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Chimneys interrupt the roof and create four different water-management sides: uphill, downhill and two sidewalls. A reliable chimney detail therefore uses layered flashing rather than a single bead of sealant.
What does chimney flashing do?
Chimney flashing directs roof water around the masonry-to-roof intersection. A durable system typically combines base/apron flashing, step flashing along the sides and counter flashing attached to the chimney. Larger chimneys may also need a cricket or saddle on the uphill side to split water around the masonry.
How do step flashing and counter flashing work together?
GAF's current field guide describes chimney flashing as a layered system. Step flashing is woven with the shingle courses along the chimney sides. Counter flashing is then set into or mechanically attached at the masonry so it covers the upper edge of the step flashing.
This lets each component move somewhat independently while still shedding water downslope. A long continuous piece of sidewall metal cannot behave the same way as correctly lapped step flashing.
What are common chimney flashing problems?
- rusted, cracked or perforated metal;
- missing or undersized step flashing;
- counter flashing pulling out of mortar joints;
- failed sealant at reglets or metal laps;
- roof replacement performed without updating incompatible flashing;
- shingles cut too tightly or loosely around the chimney;
- water concentrating behind a wide chimney without an adequate cricket;
- deteriorated mortar or masonry allowing water through above the flashing.
GAF's 2026 flashing guidance identifies damaged flashing at chimneys as a common roof-leak source and stresses proper step and counter flashing at roof-to-wall transitions.
How do roofers diagnose a chimney flashing leak?
The inspection should look at the full chimney, not only the first visible gap. Water can enter higher on the masonry or upslope roof and appear at a lower corner inside.
If interior staining is present, use the broader roof leak warning-sign guide to document when and where the moisture appears.
When can chimney flashing be repaired?
Localized repair can make sense when the metal is fundamentally sound and the defect is limited—for example, a small counter-flashing joint has opened, a short step-flashing section was disturbed, or surrounding shingles need replacement while the main flashing remains serviceable.
Repairs should restore the original drainage sequence. Simply spreading roofing cement over a large area can hide the joint and make later diagnosis harder.
When should chimney flashing be replaced?
Replacement is the better choice when the metal is rusted, cracked, hole-filled, badly distorted or too deteriorated to last with the surrounding roof. GAF's current steep-slope field guide specifically recommends replacing counter flashing when existing metal is rusted, cracked, perforated, aged or deteriorated.
Replacement is also worth considering during reroofing if the old flashing dimensions do not match the new shingle system or if removing the old roofing damages the metal.
If the surrounding roof is already near replacement age, coordinate the decision with the repair-or-replacement guide.
What if the chimney masonry is the real source of the leak?
Flashing cannot compensate for water entering through a failed chimney cap, cracked crown, open mortar joints or porous/damaged masonry above it. In that situation, both the roofer and a qualified masonry contractor may need to be involved.
A useful diagnosis separates:
- roof-to-chimney flashing failure;
- roofing failure near the chimney;
- masonry water entry;
- condensation inside a flue or chase;
- interior moisture unrelated to the roof.
Frequently asked questions
Can chimney flashing be repaired without replacing the roof?
Yes, when the surrounding shingles and deck are serviceable and the flashing defect can be accessed and rebuilt correctly.
Why does chimney flashing use two layers?
Step/base flashing integrates with the roof covering, while counter flashing protects its upper edge where it meets the masonry. Together they create a layered water-shedding detail.
Is roofing cement a permanent chimney-flashing repair?
Usually not by itself. It can be useful at specific compatible joints, but it should not replace correctly installed metal flashing.
What is a chimney cricket?
A cricket or saddle is a small peaked roof built on the uphill side of a wider chimney to divert water and debris around the masonry.
Should chimney flashing be replaced during reroofing?
Often it should be evaluated closely. Existing metal can sometimes be reused only when it remains in like-new condition and is compatible with the new roofing system.
Research basis: GAF's 2026 flashing guidance and steep-slope field guide describe chimney flashing as a step/base-plus-counter-flashing system and recommend replacement when existing metal is rusted, cracked, perforated or deteriorated. Current repair guidance also identifies flashing around chimneys as a common roof-leak location.
Rebuild the water path, not just the visible crack.
If a chimney leak keeps returning, inspect the full step-flashing, counter-flashing, uphill drainage and masonry condition before adding more sealant.
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