New England averages around 110 freeze-thaw cycles per year — days where the temperature crosses above and below 32°F. For a brick chimney, that means water absorbed into the masonry freezes and expands, then thaws and contracts, over a hundred times each year. Over a decade, that cycle opens mortar joints, pops brick faces, and works cracks progressively wider. Waterproofing doesn't stop this entirely, but it slows it significantly.
Why Masonry Absorbs Water
Brick is a fired clay product — porous by nature. The mortar between bricks is even more absorbent. A chimney that's exposed to rain, snow, and humidity year-round is continuously absorbing and releasing moisture. New masonry with sealed joints and a sound crown sheds most water. Old masonry with weathered mortar and decades of surface erosion absorbs a much higher percentage of each rainfall.
The Problem with Most Sealers
Not all chimney waterproofing products are created equal, and this is where well-meaning DIY attempts sometimes cause more harm than good. Film-forming sealers — the kind used on driveways and sidewalks — create a surface coating that blocks moisture from entering but also traps moisture already in the masonry. When that trapped moisture freezes, it has nowhere to go but outward, which dramatically accelerates spalling. These products are not appropriate for chimney use.
The right product for chimneys is a vapor-permeable (breathable) water repellent. These work by penetrating the masonry and lining the pores with a hydrophobic compound, reducing absorption while still allowing water vapor to escape. The masonry can still breathe — moisture already inside can work its way out — but water from rain and snow is largely repelled.
Never use a driveway or deck sealant on a chimney. These products trap moisture rather than allowing the masonry to breathe, which can dramatically accelerate spalling. Use only products rated specifically for chimneys and masonry.
When to Waterproof
Waterproofing works best on masonry that's in decent condition — existing repairs completed, crown intact, mortar joints sound. Applying sealant over deteriorated mortar or a damaged crown just locks in the problems. The proper sequence: repair everything that needs repairing first, then waterproof. Using waterproofing as a substitute for repair doesn't work.
Most chimneys benefit from reapplication every 5–10 years depending on the product and sun exposure. South-facing chimneys that get direct sun in addition to rain exposure may need it more frequently.
Spring Is the Right Time
We recommend waterproofing after the spring inspection season — once you've had the chimney assessed, any needed chimney repairs made, and the masonry has dried out after winter. Applying sealant to wet masonry traps moisture inside; the masonry needs to be dry throughout for the product to penetrate properly. In New England, late April through June is typically ideal.
Want Your Chimney Waterproofed This Spring?
We assess the masonry first, handle any needed repairs, then apply a proper vapor-permeable treatment. One service visit, done right.
Schedule a Spring Inspection