How to Repair Chimney Cracks

Worker repairing chimney.
Cracks in your chimney can lead to serious problems.­

­You rely on your chimney to channel smoke up and out, but when the chimney's outside or inside walls crack, you may have serious problems. Mortar joints between stone or bricks are likely to flake, wash out or crack after 25 or 30 years of exposure [source: Cornerstone Chimney].

What causes these cracks? The freeze-thaw action of colder climates is usually the culprit. Moisture can make its way into the masonry, damaging it and any attached object made of metal like a lightning rod or satellite dish [source: Cooley].


The risk of damage to your chimney increases if you live in a­n historic home. Before the use of natural gas or oil for heat, coal was used. Chimneys built to accommodate the high heat of burning coal that powered boilers are oversized and too cool for oil or natural gas conditions. This kind of chimney can lead to greater risk of condensation and resulting damage. The topmost area of the interior walls of the chimney are the most vulnerable, as well as being the most difficult to notice unless you spend time on your roof. Unfortunately, neglect and lack of inspection often lead to the need for major repairs [source: Cooley].

Your home's location may also increase your chimney's risk of damage. For example, about 50 percent of houses affected by Seattle's 6.8 Nisqually Earthquake, which occurred in 2001, sustained chimney damage. Because chimneys are essentially rigid, they don't move with a quake, as a wood-framed house can do. Damaged chimneys can allow toxic gases such as carbon monoxide to leak into your house, creating a serious health hazard [source: Perkins].

­Be sure to check the chimney at least every one or two years for damage. You'll want to scour the area for missing or loose flashing -- the metal sheeting that keeps watertight the place where the roof and chimney meet -- and any signs of damage to the masonry, especially cracking [source: Lipford]. Remember that serious problems need the attention of a professional; not all repairs can or should be done by an amateur. And, many communities require a permit for chimney work.

If you find a problem spot on inspection of your chimney, read on for more instructions.


Chimney Crack Repair Tools and Materials


Before beginning repairs, you or someone you hire will need to clear away any debris ­and nests in the chimney. To get rid of creosote, the tar-like material that burning wood creates, you will need a chimney brush. You can rent one that corresponds to the flue's inside diameter. Make certain that you seal the fireplace opening before you start repairing the chimney to keep from making a large and near impossible mess to clean.


You'll need to get safe and sturdy footing on your roof, as this is the most common way to clean the chimney. Insert the brush and move it up and down with enough pressure to remove soot without damaging the structure [source: Contractor's Solutions]. If you need a visual for this and have little luck online, you can pop in a DVD of "Mary Poppins," for inspiration.

Anyone's who worked with chimney's is probably familiar with tuckpointing, which is the replacement of fresh mortar for failed mortar joints. Done correctly, with a mortar-raking tool, the process will clean the damaged mortar. If this isn't enough, a masonry chisel and hammer should do the trick [source: Contractor's Solutions, Do It Yourself].

You may want to use a mortar hawk to get mortar into tough places, like the horizontal joints. For vertical joints, you can use a joint filler to spread the initial layer. Sometimes, it's even useful to mix a concrete fortifier or pigment in the mortar so the end result will match the rest of the chimney [source: Contractor's Solutions, Do It Yourself].

You've gathered your tools and materials. Now you can read on to learn more about repairing stone chimney cracks.



Repairing Stone Chimney Cracks

If caught soon, many cracks are easy enough to repair at home. To do this, clean the area and then simply fill in the crack with caulk or some sort of concrete bonding material. Follow the finishing steps outlined by whichever material you choose, but it's usually a standard procedure of smoothing the bonding materials with a flat edged tool, like a putty knife. If the area is uneven after drying, you can sand it to your liking [source: Repair-home].

If the mortar around the stone has begun to crumble, you'll need to remove it with a small hammer and chisel. Take out portions in a square shape. Try to go at least a half-inch (1.25 cm) deep. Clean and dry the area, so that it is moist, but not wet [source: Repair-home, Warde]. At this point, it's ready for mortar


When you mix the mortar (which needs to be weather-resistant) aim for the consistency of peanut butter. You can use a mortar hawk or a pointing trowel to spread mortar into your damp excavated areas [source: Repair-home, Warde]. The mortar should dry to a stable consistency, but you should still be able to leave a mark with a pressed thumb. Many forms of mortar mix simply need water, so follow the directions and your consistency should be OK. You should start with vertical joints and then move on to horizontal joints, making sure to clear away excess mortar along the way. Repeat this process anywhere you find damage.

To make the most of your repairs, help the mortar set properly by keeping it moist for the first half week it's setting [source: Repair-home].

If your chimney is made of brick rather than of stone, keep reading for more tips.


Repairing Brick Chimney Cracks

Perhaps you've noticed some dampness in your attic. One cause might be a leak in the chimney crown. If there are hairline cracks in your chimney, it's likely that water is entering during pounding rainstorms. These cracks can occur even during new construction, due to shrinkage [source: Carter].

Strong winds can also cause cracks in the mortar. It can take as long as a few months for mortar to reach its full strength. During this curing period, strong winds can create tension. This is why modern builders often insert steel rods into chimneys as they are building [source: Carter].


One solution for leaks in the bricks themselves is to apply a clear silane-siloxane water repellent to the bricks. The water repellent allows the water vapor to escape, but keeps liquid from entering the brick [source: Carter].

If the problem lies with mortar as opposed to the bricks, you know what to do. Head back to the previous page and follow the procedure for mortar repair. You can use this versatile procedure for many chimney types, be it stone, brick or concrete.

Always use a herbicide to kill plants before removing them from a chimney. When alive, they often have their roots or vines settled into the mortar, meaning ripping them away could cause damage. Cut them off at the ground level where they're fully rooted, and then use a putty knife to remove the dead plant [source: Warde].

Keeping your chimney in good repair will allow you to enjoy many hours of safe fireplace use. Use caution when implementing the tips and suggestions you have learned about in this article, and consider hiring a well-respected professional.

For more information, check out the links on the next page.


Lots More Information

Related HowStuffWorks Articles

  • Abstract Masonry "Chimney Problems." (Accessed 1/16/09)
  • "Frequently Asked Questions." (Accessed 1/16/09)
  • Carter, Tim. "Chimney Caps." (Accessed 1/16/09)
  • Carter, Tim. "Cracks in a New Chimney." (Accessed 1/16/09)
  • Contractor's Solutions. "Chimneys." (Accessed 1/14/09)
  • Cooley, Brian L. "Inspecting and Maintaining Industrial Chimneys." (Accessed 1/14/09)
  • Cornerstone Chimney. "Tuckpointing." (Accessed 1/14/09)
  • "Water Damage and Your Masonry Chimney." (Accessed 1/15/09)
  • Lipford, Danny. "Leak-proof Flashing: Fixing Leaks Around Chimneys." (Accessed 1/14/09)
  • Perkins, Broderick. "Chimney Damage Endangers Seattle Quake Area Residents." (Accessed 1/14/09)
  • "Masonry Cracks." (Accessed 1/16/09)
  • Warde, John. "Home Clinic: Chimney Cracks Need Repairs While Still Small." (Accessed 1/16/09)