How To: Properly Replace Chimney Flashing on a Sided Chimney

by Rhoden RoofingMar 31, 2025

Chimneys are a roof’s most common source of leaks. While the culprit isn’t always chimney flashing, that’s the first element that experienced roofers check. That system of metal is your chimney’s best line of defense against water intrusion, but only if it’s installed correctly.

Chimney flashing is the system of metal pieces that direct water away from the chimney penetration on your roof.

From reusing old flashing to an overreliance on sealant as a waterproofing method, many guides and budget roofers employ practices that reduce initial expenses while causing premature failure, costly repairs, and recurring leaks.

This article walks you through correctly replacing chimney flashing on a sided chimney while addressing some misconceptions about chimney flashing repair and replacement.

We strongly recommend consulting a professional roofer before attempting to replace your chimney flashing. Installing a proper water-shedding system for a chimney requires expertise, and the likelihood of leaks makes proper installation critical. Whether you’re planning to replace it yourself or want to make sure your contractor does it right, understanding best practices will save you time, money, and headaches in the future.

First, it’s important to note that chimney flashing should always be replaced when a roof is replaced. Reusing old flashing almost always results in premature failure.

Second, before attempting to repair your roof, always consider whether you’re under warranty or not. If you have an active warranty on your roof, DIY efforts are likely to nullify your warranty.

Step 1: Inspect and Prepare the Existing Flashing and Roof Decking

Misconception: Chimney Flashing Can Be Repaired with Roofing Cement or Caulk

Reality: Once flashing fails, it must be replaced. Caulk and cement are temporary fixes. When our roofing experts perform tune-ups, we may apply caulk or sealant to exposed fasteners or small holes in flashing as a temporary solution. Similarly, as a temporary solution, peeling flashing may be temporarily secured with concrete nails; however, the sustainable solution is complete removal and replacement.

Two roofers remove the vinyl siding from a chimney to replace its flashing.

When we replace a roof or conduct proper repairs, we ensure that no fasteners are exposed to begin with. Exposed fasteners are a vulnerability, and vulnerabilities result in leaks. Chimneys are the most common source of roof leaks, as it’s generally a roof’s largest penetration. Replacing damaged chimney flashing and installing new chimney flashing protects your roof’s biggest vulnerability.

Inspect the existing flashing for signs of rust and peeling. The NRCA recommends thoroughly documenting every step in the repair process. Documenting this step provides valuable information for personal records and contractors who need to evaluate your roof’s previous condition.
Assess siding. Because siding will be installed over the step flashing, ensure that you will be able to remove and reinstall the siding properly.
- If you have vinyl siding, all siding along the faces of your chimney should be removed. Removing the bottom piece can cause the upper siding to slide down, preventing proper reinstallation and complicating flashing installation. Ensuring that the siding can be removed and reinstalled is a prerequisite for chimney flashing replacement.
Odds are, if you’re considering chimney flashing replacement, you’ve encountered leaks. Prior to replacing any materials, check the roof decking for moisture damage, rot, or delamination. A prerequisite for any roofing work is nailable decking. If decking is deteriorated, fasteners won’t hold securely, and water will continue to find a path inside.
If your chimney has a cricket, inspect it thoroughly. A cricket is a sloped panel on the backside of a chimney that’s designed to divert water. The step flashing must integrate with the cricket so that water is effectively diverted from your chimney. If your chimney doesn’t have a cricket but it’s 30 inches wide or hosts pooling water, a cricket needs to be installed.
- Unlike the base flashing on a sided chimney, a cricket is not fastened over the shingles. The shingles neighboring a cricket are laid after the cricket is installed, meaning they will abut it.

Step 2: Remove the Old Flashing and Prepare the Surface

Misconception: You can Simply Replace the Damaged Section of Flashing

Reality: All flashing components – base, step, and counterflashing – must be removed and replaced together to maintain a watertight system. Flashing works as part of a complete system. Successful installation depends on each component’s integration, not just the material itself. Proper overlap of flashing under each course of shingles requires precision. Flashing is like a zipper. Each part is designed to interlock with the next. If you remove or misalign a few teeth and attempt to zip it closed, it won’t function properly, leaving gaps or failing to zip entirely.

A roofer removes shingles on the side of a chimney to prepare for step flashing installation.

Pry off all existing flashing – deteriorated or not. Reusing old flashing is a common mistake that budget roofers make. Old flashing introduces failure points, such as penetration holes and warping, which can lead to future leaks.
Remove shingles within 18-24 inches of the chimney and inspect the substrate.
- The Ice and Water Shield in step 3 must extend beneath surrounding shingles, so removing shingles is a critical waterproofing step.
Any roofing cement or caulk from previous repairs will need to be removed as well.
Clean the exposed area thoroughly to ensure there’s no residue or moisture that might affect the adhesion of ice and water shield.
If visible water damage is present but the decking is solid, install new synthetic underlayment after applying Ice and Water Shield in step 3 to ensure the underlayment beneath your flashing isn’t saturated. Trapping moisture between your shingles and decking can result in premature failure.
If visible water damage is present and the decking is compromised, it will need to be replaced. We highly recommend contacting a trusted roofing professional to handle decking replacement.

