Chimney Flashing Repair: How It’s Done Right

Chimney flashing – the four components

Every masonry chimney rising through a sloped roof should have these four flashing zones, each of which is a distinct piece of metal:

1. Base flashing – single piece of metal at the downhill side of the chimney that catches water flowing toward the chimney from above. Bent up against the chimney face and out under the shingles below.
2. Step flashing – running up both sides of the chimney parallel to the roof slope. Same principle as sidewall step flashing on dormers: one piece between each shingle course, stepping up the chimney.
3. Cricket (or saddle) – a small roof-like structure built into the back (uphill) side of wider chimneys (more than ~30 inches wide) to divert water around the chimney instead of letting it pool against the brick. Smaller chimneys can sometimes skip the cricket but wider chimneys always need one.
4. Counter-flashing – the upper layer of metal that’s set into a kerf cut in the chimney’s mortar joints, then bent down over the top edge of the step flashing and base flashing. This is what prevents water from running behind the step flashing from above.

All four components have to be present and properly installed for the chimney to stay dry. Skipping any one of them creates a water entry point.

Why most chimney leaks happen

The common failure modes we see in Illinois:

• Missing counter-flashing. Step flashing is installed but no counter-flashing was kerfed into the brick. Caulk was used at the top edge of the step flashing as a substitute. The caulk lasts 2-5 years; then water runs behind the step flashing.
• Counter-flashing surface-mounted with caulk. Counter-flashing applied with construction adhesive or caulk instead of being kerfed into the mortar joint. Same failure mode as above – caulk seal degrades.
• Missing cricket on a wide chimney. Water pools against the uphill face of a wide chimney without a cricket. Over time the pooling deteriorates the flashing and finds its way under.
• Failed mortar joints. Chimney brick mortar deteriorates over decades. Once a joint cracks or recedes, water enters the brick face directly – flashing can’t stop water that’s coming in through the brick itself.
• Damaged chimney crown. The crown is the concrete or stone top of the chimney chase. Cracked crowns let water enter the chimney from above; the leak shows up far below.
• Cap missing. Open chimney flue without a cap lets rain into the chimney. Not a flashing problem but often confused with one.

What a proper chimney re-flash looks like

When Trill Roofing re-flashes a chimney, here’s the full scope:

1. Remove and discard the old step flashing, base flashing, and any existing counter-flashing
2. Inspect the brick chimney itself – if mortar joints are deteriorated or the brick is failing, mason work is needed before flashing
3. Install new step flashing – one piece between each shingle course, stepping up both sides of the chimney
4. Install new base flashing on the downhill side, bent up against the chimney face and out under the shingles below
5. If chimney is wider than ~30″, build a cricket on the uphill side. The cricket is framed with 2x lumber, decked with OSB or plywood, covered with ice and water shield, then shingled per the rest of the roof. Flashing wraps around the cricket on all sides.
6. Cut horizontal kerf into the chimney mortar joints 1-2 inches above the top edge of the step flashing
7. Insert new counter-flashing into the kerf; bend down over the top edge of the step flashing
8. Re-tuck (re-mortar) the kerf joint with proper mortar to seal it
9. Re-shingle around the chimney as needed to integrate the new flashing

Total time on site: 4-8 hours depending on chimney size and complexity. Cost typically $600-$1,400 for a standalone chimney re-flash (separate from a roof replacement).

What the wrong fix looks like

Common chimney leak “fixes” that don’t last:

• Caulk applied along the visible joint between step flashing and brick. Buys 1-3 years. Doesn’t address the root cause.
• Roof cement smeared along the perimeter of the chimney. Buys 6-18 months. Cracks and lets water in once the asphalt dries out.
• New counter-flashing surface-mounted to the brick with adhesive. Buys 3-5 years. Adhesive degrades.
• Step flashing replaced but counter-flashing skipped. Looks better but the underlying water-entry path is unchanged.

The pattern: anything that doesn’t kerf the counter-flashing into the mortar and re-seal the kerf with proper mortar is a band-aid.

When the chimney itself is the problem

About 30% of “chimney leak” calls we get turn out to be chimney structure problems, not flashing problems. Indicators:

• Mortar joints visibly receded or cracked – finger-tip recession means water is entering the brick face directly
• Brick face spalling (faces of bricks popping off) – moisture damage from freeze-thaw
• Chimney crown cracked or missing – water enters from the top
• No chimney cap installed – rain falls directly into the flue

When the chimney structure is the issue, flashing repair alone won’t fix the leak. The fix is tuckpointing the mortar (a mason job), repairing the crown, or installing a chimney cap. Then re-flash. We can identify whether flashing or chimney structure is the cause during inspection and recommend the right scope.

Get an inspection

If you’ve got an active or recurring chimney leak, schedule a Trill Roofing free inspection at /free-inspection/. We’ll get on the roof, examine each of the four flashing components, evaluate the brick and crown, and tell you whether you need flashing repair, chimney repair, or both.

Related Trill Roofing Services

More guides: Asphalt Shingle Types Compared · Class 4 Impact-Resistant Shingles · Attic Ventilation Sizing

Get a free roof inspection from Trill Roofing

No-pressure, written estimate. Family-owned. IL-licensed. Serving Godfrey and the Riverbend.