chimney flashing
A strip of material around the base of a chimney, traditionally lead and sometimes including a short length of flue, that prevents rainwater from entering a building.
chimney flashing: the seal that keeps rain out of your roof
Chimney flashing is a barrier of sheet material installed where the chimney passes through the roof. Its job is straightforward: block rainwater from running down the outside of the chimney and into the gap between the chimney and the roof opening. Without it, water penetrates the roof structure, rots framing, and causes interior damage that costs far more to repair than the flashing itself.
The material sits in two distinct zones. The upper section is embedded into the mortar joints of the chimney itself, typically driven 4 inches into the brick or stone. The lower section slides under the roof covering on the upslope side and over the roof covering on the downslope side. This overlapping arrangement, combined with proper slope and drainage channels, forces water to run off rather than collect.
Materials and variants
Lead was the traditional choice for centuries because it is soft, durable, and easily shaped. It forms a lasting seal without cracking as the chimney and roof expand and contract at different rates. Copper is more expensive but lasts 50+ years and weathers to a stable patina. Aluminum is cheaper and lightweight but less durable in freeze-thaw climates. Galvanized steel flashing lasts 20 to 30 years; stainless steel lasts longer but costs more. Step flashing, used on sloped roofs, consists of overlapping rectangular pieces that step down the roof line rather than a single large sheet.
The critical failure point is the junction between flashing and chimney. If the sealant or mortar cracks, water follows the flashing like a gutter straight into the structure. Many problems originate from flashing installed too low or with inadequate overlap, or from settling that breaks the seal. Poorly maintained mortar joints in old flashing allow water to work sideways behind the flashing itself. Regular inspection of mortar, sealant, and the condition of the material itself is far cheaper than remedying interior water damage.
Chimney flashing is part of the broader class of penetration flashing used around skylights, vents, and other roof intrusions. The principles are identical: overlap materials in the correct order, slope everything to drain away from the building, and seal the upper edges into the structure that passes through the roof. Building codes specify minimum lap distances and overlap requirements; these vary by climate and roof pitch. Poor flashing installation remains one of the most common sources of roof leaks.