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Step flashing around a chimney: how the sides are sealed and how they fail

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The sides of a chimney are sealed with a staircase of small metal pieces. When one lifts or rusts, water finds the gap.

Step flashing around a chimney is a row of small L-shaped metal pieces, one per course of shingles, that seals the sides of the chimney where the slope runs into the brick. Each piece laps over the one below and under the shingle above. Water is handed downhill from piece to piece and never gets a straight path inward.

How the sides of a chimney are built

A chimney has four faces, and each one is flashed differently. The low face gets a base flashing (an apron). The high face gets a cricket and a saddle. The two sides, where the roof slope climbs past the brick, get step flashing.

The bottom leg of each piece sits flat under the shingles. The upright leg rises against the brick. Above it, a second layer of metal called counterflashing is tucked into the mortar and hangs over the upright leg. The two layers together are what keep the side dry. Sealant is only a backup at the top edge.

Where step flashing fails

Most side leaks come from a handful of causes. They are easy to describe and sometimes hard to see.

  • A piece that has rusted through, usually at the lower end where water lingers.
  • Counterflashing that has pulled out of the mortar joint and no longer covers the upright leg.
  • A continuous strip of metal or a long bead of sealant used in place of individual pieces.
  • Pieces that were cut too short and do not lap far enough under the shingle above.

A strip or a smear of sealant is a warning sign. It means somebody has already patched this side before.

What you can check without going up

Stand in the yard with binoculars and look along each side of the chimney. You are looking for rust streaks on the brick, a gap between the metal and the masonry, or black tar smeared over the joint. Compare the two sides. If one side looks very different, start there.

Then go inside. Look at the ceiling and wall on the side of the house where the chimney rises. And check the attic framing beside the chimney with a flashlight, staying on the joists. A stain that follows the length of one side points at step flashing. A stain below the high face of the chimney points somewhere else, usually the cricket.

Why we stop at looking

Replacing step flashing means lifting shingles beside the chimney, working at the steepest part of the roof, with brick above you. We do not publish those steps. No saved repair bill is worth the fall.

The more precisely you describe the problem, the better the quote you get. Photograph the metal-to-brick line from the ground, note which ceiling shows the stain, and ask the roofer to show you the flashing they plan to replace. If the quote says only "reseal the chimney", ask why the flashing itself is not being replaced. Resealing alone is usually a repeat of the last patch.

If you are not sure which part of the chimney is leaking, start with water around the chimney: four different failures. For the wall version of the same detail, see step and kick-out flashing.

Frequently asked questions

What is step flashing on a chimney?

It is a series of small L-shaped metal pieces woven in with the shingle courses along the sides of a chimney. Each piece overlaps the one below, so water runs down the slope instead of behind the brick.

How do I know if my chimney step flashing is failing?

From the ground, look for rust streaks, a visible gap between metal and brick, or sealant smeared along the side. Inside, a stain on the ceiling or attic framing beside the chimney points the same way.

Is step flashing the same as counterflashing?

No. Step flashing lies under the shingles and runs up the brick. Counterflashing is the upper layer set into the mortar that covers the top of the step flashing. They work as a pair.

Can sealant replace step flashing?

No. Sealant is a backup at the top edge. If the metal has rusted or lifted, sealant hides the problem for a season and the leak returns.

Can I replace chimney step flashing myself?

We do not publish steps for it. The work is done on the roof beside the chimney, where a fall is a real risk. Document what you see from the ground and the attic, and have a roofer do the repair.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

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