Chimney Flashing

What Is Chimney Flashing

Chimney flashing is a multi-component waterproofing assembly installed around the perimeter of a chimney where it penetrates the roof. It is not a single piece of metal but a system of coordinated parts: front apron (base) flashing, step flashing along both sides, counter flashing that overlaps the step and base flashing from above, and often a cricket or saddle behind the chimney. This is the most complex flashing detail on most residential roofs because it must seal a large, rigid structure that moves independently of the surrounding roof framing. The assembly must handle water approaching from upslope, prevent lateral water entry at the sides, and manage the debris and water that collect behind the chimney.

Chimney Flashing Components

The front apron flashing is a single bent piece that runs across the downslope face of the chimney. Its lower leg extends under the shingles, and its upper leg extends up the chimney face by at least 4 inches. Step flashing runs up each side of the chimney, with individual L-shaped pieces woven into the shingle courses exactly as they would be at any sidewall. Counter flashing overlaps the top of the base and step flashing. It is typically embedded at least 1 inch into a mortar joint (a slot cut with a grinder called a reglet) and then sealed with polyurethane sealant. Counter flashing folds down over the base and step flashing, creating a two-layer system that allows the roof and chimney to move independently. Behind the chimney, a cricket diverts water around both sides. The IRC requires a cricket when the chimney is wider than 30 inches, but best practice includes one at any width.

Common Chimney Flashing Problems

The most frequent failure point is the sealant at the counter flashing reglet. Polyurethane sealant exposed to weather typically lasts 5 to 10 years before it shrinks, cracks, and pulls away from the mortar, allowing water behind the counter flashing. Mortar joint deterioration can loosen the counter flashing entirely. Behind the chimney, the absence of a cricket allows water and leaf debris to pool against the upslope chimney face, eventually saturating the base flashing and underlying deck. The differential thermal movement between masonry and wood framing stresses every joint in the assembly. In older homes, tar or roof cement was often used in place of properly embedded counter flashing, and these repairs degrade rapidly.

Chimney Flashing Insurance Claims

Chimney flashing claims require careful documentation because the assembly has so many potential failure points, each with different coverage implications. Storm-related damage such as wind-displaced counter flashing, hail dents in the base or step flashing, or a fallen branch that damaged the cricket is typically covered. However, sealant deterioration at the reglet, mortar joint erosion, and general corrosion are wear-and-tear conditions that insurance does not cover. When documenting chimney flashing for a claim, photograph each component separately: the front apron, both side step flashing runs, the counter flashing and reglet, and the cricket or rear transition. Measure each section independently for Xactimate line items. Chimney flashing is one of the most frequently supplemented items because initial estimates often under-scope the work required.