Waterproofing a chimney is a preventative maintenance measure that will help maintain the structure and integrity of your chimney.  Water can bleed through the brickwork and into the chimney flue, which could then end up in a fireplace, run along an interior facial wall, or puddle on the basement floor.  Water is what causes efflorescence stains on the exterior of the chimney.  This is the white, powder-line stain that you see on some chimneys.  The stain is caused by water penetrating and leaching salts from the brick and mortar.  The appearance of mildew or moss growing on the brick and mortar of a chimney is also an indication that the chimney is retaining water and is susceptible to water damage.  This is commonly seen on a chimney that receives very little sunlight, resulting in the brick not being able to dry out completely.  Water can also damage the brick with freeze/thaw damage, causing parts of the brick to crack and spall (chip and break off).

For waterproofing, we use a product called Chimney Saver®.  This is a permeable solution that prevents water from getting inside the chimney but also allows the chimney to release any water that may have built up inside the flue from condensation.  Chimney Saver® is a water repellent, not a water sealer.  Waterproofing sealers can actually cause damage to masonry products because they can trap water vapor inside.  Chimney Saver® is warranted by the manufacturer for 10 years.