Homeowners don’t often think about chimney flashing — the topic usually only comes up when something has gone wrong, like after water has seeped in along the chimney wall, causing mold, wood rot, or other serious issues. But a roof is a connected system; even a small vent can be the source of water damage if it’s insufficiently sealed and left unmonitored. And the larger the penetration through a roof, the more opportunities water has to find ways to get inside the home, and these issues can remain hidden for years.
That’s why as much attention must be paid to the chimney flashing as is paid to the chimney itself.
Flashing is a water-impervious material designed to redirect water away from corners, edges, and penetration points in a building. Flashing can be found most often around chimneys, windows, doors, and the valleys on a roof.
Chimney flashing is a specific system of flashing products designed to work together to redirect water around a chimney. Chimney flashing systems are complex because of the role that they play in connecting two dissimilar materials that are often supported by different foundations.
Whether built of brick, stone, or metal, chimneys are often constructed to stand on their own base. A chimney may be located along an outside wall, or emerge from the center of a building, particularly if it was originally designed as the primary heat source for the structure.
The free-standing nature of most chimneys often means that changes to the weather won’t cause them to move in the same way that the rest of the house moves. It’s most noticeable in the winter months; the peak time when heat flows through many chimneys is when the weather outside is at or below freezing.
So, to make sure that expansion and contraction along the junction between the chimney and the adjoining roof panels doesn’t break the waterproof seal, chimney flashing has to be designed to withstand movement. Even if you’re not experiencing water infiltration today, it may be a good idea to get your roofing system assessed for weak points to stop damage before it happens.
Chimney flashing works with other components to protect the roof. The flashing, the shingles, the decking, the underlayment, and the ice and water shield are all working together to move debris and water down to the gutters and downspouts and away from the house. Importantly, the ice and water shield acts like a flexible membrane, able to bend in right angles and climb the lower part of the chimney where it connects with the roof. This shield membrane is often the first step in protecting those spots where a vertical structure meets the roof line.
Counter flashing is a metal component attached to a chimney that redirects water, snow, and ice over the top of the step flashing and down to the roofing shingles, away from the base of the chimney. Counter flashing may be made from a variety of metals — commonly steel, copper or aluminum. Decorative value may play a role in the choice of counter flashing, but chemical compatibility with the chimney mortar is an important consideration that masonry experts must also consider. Counter flashing overlays, but is not directly attached to, the step flashing or the roof itself to allow for thermal and weather-related movement between the roof and the chimney.
In brick chimneys, it’s not uncommon to see counter flashing embedded into the mortar between bricks, attached with masonry nails to the brick chimney. Having the metal counter flashing curve out from the masonry allows it to collect accumulated ice or snow and move it away from the base. These installations are fairly common, with linear cuts and repeating patterns to the counter flashing making the process repetitive and fairly quick to install.
With stacked stone chimneys, counter flashing may be even more complex. Some stacked stone chimneys may have been installed with counter flashing buried beneath the stone facade, while others may have required complex shapes to be cut into the flashing to follow the mortar lines between stones. In both cases, the goal remains the same — move water away from the chimney — but the solutions may differ significantly, particularly if there is an existing water problem that is being addressed.
Stacked stone that was mortared in place over top of counter flashing may require a removal and rebuild of part or all of the chimney facade, depending on the source of the water. Counter flashing installed over the top of the chimney will likely require a customized approach that matches the shape of the mortar between the natural stones.
Chimney flashing isn’t typically the detail most homeowners focus on during a home restoration, yet the attention to detail can make a serious difference in the preservation of a historic home. If you are seeing signs of water infiltration adjacent to your home’s chimney, or if you’re planning to install a new historic roof, let one of our restoration specialists handle your inspection and diagnosis and offer you the best plan of action.