Home  Basic Chimney Flashing
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Monday, 14 January 2008 |
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Basic Chimney Flashing Installation
At Roofing Contractor Review we recognize that there is more than one way to install chimney flashing. But there are basic requirements that are important in order to insure a problem free installation that will last the life of the roofing material installed. Compatible materials, separate sub and counter flashings, appropriate fasteners and sealants, etc., are all part of a good installation and are evaluated accordingly. This page on a simple, basic chimney installation illustrates and explains some of the techniques and practices we look at when reviewing a contractor’s workmanship. Different roofing materials such as slate, tile or metal roofing, on stone or stucco chimneys may require different but similar techniques.
 Basic chimney flashing
This is a basic chimney flashing installation. The chimney here is at the top ridge of a contemporary style house and has no saddle or cricket. The previous roofing and flashing has been removed. The excess tar that was smeared around the previous flashing and on the bricks has been scraped off as much as possible. New felt/tar paper has been installed over the sheathing and the new asphalt roof shingles have been run up to the chimney.
Chimney Apron Flashing
 Chimney apron flashing
First, measurements are taken of the chimney after the roofing and flashing have been removed. Measure the width of the chimney and add 3 inches to either side. The apron should extend approximately 5 inches onto the roof and cover the shingle below at least to the exposure line. This will match the roofing material exposure, and prevent rain from blowing under the flashing. The apron extends vertically 4 inches onto the chimney. The corners are cut diagonally from the outside of the vertical section to the corner of the chimney and roof. The upper tab is bent around the chimney and the lower tab is flattened against the roof and secured with a galvanized nail.
Shingle Flashing Tins
 Flashing tins
Shingle flashing tins are cut from painted aluminum sheet metal stock at 7 inches wide by 8 inches long. Bent in a brake to allow 4 inches on the roof and 3 inches up the chimney. The tins will overlap 3 inches under the shingles. The first tin is installed past the lower corner of the chimney by approximately one and one half inches. Cut this piece about one half inch on the vertical leg and bend the upper tab around the front of the chimney. A bit of quality flashing sealant in the corner will keep out any water. Alternating the shingles and flashing tins will ensure that any water that blows against the chimney and under the shingles will run onto the tin and out onto the shingle below. Leave a bit of space between the edge of the shingle and the corner of the tin. This should allow any debris that blows against the chimney to be washed away and not block the water flow.
Installing the Counter Flashing
 Counter flashing
First measure the face of the chimney and add three inches to each side. Then measure the height from the roof line to the mortar joint you want to cover. Deduct about one inch to allow for roof movement and the height of the side shingle tins. This counter flashing was made to cover the existing old joint and the tar that was smeared on the brick. Make a bend on the counter flashing both top and bottom one half inch from the edge. This will stiffen the aluminum and remove some of the waviness inherent in aluminum sheet metal. It will also make for a tight fit over the apron flashing and will allow the top edge of the metal to sit into the mortar joint. Cut a slight angle on each end to allow the excess metal to follow the roof pitch and cut a small notch at the top corner, then bend the tabs around the chimney. Over bending the metal will allow the counter flashing to hold itself temporarily in place. About every foot along the top edge of the flashing, at the mortar joint, punch or drill a small hole in the metal. Then use a carbide tipped masonry drill to drill a pilot hole into the mortar joint about one inch deep. Tap a hardened concrete nail through the metal and into the drilled hole to secure the flashing.
Installing the Chimney Side Counter Flashing.
 Side counter flashing
First determine the mortar joint you will follow. In the picture we are making a large piece of flashing to cover the old tar that was smeared on the chimney. Measure from the front face along the morter line to a couple inches past the roof peak. Take a measurement vertically from the mortar joint down to the bottom corner of the apron flashing. At the other end, measure from a point on the top mortar joint even with the peak of the roof. Measure down to approximately one inch above the shingle flashing. Add three inches to the front of this flashing to wrap around the front of the chimney. The front corner should be cut at an angle to match the horizontal line of the apron flashing.Put a half inch bend top and bottom to stiffen the aluminum and allow the top of the flashing to set into the mortar joint. Secure with hardened concrete nails tapped into pre drilled holes.
The Finished Chimney Flashing.
 Finished chimney flashing
Finish the flashing by cleaning any dust from the mortar joint and applying a good bead of flashing sealant. Here we've used a high quality polyurethane sealant. Press the sealant against the flashing and mortar joint with a scrap piece of aluminum to ensure a tight seal. It may be necessary to nail the front of the apron flashing to keep it from buckling. Be sure to place a heavy bead of sealant on these nails.
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