Chimney Flashing Explained

Where two different structures meet

A masonry chimney and the roof around it are separate structures. Brick Industry Association guidance describes the chimney resting on its own foundation, sized to carry the full weight of the fireplace and chimney. The roof sits on the house framing. The two expand, shrink and settle on different schedules, so the joint between them can never be a rigid, glued seam for long.

Flashing handles that by splitting the job. One set of metal pieces belongs to the roof and moves with it. A second set belongs to the chimney and moves with the brick. They overlap without being fastened to each other, so they can slide a little while still shedding water downhill.

Following the water: four places rain can land

The easiest way to understand flashing is to follow rain to each side of the chimney and ask which piece of metal handles it.

1. The uphill sideWater running down the roof hits the back of the chimney first. On a narrow chimney, a back pan turned up the brick sends it to the sides. On a wider chimney, a cricket (also called a saddle) divides the flow before it reaches the brick.
2. The two sidesStep flashing takes over. These are short L-shaped pieces, one per course of shingles, each lapped under the shingle above and over the shingle below, with one leg turned up against the brick. The Copper Development Association's flashing details show each piece extending 2 inches above its shingle course.
3. The downhill faceBase flashing, sometimes called the apron, runs across the front of the chimney, turned up the brick and laid out over the shingles below, so the water that came around the sides drains onto the roof surface.
4. The brick face above the roofRain hitting the chimney runs down the brick. Counterflashing anchored in the masonry laps down over the upturned legs of all the roof-side flashing, so that water lands on metal instead of slipping behind it.

There is also a fifth path that no outside metal can stop: water that soaks into the brick and mortar and travels down inside the wall. That is the job of through-wall flashing, covered below.

The layers in one picture

Counterflashing: how it is held in the brick

Counterflashing only works if it is locked into the masonry. The Brick Industry Association's chimney note describes counterflashing inserted 3/4 to 1 inch into a mortar joint, mortared solidly in place, and lapping the base flashing by at least 3 inches. It also describes base flashing running at least 4 inches up the chimney face and along the roof.

There are three common ways to anchor it:

• Into a mortar joint as the chimney is built or repointed. On the sides of a chimney this produces the familiar stair-step pattern, because each piece follows a horizontal joint.
• Into a reglet. A slot is cut into the masonry and the flashing edge is set into it. The Copper Development Association shows one-piece cap flashing held in a reglet with lead wedges, with the reglet filled with sealant.
• As a two-piece receiver and counterflashing. A receiver is anchored in the masonry and a separate counterflashing clips into it, which makes later roof work easier because the outer piece can be removed and reinstalled.

A strip of metal nailed or glued to the face of the brick, with caulk along its top edge, is held only by the sealant. That arrangement depends entirely on a material that dries and cracks in sun and frost.

The cricket: when the back of the chimney becomes a dam

A chimney that is wide across the slope blocks a lot of roof. Rain, leaves and snow slide down and stop against its back wall. A cricket is a small peaked roof built behind the chimney to split that flow and send it around both sides.

Brick Industry Association guidance mentions crickets usually for chimneys wider than 30 inches parallel to the ridge. The 2003 International Residential Code text, as reproduced by Buckley Rumford Fireplaces, called for a cricket when that dimension is greater than 30 inches and the chimney does not intersect the ridge, with the cricket's height set by a table based on roof pitch. New Jersey enforces a later code edition, so the local construction office has the final word on what applies to a given roof.

A cricket has its own flashing: its valleys tie into the roofing, and the edge where it meets the brick is counterflashed like every other side.

Through-wall flashing: the layer you cannot see

Brick and mortar absorb water. In a heavy, wind-driven rain, the part of a chimney above the roof can take in enough water to carry it down inside the masonry, past all the outside flashing, and into the attic or the ceiling below.

Through-wall flashing is a sheet of impervious material built across the masonry, just above the roof line. The Brick Industry Association defines it as a material that contains water that has gotten past the outer wythe and directs it back to the exterior. Its outer edge exits the face of the brick, and small openings called weeps let collected water drain out. The same BIA note says that at a roof and wall intersection, through-wall flashing and counterflashing may both be needed, depending on the wall.

Through-wall flashing goes in while the chimney is being built or rebuilt. Not every chimney has it, and whether yours does usually cannot be confirmed from outside unless you can see a flashing edge and weeps along a mortar joint above the roof.

How each layer fails

Because the system is layered, a leak usually traces back to one layer that stopped passing water to the next.

• Step flashing fails when it is missing, when a single long piece was used instead of one piece per course, when it sits on top of the shingles instead of woven between them, or when old rusted pieces were left in place during a reroof.
• Base or apron flashing fails when it is too short to reach out over the shingles, when it has rusted through, or when it ends under the roofing instead of on top of it.
• Counterflashing fails when the mortar holding it erodes and the edge pulls out of the joint, when it was bent up for roofing work and never seated again, or when it is too short to lap the flashing below.
• The cricket fails when it is missing behind a wide chimney, too low to clear debris, or flashed poorly where its valleys meet the main roof.
• Through-wall flashing fails when it is absent, when it does not extend through to the face, or when its weeps are filled with mortar.
• Sealant is not a layer. When caulk or roofing cement is the main thing keeping water out, it is a sign that one of the layers above is missing or misplaced.

Water above the roof line can also enter through the crown, a cracked flue tile or porous brick, and the stain inside can look identical. Our guide to why chimneys leak covers those other paths.

What you can learn without climbing

From the ground, with binoculars or a phone zoom, you can usually see:

• Whether the counterflashing on the sides follows a stair-step pattern or runs in a straight diagonal line across the brick face.
• Shiny or dark smears along the top edge of the flashing, which usually means a sealant bead.
• Rust streaks below the flashing, or a gap you can see daylight through.
• Whether there is a peaked cricket behind a wide chimney.

From the attic, with a flashlight and only where flooring makes it safe to stand, look at the roof boards and rafters right beside the chimney after a rain. Note which side is darkest. That observation, plus the weather that caused it, is useful to whoever checks the roof. It cannot tell you which layer failed. Our assessments include a roof check when it is safe and accessible, a look in the attic, and controlled water testing when the source is unclear. See chimney flashing repair, and if new shingles are in your plans, planning the chimney with a roof replacement.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 19B: Residential Chimneys, Design and Construction (archived PDF copy)
2. Brick Industry Association, Technical Note 7: Water Penetration Resistance, Design and Detailing (PDF)
3. Copper Development Association: Stepped and Chimney Flashings (Copper in Architecture design handbook)
4. 2003 International Residential Code, Chapter 10, Chimneys and Fireplaces (text reproduced by Buckley Rumford Fireplaces)
5. Brick Industry Association, Technical Note 19A: Residential Fireplaces, Details and Construction (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.