How to Waterproof Your Chimney and Prevent Chimney Leaks

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If you are like most homeowners, your chimney is not something you think about until you have a problem. Because chimneys project above the roof line, they are exposed to more severe weather than any other part of your home. Spending a little time to protect your chimney now may save you thousands of dollars down the road after more serious damage occurs.

Chimneys take a beating from severe weather year round. Water penetration is the single biggest cause of deterioration and damage. Waterproofing your chimney is a simple and easy task that will extend the life of your chimney for years and protect against expensive repairs.

Chimney leaks are a frustrating problem for many homeowners. Waterproofing your chimney is the best way to solve chimney leaking problems. While there are several potential sources of water intrusion on a chimney, the brick and mortar joints is usually the most obvious place to start.

Note: Before you start, make sure the pitch of your roof isn't too steep, and you are comfortable climbing up on your roof. Also, make sure to use some type of fall protection to prevent serious injury. If you're not comfortable with this, it might be a good idea to hire a professional chimney sweep. You can search for a qualified sweep in your area by going to

Step 1 – Measure the Surface Area of Your Chimney

The first step to waterproofing your chimney is to determine how much material is needed. Measure all the sides of your chimney to determine the total area. Calculate the area of each side of your chimney then add all 4 sides together to come up with the total area.

Step 2 – Clean the chimney

chimney-moldIf the chimney has any mold, dirt, rust, creosote stains, efflorescence or any other foreign matter, you will want to remove it first. Use ChimneyRx Safer Brick & Masonry Cleaner to thoroughly clean the surface of the chimney. You can mix it 4 parts water to 1 part cleaner and apply it with a pump-up garden sprayer.

One concentrated gallon makes 5 gallons of diluted cleaner and will cover up to 625 sq. ft. If necessary, scrub any tough areas with a stiff bristle brush and then rinse thoroughly with a hose. A power washer can be substituted for scrubbing if desired. Once cleaned, allow the chimney to dry out for 24 hours.

Step 3 – Protect Non-Target Areas

This step is important because getting overspray on the shingles can cause discoloration. Use a canvas drop cloth or a material that absorbs well around the base of the chimney. Don't use plastic drop cloths because the material can become slippery and the overspray can run off onto the roof. Also, the slick surface can potentially be a slip hazard if you accidentally step on it while on the roof.

Also, if there are any windows or siding within range of overspray you will want to protect them as well. Overspray on these areas can easily be wiped off while still wet, but once the water repellent is allowed to dry it becomes nearly impossible to remove.

Step 4 – Minor Chimney Repairs

If the chimney has minor cracks, repair these before you waterproof it. ChimneyRx Crack & Joint Sealant is a great product for repairing minor cracks up to 1/8" wide on mortar joints. It's kind of like paintable caulk that dries to a translucent, milky-white color. You won't notice it on the light gray mortar joints, but try to keep it off the bricks as it can become more noticeable over time. One pint of Crack & Joint Sealant is enough to treat the mortar joints surrounding approximately 137 bricks.


If the chimney has more severe damage that requires tuckpointing or rebuilding, you may consider hiring a pro. Tuckpointing, brick replacement and chimney rebuilding are beyond the scope of this article.

If your chimney has recently been tuckpointed, allow 3 or 4 days for the mortar to cure before applying a water repellent. If major areas of the chimney have been totally rebuilt, allow a full 28 days for the new masonry to cure before applying a water repellent. Applying a water repellent on masonry that has not cured will result in a shortened lifespan as the water repellent won't bond properly.

Step 5 – Choose a Water Repellent

sealers-vs-water-repellentsBefore we get to the how-to, one important point to consider is the difference between a sealer and a water repellent. Sealers will form a film on the surface and will trap moisture inside. Water repellents such as ChimneyRx Masonry Chimney Water Repellent are the opposite. They are breathable and will allow the surface to breathe, while keeping water out.

When treating chimneys, it's very important not to use a sealer. As chimneys heat up and cool down, moisture is created inside the chimney. This moisture will normally escape through the bricks and mortar joints. If a sealer is used, the moisture will remain trapped. When freezing weather occurs, this trapped moisture will freeze and then thaw as it warms up. When water freezes, it expands. As it expands, it will cause the faces of the bricks to pop off (called spalling). Once this happens, you're getting into some major expense as the bricks cannot be repaired. You will have to have the bricks replaced or the chimney rebuilt if enough of the bricks are spalling. Treat your chimney with a breathable water repellent and you won't run into this problem.

Step 6 – Apply the Water Repellent

Once your drop cloths are in place, apply the water repellent using a pump-up garden sprayer with a fan type nozzle. One gallon of ChimneyRx Masonry Chimney Water Repellent covers about 100 sq. ft. (for 2 coats). Use your area calculation in step 1 to determine how much water repellent material you'll need.

applying-water-repellentStart at the bottom of the chimney and gradually work your way to the top. Apply a flood coat, meaning spray it on heavy enough so that it runs down about 6"-8" from where you're spraying. Move side to side, gradually moving towards the top of the chimney.

Applying a flood coat will cause the surface to become totally saturated and allow as much water repellent to soak in as possible. It's also a good idea to keep a soft bristle brush handy. Agitating the wet surface with this brush will help ensure even distribution and break the surface tension to allow even more of the water repellent to absorb. The deeper the absorption, the longer the waterproofing job will last.

For optimum performance, apply a second wet-on-wet coat within 3-5 minutes. The water repellent will take about 2-6 hours to dry. Make sure there's going to be at least a 6 hour window of dry weather before waterproofing your chimney.

Note: Over applying the water repellent can result in a slight darkening of the surface.

Step 7 – Cleanup

Once you've waterproofed your chimney it's time to clean up. Carefully remove the drop cloths and any supplies you have on the roof top. It's a good idea to have a second helper to hand things to down on the ground. Clean out your sprayer and brush with soap and water and you're done.

Further Reading

Once you've successfully waterproofed your chimney, you may want to take a look at chimney crown repairs, or repairing chimney flashing.

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