How to Install Metal Flashing Around a Chimney
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How to Install Metal Flashing Around a Chimney

Replacing metal flashing can be a necessary part of controlling chimney leaks and prevent further wear damage. Precautions should be taken through careful inspection prior to repair or installation, followed by safe procedures and implementation of proper steps to ensure installation of a safe durable replacement flashing around the chimney where there is a needed repair.

Why Replace Metal Chimney Flashing?

Chimney damage can lead to leakage of harmful gases and potential fire hazards, so it is important to identify and repair damaged metal chimney flashing around the base.  Proper detection of any blocks or leaks due to damaged flashing or mortar is necessary first, then if flashing replacement is needed follow these steps for proper repair and installation. 

Be Safe and Smart

Always inspect the chimney first to determine cause of leaks by looking for blocks or damage and testing for gases using a carbon monoxide detector.  Determine if mortar needs repair as well as metal flashing as well as any other possible causes of blocks and leaks.  Look for hot spots in cracks as well as leaking smoke wherever chimney is exposed to the interior attic area.  Use protective goggles, heavy work gloves, and the proper tools to prepare to install your metal flashing.

Installing Replacement Flashing

Follow these steps to remove any old and damaged flashing around the base of the chimney.  You may have to replace some of the adjacent shingles as they may be worn as well. 

Chisel out any mortar along the chimney using a masonry chisel.  Try to leave about a 1 1/2 inch groove.  Then use a pry bar or large hammer to pull out any nails holding the metal chimney flashing in place.

Cut out new sheets of flashing identical in shape and size to the old using metal snips by laying the old pieces of flashing on top as a template.  Decide what material of metal flashing to use. Aluminum flashing is more flexible and easier to work with, however galvanized steel is harder and thus more durable.  Cutting aluminum and other metals can be sharp as a razor so use heavy duty work gloves to protect your hands.

Install your roof attached flashing first by either sliding the edges into the chiseled out mortar groove and nailing down the sides with rust proof galvanized roofing nails.  If shingles are in tact nail the flashing directly beneath them. 

Attach your side flashing next around your chimney by again bending and sliding the metal into the chiseled out mortar grooves and nailing the overlapping piece through the base flashing working from bottom to top. Use a saw horse or other hard straight edged surface to bend your metal into place.  Always begin with the base flashing first the work your way up by overlapping each side flashing, finishing with the up roof piece last.

Seal your metal sheets into the chimney groove using fresh mortar and a hand towel.  Apply roofing cement to the vertical metal chimney flashing edges to insure water protection if they cannot be mortared in. 

Replace any damaged or worn shingles that were removed and make sure they overlap the nails used to secure the metal chimney flashing around the base of the roof.

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