Chimney Waterproofing and Water-Damaged Masonry

How water gets into a chimney

• From the top: a cracked crown, a missing or loose cap, or a damaged chase cover lets rain reach the flue and the masonry below.
• Through the masonry: brick is porous and mortar joints wear down over time. Water soaks in, and freeze-thaw cycles can push it deeper and widen cracks.
• At the roofline: gaps in flashing let water run behind the metal.
• From inside: condensation can form when warm flue gases meet cooler flue surfaces, especially in a liner that is not suited to the appliance.

Signs of water damage

• White, powdery deposits on the brick (efflorescence) as dissolved salts are left behind
• Spalling, where the face of the brick flakes or pops off
• Soft, sandy, or missing mortar
• Rust stains, a damp smell, or stains on interior walls and ceilings near the chimney
• Rusting damper or firebox parts

These signs show that moisture is moving through the masonry. They do not by themselves say where it enters.

What a water-repellent treatment can and cannot do

A breathable water-repellent product is applied to the outside of sound masonry to reduce how much rain it absorbs while still letting vapor escape. Whether it is appropriate depends on the condition of the brick and mortar, and on whether the product is compatible with the masonry. Treatment generally cannot:

• close a cracked crown or open mortar joints
• replace failed flashing or a missing cap
• reverse brick that has already spalled
• stop moisture that comes from inside the flue

Sealing a chimney with a coating that traps moisture can also cause damage, which is another reason for an assessment first.

A sensible order of work

• Find and correct the entry points: crown, cap or chase cover, flashing, and joints.
• Repair or replace deteriorated mortar and brick. See masonry repair and repointing.
• Only then consider whether a breathable water-repellent treatment makes sense for the condition of the masonry.

Questions to ask

• Where do you think the water is entering, and how did you confirm it?
• Is the masonry sound enough to be treated, or does it need repair first?
• Is the product breathable, and what does the manufacturer say about its use on this type of masonry?
• What will we look at after the next few rainstorms to see if the repair worked?

About our services

AM Chimney Sweep inspects chimneys, repairs crowns and masonry, replaces flashing, and installs caps and chase covers. Because every chimney is different, an inspection is the way to find where water is entering and which repairs apply.

Related pages

Chimney flashing and roof leaks, crown repair, caps and chase covers, and projects in Our Work.