Proper flashing around a brick chimney is important to ensure a water-resistant rooftop. Chimney flashing comes in a series of prefabricated sheet-metal pieces. These pieces overlap one another, and are in turn overlapped by the roofing shingles, to create a cohesive membrane over the house's structure. Flashing base pieces are fitted onto the front, back, and around the sides of the chimney, and a specialised saddle directs water away from the backside of the chimney.
- Skill level:
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Things you need
- Tin snips
- Wood block
- Galvanised roofing nails
- Cordless drill
- 3/16-inch masonry bit
- Plastic expanding anchors
- Angle grinder
- Diamond cutting blade
- Caulking gun
Cut the front base for the flashing. This piece of material is pre-bent in half lengthwise, and should measure 8 inches wider than the chimney. Measure and make a mark 4 inches from each end. Use tin snips to cut from the back corner to the 4-inch mark at the bend. The angle of your cut should be about 45 degrees.
Place the front-base flashing against the front of the chimney and hand-bend the cut corners around the sides of the chimney.
Cut and shape the front pieces of step flashing. At the prefabricated bend, use tin snips to make a cut about 1 inch long. Bend the top edge to fit around the chimney corner. Use a single dollop of caulk on the corner of the front base flashing and set the fitted corner piece of step flashing into place.
Place each piece of step flashing, and fasten the shingles in place over them as you work your way up each side of the chimney.
Fit the chimney saddle flashing into place on the backside of the chimney and nail it into place. Drive galvanised roofing nails every 6 inches along the saddle edges.
Cut grooves into the mortar joints. Use an angle grinder fitted with a diamond blade. Set your cap flashing into place to determine where the grooves should be cut. As you work your way up the chimney, the grooves in the mortar joint will be cut one or two bricks higher for each piece of cap, depending on the slope of the roof.
Cut and bend your front cap flashing piece to fit around corners and into the grooves in the mortar. Use a block of wood and hammer to pound the front cap flashing into place without denting the sheet metal. The flange in the flashing will fit into the cut mortar grooves.
Drill a hole in the top corner of the front cap flashing and into the brick behind it. The hole should measure 3/16 inch wide, and about 1½ inch deep. Insert an expanding plastic anchor into this hole.
Cut the side cap flashing so that each piece overlaps the piece below by about 2 inches. Knock these pieces into place, so that the flanges fit into the mortar grooves. Drill and anchor each piece with expanding anchors.
Install the back cap flashing. Knock the flange into the grooved mortar and fasten the flashing with expanding plastic anchors.
Seal all the points where the cap flashing meets the mortar with polyurethane caulk.
Tips and warnings
- Always take care when working on a rooftop.
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