Waterproof a masonry chimney exterior within a chimney-sweep service scope (repellent, crown sealer, flashing check)

Steps

1. Inspect the chimney crown, masonry, mortar joints, and flashing for existing water damage (spalling brick, efflorescence, cracked crown, rust staining) before applying any sealant, since sealing over active damage does not fix the underlying problem
2. Check the flashing where the chimney meets the roofline for gaps, rust, or improper installation, and refer flashing repair to a roofer/mason if it is outside your scope
3. Select a vapor-permeable masonry water repellent (breathable, not a film-forming sealer) for the vertical masonry surfaces so trapped moisture inside the masonry can still escape as vapor
4. Apply the repellent per the product manufacturer's instructions and coverage rate, avoiding application to areas with active efflorescence or spalling until those issues are addressed separately
5. Seal or repair the chimney crown separately from the vertical masonry treatment, since crowns need a different, more heavy-duty waterproof/crack-resistant coating or a proper crown rebuild if cracked
6. Advise the client this is a maintenance/repellent service, not a substitute for mortar repointing, crown rebuilding, or flashing replacement if those structural issues are present