How to Waterproof a Chimney: The Ultimate Defense Against Leaks & Decay
By Chimney Insight Team | Updated for Winter Preparedness
To waterproof a chimney effectively, you must address three zones: the Cap, the Crown, and the Masonry. Start by repairing mortar joints and installing a full-coverage chimney cap. Finally, apply a breathable, siloxane-based water repellent (not a silicone sealer) to the bricks. This allows trapped moisture to escape while blocking rain from entering.
Water is the single most destructive force against masonry. While homeowners often worry about how often they should clean a chimney to prevent fires, they frequently overlook the silent destroyer: moisture.
A waterlogged chimney leads to spalling bricks, rusted dampers, and even structural failure. If you have noticed fireplace smoke coming into the house or damp smells during summer, water entry might be interfering with your draft. This guide covers the professional approach to sealing your stack for good.
Step 1: The Inspection (Diagnose Before You Treat)
You cannot waterproof a dirty or damaged chimney. Sealing over cracks just traps moisture inside, accelerating the freeze-thaw damage cycle.
What to Look For:
- Efflorescence: White, powdery stains on the brick? This is salt left behind by water evaporation.
- Cracked Crown: The concrete slab at the top often cracks first.
- Missing Mortar: If the joints are crumbling, you need the best mortar for chimney repair before applying water repellent.
- Flashing Gaps: Check where the chimney meets the roof. This is a common leak point. See our review on the best chimney flashing sealant.
Step 2: Install or Replace the Chimney Cap
Think of the cap as the umbrella for your chimney. Without one, you have a 12-inch hole in your roof inviting rain, snow, and critters.
A good cap also prevents animals from nesting. If you’ve ever wondered how to get a bird out of your chimney, you know prevention is better than the cure. We highly recommend stainless steel over galvanized metal, as it resists rust and pairs well with the best chimney brush for stainless steel liners during cleaning.
Recommendation: Read our detailed breakdown of the best chimney caps for rain.
Step 3: Repair the Crown and Flashing
The crown is the cement “lid” that sheds water away from the flue. If it has hairline cracks, water will seep in and freeze, blowing the crown apart.
For small cracks, use a brushable crown repair sealant. For larger cracks, you may need a trowel-grade cement. While you are up there, check the flashing. If you are seeing leaks in the attic, it’s likely the flashing, not the brick.
ChimneyRx Brushable Crown Repair
Flexible waterproof coating that bridges cracks on the chimney crown. Prevents water intrusion instantly.
Check Price on AmazonFlashSeal Elastomeric Flashing Sealant
Reinforced with fabric, this fixes leaks where the chimney meets the roof shingles. Essential for old flashing.
Check Price on AmazonStep 4: Applying the Water Repellent
This is the main event. Unlike standard paint, which traps moisture, a siloxane/silane water repellent penetrates deep into the masonry pores. It creates a hydrophobic barrier that allows the chimney to “breathe” out internal moisture while blocking external rain.
The Application Process:
- Clean the Brick: Use a pressure washer on low setting (be careful not to damage mortar) or a stiff brush. If you have soot stains, consult our guide on cleaning agents.
- Protect the Roof: Cover your shingles with a drop cloth. Repellents can discolor asphalt shingles.
- Spray Low to High: Using a garden sprayer, apply the repellent starting at the bottom and moving up. This ensures a “flood coat” that saturates the pores.
- Wait for Absorption: The milky white liquid should turn clear as it dries (usually within 10-20 minutes).
Note: Do not use standard exterior paint unless you are looking for a specific aesthetic. If you must paint, ensure it is a masonry-specific formula. Read our comparison of the best paint for chimney brick.
Top Pick: ChimneySaver Water Repellent
The industry standard. 10-year warranty, 100% vapor permeable, and prevents spalling/freeze-thaw damage.
Check Price on AmazonIs the Roof Too High? Let Us Handle It.
Don’t risk your safety on a ladder. Our certified technicians can inspect, repair, and waterproof your chimney for you. Get a free quote today.
*We typically respond within 24 hours.
Frequently Asked Questions
How often should I waterproof my chimney?
If you use a high-quality Siloxane repellent, it typically lasts 10 years. However, you should inspect the crown and cap annually as part of your winter home comfort prep.
Can I use Thompson’s Water Seal on my chimney?
We generally do not recommend it for chimneys. Thompson’s is often silicone-based or paraffin-based, which forms a film that traps moisture inside the brick. This trapped moisture can freeze and pop the face off your bricks (spalling). Always use a “breathable” masonry repellent.
Does homeowners insurance cover chimney leaks?
Typically, insurance covers “sudden and accidental” damage (like a lightning strike hitting the chimney). They usually deny claims related to “maintenance neglect,” which includes slow leaks from lack of waterproofing. For more details, read: Does Homeowners Insurance Cover Chimney Repair?