Water and moisture
Do chimneys need a water repellent?
Water repellents are sold as a cure for leaking chimneys. They can help, but only after the real problems are fixed.
Short answer. A breathable masonry water repellent reduces how much water brick absorbs. It does not fix cracks, open mortar joints or a failed crown, and a non-breathable sealer can trap moisture and make damage worse. Repair first, then consider a repellent made for masonry.
What a repellent does
Brick and mortar are porous and absorb water. A repellent soaks into the surface and makes it shed water while still letting water vapor out. That is the important property. A coating that seals the surface shut can trap moisture inside, where freezing weather can then damage it.
What it does not do
- It does not close cracks or open joints.
- It does not repair a failed crown or flashing.
- It does not remove existing moisture or salts.
When it might make sense
| Situation | Repellent? |
|---|---|
| Sound masonry, sound crown, still absorbs water | May help |
| Open joints or a cracked crown | Repair first |
| Brick already spalling badly | Repair first; consult a mason |
| White staining keeps returning | Find the moisture source first |
| Painted or sealed surface | Ask the manufacturer what is compatible |
Questions to ask
- Is the product breathable?
- Is it intended for chimney masonry?
- What repairs should be done first?
See What causes white stains on chimney brick? for how moisture moves through masonry.
Common misconceptions
"It seals the chimney." A suitable product should let vapor out.
"One coat forever." Treatments wear and may need reapplying.
About this guide. Published by Dr. Sweeper as general homeowner education, last updated October 1, 2026. Sources are linked where they are cited. What applies to your home can depend on your local building code, your appliance and its manufacturer's instructions, and the actual condition of your chimney, so this guide is not a substitute for an inspection.
If you need help
Related services
Repointing
Replacing deteriorated mortar joints.
Learn moreCrown repair
Repair of the cement or masonry top of the chimney.
Learn moreChimney inspection
A condition check of the chimney system.
Learn moreChimney Leak Repair
Trace water to its source.
Learn moreChimney Maintenance
A simple yearly routine.
Learn moreChimney Waterproofing
A breathable repellent on sound masonry.
Learn moreReady to have your chimney looked at?
Call us, or send a scheduling request. We will contact you to confirm a time. Your appointment is not confirmed until we do.