Because they are tall and frequently exposed on three or four sides, chimneys are especially vulnerable to water problems. Efflorescence, freeze-thaw damage and its attendant spalling, and sometimes even structural damage can occur unless appropriate steps are taken to prevent it. One such step is to install flashing at the base, roof intersection, and crown of masonry chimneys. ASSESS RISK OF DAMAGE Different sources recommend different chimney flashing details. To select the appropriate details for any project, the designer has to understand the flashing's function in each location and analyze the risk of water penetration and resultant damage. When in doubt, the designer and contractor should opt for more protection rather than less. FLASH THE CHIMNEY BASE For exterior chimneys, built outside the building's exterior wall, base flashing is needed. The flashing should extend beyond the face of the foundation and turn down to form a drip. ROOF INTERSECTION Flashing at the intersection of the roof and the chimney prevents water from entering the gap between these elements and reaching the interior of the building. CHIMNEY CROWN FLASHING A flashing system is needed at the chimney crown (or cap) to prevent water from entering the gap between the crown itself and the flue tile. MATERIALS AND WORKMANSHIP Once installed, flashing should remain serviceable for the life of the masonry. Always consider quality and durability first when selecting flashing materials. Even well-designed flashing systems using high-quality materials are effective only when installed correctly.