The Best Prevention You Can Buy — If It's the Right Product
Water causes nearly every chimney problem in Maryland. Waterproofing the masonry is the best preventive step available. But the product matters more than the application — the wrong sealer actively destroys brick.
Breathable: What You Want
Vapor-permeable silane/siloxane sealers soak into the brick and line the pores. Liquid rain beads and runs off; water vapor from inside still escapes. The chimney stays dry and keeps drying. These last 7 to 10 years, don't change the brick's appearance, and are what professionals use.
Film-Forming: What Destroys
Paint, silicone coatings, and most hardware-store "masonry waterproofers" form a skin on the surface. Rain stays out — but so does every bit of moisture already inside the wall. Trapped water freezes in winter, expands, and blows the coating off along with the brick faces underneath. We've seen chimneys damaged worse by the wrong sealer than by no sealer at all.
The Test
If a product changes how the brick looks — sheen, darkening, visible coating — it's film-forming and wrong for chimneys. The right product is invisible after application.
When to Seal
Right after repointing or a rebuild (fresh mortar is porous — protect the investment immediately). When efflorescence appears (white staining = water traveling through). Or as maintenance every 7-10 years on a sound chimney. Never as a substitute for fixing structural damage first.