It is important to have your chimney carefully inspected periodically. Masonry chimney problems are easy to overlook because they are often hidden from view, and they can be dangerous. Avoid walking around on your roof; use binoculars to have a close look from the ground.
What are you looking for when doing this inspection? Large cracks in mortar, cracked or missing bricks, loose or damaged flashing around the base, a damaged chimney cap, or a leaning chimney stack can all signal it is time for a chimney repair project.
Chimney Brick Repair
For crumbling or cracked mortar between bricks, repointing is required. If your chimney has a few cracked or missing bricks, then replace them. Before starting any chimney repair work, the damper should be closed in all flues to avoid getting excess soot and debris in your fireplace.
For repointing, all deteriorated mortar needs be raked out to a constant depth across the mortar joint width. The ideal depth for best adhesion is 2 to 2 ½ times the width of vertical joints, which works out to typically ½ inch to ¾ inches deep. Never use power tools for this step, all joint cleaning should be by hand tools, as time consuming as that may be, otherwise you risk damage to the bricks and underlying mortar. Follow the repointing procedure on our brick repointing page.
For brick replacement, make sure the brick that is missing or to be replaced is not in a critical load-bearing area; consult a mason if you are not sure. Carefully remove the mortar around the deteriorated brick with a small hammer and chisel (make sure you are wearing safety goggles for this step). Remove the mortar to a depth of at least an inch, and then break the brick with the chisel so it can be removed in pieces.
When brick and mortar have been removed all the way to the back of the chimney wall, mix a batch of mortar up, then apply it in a layer to the base of the cavity. Next apply mortar to the top and end sides of the replacement brick, and placing the brick on your hawk, slide it into the hole.
Where required, add mortar to fill gaps or remove excess. When the mortar has hardened enough to take a thumbprint, tool finish the joints per the instruction on this page.
Once a year, chimneys should be inspected for leaks in the flue liner. You can test for leaks by making a smoky fire in the fireplace with damp newspapers or wood, and covering the chimney cap with a piece of heavy wetted cloth. Any leaks will be evident from the smoke issuing from them into the interior of the house.
If needed, a chimney can be flue relined with rigid steel liner or poured concrete. See our page on installing a flue liner. This is not a DIY job unless you are well experienced with masonry repair.
Chimney Flashing Repair
Chimney flashing typically consists of two pieces, a base flashing attached to the roof, and a counterflashing layer attached to the chimney which overlaps the base flashing. If the flashing has been damaged or is deteriorated, it may need replacement, but often it is just that the counterflashing is loose and needs to be repositioned.
If the latter is the case, remove the loose mortar from the joint the flashing was inserted into. When the joint is cleaned out, reinsert the flashing to a depth that will give it a good tight fit, then reapply fresh mortar or recaulk the joint with a good outdoor caulk.
To replace deteriorated or damaged chimney base flashing, chisel out the old mortar in the joints the flashing is inserted into, using a hammer and chisel; always wear safety goggles and heavy gloves and never use power tools for this step.
First remove the counterflashing. Then remove the base flange of the flashing; you will need to lift up the shingles around the base of the chimney in order to find the nails fastening the flashing to the roof sheathing. Use a pry bar and work in temperatures warmer than 50 degrees F so the shingles will be more pliable.
For metal shingles, panel or roll roofing, you may need to call in a flashing contractor. The nails can be pulled out by lifting the proper shingles with the pry bar, taking care to not damage the underlying lining of roofing felt or paper.
Installing the replacement flashing is a reverse of this process. You will need to have new base flashing made from sheet metal, using the old flashing as a pattern.
A good sheet metal shop or contractor will go a long way in helping get the best fit. They can also suggest the best counterflashing pattern for the exact dimensions of your roof and chimney if you provide them with these. After fitting and nailing the flashing to the roof, insert the counterflashing into the mortar joints and apply fresh mortar. The flashing and counterflashing seams should be sealed with mortar or exterior caulk.
Photo by Bryn Pizgauer, Creative Commons Attribution