Step Flashing vs Counter Flashing on a Chimney

Why a chimney needs two layers

A chimney and a roof are built from different materials and move in different ways. The roof framing expands and contracts with temperature and moisture, and settles over time. The chimney is a heavy block of masonry standing on its own foundation, and it hardly moves at all. A single piece of metal fastened to both would tear, buckle or pull loose within a few seasons.

So the seal is split in two. The bottom layer belongs to the roof and moves with it; the top layer belongs to the chimney and stays with it. They overlap without being fastened to each other. Water running down the brick lands on the counter flashing, sheds onto the step flashing, and runs off onto the shingles below.

Step flashing

Step flashing is made of short L-shaped pieces, usually aluminum, galvanized steel or copper. One leg lies flat on the roof deck, the other stands up against the side of the chimney. Each piece is tucked under the shingle above it and over the one below, so the flashing steps up the slope with the shingle courses, which is where the name comes from. Because each piece overlaps the next like the shingles themselves, water is always shed downhill onto a lower piece.

Step flashing is fastened to the roof deck only, never to the chimney, so the roof can move without pulling it off the brick.

On older homes, step flashing is often missing altogether. Instead, a single continuous strip was bent along the side of the chimney and tarred over, or the shingles were simply butted against the brick and sealed with roofing cement. Those details can hold for a while on a sheltered chimney, but they rely on the tar staying flexible, which it does not through many winters of sun and frost.

The rest of the flashing set

Around the rest of the chimney, the same idea continues with different pieces. Each face of the chimney has its own job, and a leak can start at any of them.

• Step flashing: L-shaped pieces woven with each shingle course on the sides
• Apron flashing: one piece across the low side of the chimney
• Back pan or cricket: on the high side, steering water around the chimney
• Counter flashing: set into the brick and lapping over all of the above
• Corner pieces: folded or soldered to close the corners where faces meet

Counter flashing

Counter flashing is cut to follow the brick courses, often in stepped pieces that match the slope of the roof. A groove is ground into a mortar joint, the top edge of the flashing is bent and fitted into it, and the joint is sealed with mortar or a suitable sealant. The flashing hangs down over the top of the step flashing like a cap over a collar. Properly done, it covers the upturned legs of the step flashing completely, so rain cannot run behind them.

A common shortcut is surface-mounted counter flashing: a strip screwed to the face of the brick and sealed along the top with caulk. It can work for a while, but it depends entirely on the sealant, which dries out and cracks in sun and frost, and water running down the brick gets behind it as soon as the bead fails.

Side by side

The two layers do different jobs, attach to different things and fail in different ways.

Flat roofs are different

Many rowhouses and twins have flat or low-slope roofs, and there are no shingles to weave step flashing into. On those roofs the base flashing is usually the roofing membrane itself, turned up the side of the chimney and sealed to it. Counter flashing still goes into the brick above and laps down over the membrane edge, doing the same job of covering the joint.

The weak point on flat roofs is coating. Each time the roof is recoated, the coating tends to be smeared further up the brick, burying the counter flashing or replacing it altogether. The smeared coating cracks as the chimney and roof move, and water runs behind it.

Which layer is usually the problem

In the chimneys we inspect, leaks most often trace to the counter flashing: pieces pulled out of their joint, sealant failing on surface-mounted strips, or mortar crumbling around the groove so water gets in behind. Step flashing problems are more often rust, missing pieces, or old roof work where the flashing was simply buried in tar instead of woven in.

The signs from inside are similar either way: stains on the ceiling or wall beside the chimney, damp framing in the attic, or water dripping during wind-driven rain. Tracing it takes a look at both layers and at the brick and crown above, since water can enter higher and run down behind the flashing.

Timing helps narrow it down. A leak that appears only in wind-driven rain from one direction often points to the flashing on that face. A leak that follows long, steady rain or snowmelt may be coming from the crown or the brick above. A leak that started right after roof work is worth raising with the roofer first.

How flashing is repaired

When flashing fails, the right repair usually replaces both layers together so they fit each other, along with repointing the joints the counter flashing sits in. On a shingle roof, the shingles beside the chimney are lifted and the new step flashing is woven in; on a flat roof, the membrane is patched or renewed at the base and new counter flashing is set into ground joints above it. Wide chimneys on the uphill side may also need a cricket.

From the ground, a homeowner can spot thick black tar around the chimney base, bent or lifted metal, and gaps between the flashing and the brick. The close inspection belongs to someone on the roof with proper gear.