How to Repair a Chimney Flue Crack

You will need a ladder and a safety harness to secure your trip to the roof to access the top of the chimney. Ideally, you’ll want a partner around to spot you as you ascend and be ready on the ground for any further assistance.

Aside from making the actual climb to the top, you’ll want to feel comfortable standing on the roof by your chimney to do this work. If you have a steeply shaped roof or have any misgivings about spending some time high above the ground for however long it will take to repair the chimney flue crack, then call a chimney repair professional who has done the job safely countless times.

Clay chimney flue tiles are typically two-foot squares. The tiles located at the top of the chimney are often the ones to fail first during your flue’s lifetime. This is good news because these are the only tiles you’ll likely be able to reach as a DIYer; remember that chimney professionals have all the specialized tools and techniques needed to inspect further into the flue. 

Flue tiles can crack due to weather conditions and your home settling over the years. Spalling could be to blame if the tiles aren’t cracked but look pitted or otherwise rough in texture. Condensation causes spalling, so you’ll usually see evidence of spalling tiles at the top of your chimney where moisture collects.

Call a chimney repair professional if you suspect there are more than one or two cracked tiles or if the damaged tiles are too far out of reach to repair safely. At this point, it might be better to install a completely new liner rather than simply make repairs.

Sometimes, it’s not cracked tiles causing the trouble but rather old mortar joints that are the culprit. Mortar joints can erode over the years due to moisture and heat, leaving gaps between the clay tiles in your chimney flue. Without mortar to keep them in place, those tiles will eventually loosen. If this is the case in your chimney, you must contact a pro to have them refilled. 

Typically, the process involves applying fresh mortar to the entire chimney. Then, a foam plug the size of your chimney’s interior is attached to a winch and pulled up through it, spreading the mortar into the joints, where it will dry and harden.

Gently bang the cracked flue tile with the masonry hammer. It should only take a few direct strikes to crack it off entirely, allowing you to reach in to pull out the pieces. If any fall down the chimney instead, you can retrieve and discard them later.

Apply mortar to the back of the new tile and press it into place where the old tile was previously. Add more mortar to the joints surrounding the tile to secure it, then scrape off extra. 

Carefully descend the ladder and allow the fresh mortar time to dry, per the manufacturer’s guidelines.