches, then adding the two measurements provides 14 inches. Add an additional inch to arrive at a 15-inch wide strip that is needed.

After measuring and marking a line with a scribe or marker that indicates the width along the strip, use a pair of tin snips to carefully cut the aluminum to size; be sure to wear leather gloves to protect your hands from cuts.

3. Make bends in the flashing - In order to fit the flashing onto the chimney, you will need to make pre-bent patterns in the flashing using a hand seamer. This will enable the flashing installation to be easy and fast.

Begin by laying out the strip of flashing that you cut in step 2, and measure ½-inch inward from each edge. Draw parallel lines along the strip at both points, then measure inward from one edge of the flashing a distance equal to the groove-to-rooftop. After measuring and marking these distances, use a hand seamer to bend the aluminum slowly along the three lines you just made. The final profile of the flashing after bending should resemble an "L" with two tabs at each end of the letter.

4. Attach the flashing to the brick - After bending the flashing into position with the hand seamer, hold the metal strip up to the grooved chimney and insert the tab you just made in step 3. Lightly tap the tab into the groove with a nylon mallet to prevent cosmetic damage.

Next, to hold the tab in position and prevent slippage of the flashing, tear off a small dime-sized amount of lead wool and roll it in between your gloved fingers to create a small cylinder. Stuff the lead wool into the space just above the tabs you inserted, then use a chisel and hammer to firmly tap the lead material into the gap. Don't tap too hard, or you may crack the bricks or do other damage. Repeat this process of tucking lead wool into the gaps every 8-to-10 inches. After the lead wool has been used, mix a small amount of fast mortar mix with a little water. Next, stir the mortar with a pointing trowel blade until the consistency of the mortar is equivalent to peanut butter. Apply mortar into the crevice you cut earlier to help hold the flashing in place once it hardens.

5. Attach the flashing to the shingles - After the tab is resting inside the groove and has been firmly joined, adjust the second bend you made in the strip so the bottom side of the strip just passes the bottom of the first row of shingles beneath the chimney. Use the seamer to fold the bottom edge of the strip down and underneath the edge of the shingles. This will help provide additional protection from wind damage and help hold the flashing in place.

If your roof needs to have shingles replaced or other repairs done as well, click here for info on roofing contractors.