How to Inspect Chimney Flashing
Chimney flashing is one of the most leak-prone areas on any roof. The junction between a vertical masonry structure and a sloped roof surface creates a natural water-collection point that relies entirely on properly layered metal and sealant to stay dry. Understanding the anatomy of chimney flashing—step flashing, counter-flashing, base flashing, and the cricket—lets you pinpoint exactly where failures occur and decide whether a quick re-seal or a full re-flash is the right repair.
Quick Summary
Time Required
30-45 minutes
Difficulty
Moderate
Cost
$15-$50 for sealant; $300-$800 for full re-flash
Understanding Step Flashing vs Counter-Flashing
Chimney flashing is a two-layer system. Each layer has a specific job, and a failure in either one allows water to penetrate the roof deck. Knowing which layer has failed determines the scope of your repair.
Step flashing
Individual L-shaped metal rectangles—typically 4 by 5 inches—installed at every shingle course along the chimney's sides. Each piece overlaps the one below by at least 2 inches, creating a stair-step waterfall effect that directs water outward onto the shingle below. When a piece slides out or corrodes through, water runs behind the shingles and into the roof deck.
Counter-flashing
A continuous strip of metal set into a reglet (groove cut into the mortar joint) or surface-mounted to the chimney face. It bends downward over the top edge of the step flashing so rain running down the chimney wall cannot get behind the step flashing. When counter-flashing pulls free of the mortar, water bypasses the entire system.
Base flashing and front apron
The front apron is a single piece of bent metal that covers the joint where the downhill side of the chimney meets the roof. It sits under the shingles and extends up the chimney face at least 4 inches. Corrosion or nail holes in the apron are a common leak source that homeowners overlook.
Cricket (saddle)
A peaked diverter built on the uphill side of chimneys wider than 30 inches. It prevents water and debris from pooling behind the chimney. A missing or poorly flashed cricket is the number-one cause of chronic chimney leaks on roofs with larger chimneys.
Common Chimney Flashing Failure Points
During your inspection, pay special attention to these areas where chimney flashing most commonly fails. Look for stains on the ceiling below the chimney inside the attic as well—interior evidence often reveals leaks that are invisible from the roof surface.
- Mortar joint separation: Counter-flashing pulls out of deteriorating mortar joints, especially on older brick chimneys. You can often push a screwdriver into the gap where the metal was embedded.
- Sealant cracks: Roofing caulk and tar used to seal flashing edges become brittle after 5 to 10 years of UV exposure and temperature cycling. Look for visible cracks, gaps, or sealant that has pulled away from the metal.
- Galvanic corrosion: When dissimilar metals touch—such as aluminum flashing against copper nails—electrolytic corrosion accelerates. Check for white powdery deposits or pinholes in the metal.
- Displaced step flashing: Thermal expansion, wind uplift, or careless shingle replacement can shift step flashing pieces out of alignment. Even a half-inch displacement breaks the overlap and creates a leak path.
- Missing cricket: Chimneys wider than 30 inches without a cricket trap water and debris on the uphill side, causing accelerated deterioration of both shingles and flashing in that area.
How to Re-Seal Chimney Flashing
If the metal flashing is still in good condition but the sealant has failed, a re-seal is an effective and affordable repair. This is a DIY-friendly project as long as you are comfortable on the roof and follow proper safety practices.
Remove old sealant completely
Use a putty knife or 5-in-1 tool to scrape out all old, dried caulk and roofing cement. New sealant does not bond well to old sealant—you need bare, clean metal and masonry for a lasting seal. Wire-brush the surfaces to remove loose debris.
Apply polyurethane roofing sealant
Use a high-quality polyurethane sealant rated for roofing use—not basic silicone caulk, which does not adhere to asphalt shingles. Apply a continuous bead along every joint where flashing meets masonry, where counter-flashing overlaps step flashing, and along the front apron edges.
Reseat counter-flashing in mortar
If counter-flashing has pulled free, clean the mortar joint with a cold chisel, press the metal back into the joint, and pack it with fresh mortar or polyurethane sealant. Lead wedges hammered into the joint every 12 inches hold the metal in place while the sealant cures.
When to Fully Re-Flash the Chimney
Re-sealing is a temporary fix when the underlying metal is compromised. A full re-flash is a larger job that involves removing shingles around the chimney, pulling out all existing flashing, and installing a completely new system. Consider a full re-flash in these situations:
- Visible rust or corrosion holes: If you can see daylight through the metal or if rust has thinned the material so it flexes easily, the flashing cannot be saved with sealant alone.
- Repeated leaks despite re-sealing: If you have re-sealed the same area more than twice and the leak returns, the flashing geometry is wrong or the metal has hidden damage that sealant cannot bridge.
- During a roof replacement: Any time you are replacing shingles around the chimney, it makes financial and practical sense to install new flashing at the same time since the surrounding material is already removed.
- Missing or damaged cricket: Adding a cricket requires rebuilding the flashing system on the uphill side of the chimney, which means a partial or full re-flash of that section.
Pro Tips
- •Check from the attic first: Before climbing onto the roof, inspect the underside of the roof deck around the chimney from inside the attic. Water stains, damp insulation, or daylight peeking through reveal the leak location without needing binoculars or risking a fall.
- •Use a garden hose to simulate rain: Have a helper spray water at the chimney base while you watch from the attic. Start low and work upward—this isolates exactly which flashing joint is leaking so you do not waste time sealing areas that are still sound.
- •Never use roofing tar as a permanent fix: Black roofing cement is tempting because it is cheap and easy to apply, but it dries brittle, cracks within a few seasons, and makes the next repair harder because it is difficult to remove. Use polyurethane sealant for lasting results.
- •Match flashing metal to existing materials: If your current flashing is aluminum, use aluminum for patches. Mixing metals (for example, aluminum and galvanized steel) causes galvanic corrosion that accelerates deterioration of both metals at the contact point.
Frequently Asked Questions
What is the difference between step flashing and counter-flashing on a chimney?
Step flashing consists of individual L-shaped metal rectangles installed at each shingle course along the sides of the chimney, with each piece overlapping the one below it. Counter-flashing is a continuous piece of metal embedded into the chimney mortar joints that folds down over the top edge of the step flashing, creating a two-layer waterproof system. Both layers must be intact for the flashing to work properly.
Can I re-seal chimney flashing myself or do I need a professional?
Minor re-sealing is a reasonable DIY project if you are comfortable working on a roof. You can scrape out old, cracked sealant and apply fresh polyurethane roofing sealant to gaps. However, if the metal flashing itself is corroded, bent, or missing pieces, or if the counter-flashing has pulled out of the mortar joints, a professional roofer should handle the repair to ensure a watertight result.
How often should chimney flashing be replaced?
Chimney flashing typically lasts 20 to 30 years when properly installed with quality materials. However, the sealant joints need re-sealing every 5 to 10 years. If you notice recurring leaks around the chimney despite re-sealing, the flashing itself has likely reached the end of its service life and should be fully replaced.
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