By Johnathan Mason, Ace Chimney Service.
Water is what destroys masonry chimneys in northern Illinois — not fire, not age, not use. Brick absorbs it, winter freezes it, and each cycle pushes the material apart a little more. Waterproofing is the cheap step that slows that down, and it is also the step most often done with entirely the wrong product. In Cary, where plenty of chimneys are exposed on all four sides, it earns its keep.
The rule: breathable, not sealed
A chimney has to be able to let moisture vapour out. Masonry always has some moisture in it, and combustion adds more from the inside. A vapour-permeable water repellent — typically a siloxane or silane formulation made for masonry — stops liquid water from soaking in while still letting vapour escape.
A film-forming sealer does the opposite. Products like acrylic sealers, waterproof paint, and general-purpose coatings lay down a skin. Whatever moisture is already in the brick, or arrives from inside, is now trapped behind that skin. It freezes there, and it takes the face of the brick off. We see chimneys every year that were spalling badly, and the cause was a coating applied with good intentions.
Sequence matters more than the coating
Waterproofing is the last step, not the first. Applied over failed mortar joints, a cracked crown, or bad flashing, it seals nothing useful, because water is entering through the gaps rather than through the brick face. The correct order is: repair the crown, repoint failing joints, correct the flashing, then apply the repellent to sound masonry. Our chimney repair page covers the steps that come first.
How it is applied
Low-pressure saturation, working bottom to top, applied heavily enough that the material runs down the face several inches ahead of the spray. Masonry needs to be clean and dry, and the weather needs to be dry and above freezing for the cure. That is why this is autumn or spring work here and not a January job.
What you get out of it
A properly repelled chimney sheds water instead of drinking it, which slows freeze-thaw damage, reduces efflorescence staining, and helps keep water out of the space around the flue. Reapplication is generally measured in years, not seasons. It is not a substitute for a cap, a sound crown, or good flashing — it is the layer that protects the brick between them. See the full list on chimney services.
Ace Chimney Service is insured and bonded with 29 years in the trade, gives the price up front, and serves Crystal Lake and McHenry County seven days a week, 7 AM to 7 PM. Call (815) 320-0883.
Frequently asked questions
Can I use a hardware-store waterproofing sealer?
Only if it is a vapour-permeable masonry repellent. Film-forming sealers and waterproof paints trap moisture and cause spalling.
How often does waterproofing need redoing?
Typically several years between applications, depending on exposure and the product used.
Will it fix an existing leak?
No. Leaks come through the crown, flashing, cap, or open joints. Those get repaired first, and the repellent protects the brick afterwards.
Can I waterproof a chimney that is already spalling?
Damaged brick must be addressed first. Coating failing masonry locks the problem in.
Does it change the look of the brick?
A proper repellent soaks in and dries clear without gloss. Coatings that visibly change the surface are the kind to avoid.
What time of year is best?
Dry weather above freezing, with dry masonry. Autumn and spring suit our climate.
Should the crown be sealed too?
The crown gets its own treatment, and cracks in it are repaired rather than coated over.
Is waterproofing worth it on a newer chimney?
Often yes. Protecting sound masonry from the start is far cheaper than repointing it in fifteen years.
Published September 9, 2026