Step 3: Apply Ice and Water Shield for Added Protection

Misconception: Flashing Alone Is Enough to Prevent Leaks

Reality: Some DIY guides skip this step, but NRCA guidelines emphasize it as a critical waterproofing barrier. It’s designed to protect against wind-driven rain, ice dams on a home’s perimeter, and water intrusion at critical roof penetrations, such as chimneys. A front door has the most security because it’s an obvious point of entry, and chimneys demand proper waterproofing for the same reason. Ice and water shield seals around fasteners like a gasket. The fasteners used to attach your flashing to the roof deck will be insulated by the waterproofing membrane, mitigating the risk of future leaks. While well-installed chimney flashing will offer excellent protection, no place requires more redundancies than a chimney.

Two roofers apply ice and water shield to provide superior protection around a cladded chimney.

Lay the ice and water shield over the roof deck around the chimney, extending at least 12 inches past each side of the chimney and up the adjacent wall by 4-6 inches.
The membrane should be taut to the substrate with no air pockets or wrinkles, providing a continuous barrier at the chimney’s base.

Step 4: Shingle up to the Chimney Base and Install Base Flashing

Misconception: Base Flashing Is a Minor Detail that can Be Installed Loosely Because Sealant Will Cover any Gaps

Reality: Relying on sealant is like using glue to create origami: it shouldn’t be necessary. Flashing is not a single unit, and it doesn’t have elective parts. It’s a multi-layered system that includes:

Base Flashing: Also called apron flashing, it’s applied on the downslope side of a chimney.
Step Flashing: Step flashing is base flashing that’s applied on the sloped side of a chimney.
*Counterflashing: Secured on top of base and step flashing. *Applied on all brick chimneys but rarely used for sided chimneys.

Each type of flashing serves a distinct role. Budget roofers often rely on sealant to create a watertight seal atop the flashing rather than properly integrating the flashing.

Properly installed base flashing is secured to the deck through fasteners and through proper overlap. Proper installation of chimney flashing uses sealant as a redundancy – not to make the system watertight.

Prior to installing base flashing, you’ll shingle up to the chimney’s downslope base. The base flashing leaves an apron of metal that extends at least 3 inches over the downslope shingles.

Base flashing on a sided chimney offers optimal protection when it extends over the downslope shingles, just like an apron is worn over clothes, not under. If your existing base flashing is underneath the shingles or abutted by shingles, flashing replacement is a good opportunity to rectify the base flashing’s placement. At least 3 inches of apron flashing metal extending over the shingles ensures that water isn’t slowed or pooled en route to your gutters.

If your chimney extends flush to your eave or past your eave, base flashing won’t be necessary, although we suggest using kickout flashing to divert water from your chimney. Kickout flashing is installed at the end of the step flashing where it extends from the wall and directs water runoff into the gutter. This prevents water from running down the side of the chimney.

Base flashing should envelop the chimney base, extending at least 4-6 inches up the side of the chimney and at least 3 inches onto the roof deck so it overlaps the ice and water shield and shingles. The greater the slope, the greater the lap should be. It acts as the first line of defense, diverting water from the chimney.
Mechanically fasten the base flashing to the roof deck. Because ice and water shield is self-sealing, fastening through it doesn’t create additional vulnerabilities.

Step 5: Layer Step Flashing with Each Shingle Course

Misconception: Step Flashing can Be Installed Without Removing Siding

Reality: Some installers attempt to tuck step flashing under the siding instead of removing it. This can lead to improper water shedding, potential siding damage, and future leaks. To ensure proper step flashing integration, always remove obstructive siding around the chimney so that step flashing can be properly installed behind it.

Sealant can temporarily make gaps watertight, just like a Band-Aid can make a cut disappear underneath it. But stitches – enabling your skin to regrow – protects a deep cut. Mechanical fastening and proper overlap are stitches that protect your chimney, not Band-Aids — beads of sealant and warped siding from manipulation.

A roofer removes shingles on the side of a chimney to prepare for step flashing installation.

For sided chimneys, the siding affecting your work area will need to be completely removed.
To layer the shingles properly, begin at the bottom of the chimney and work upward, installing a piece of step flashing with each new shingle course. By this point, the shingles surrounding the base flashing have already been reinstalled – this is where you’ll start layering the shingles with step flashing.
- - Each piece of step flashing needs to overlap the one below it to direct water off the roof.
  - All flashing is fastened to the roof deck, not the wall or chimney.
Inspect the alignment after installation. The step flashing should integrate seamlessly with the shingle courses, maintaining a continuous water-shedding path.
Using sealant at the chimney’s corners, where the step flashing meets the base flashing, can offer additional protection against water intrusion.

Step 6: Reinstall Siding with Proper Clearance

Misconception: Siding Can Sit Flush Against the Roofline

Reality: Where siding meets the roofline, water gets trapped between the siding and step flashing, which can invite water into the substrate. The NRCA suggests that the siding sits a minimum of 1/4-inch above the roofline to allow for proper drainage. We generally suggest a minimum of a 1-inch gap to ensure water has adequate clearance. Properly integrated step flashing and shingle overlap further ensure that water flows away from the chimney. However, if your siding is flush to the roof, water can get trapped behind your siding’s corner board where it can pond and result in water infiltration.

Re-placing vinyl siding on a cladded chimney.

If a 1-inch gap is untenable, a 1/4-inch gap is acceptable. Just keep in mind that greater clearance, up to 2 inches, is always preferable to a 1/4-inch gap.
Carefully reattach the siding to restore the original appearance of your chimney.
Counterflashing isn’t usually necessary for sided chimneys. Counterflashing prevents water from entering the space between a roof and a vertical surface, like a chimney. It’s installed over the base flashing, which includes step flashing. The siding on a chimney serves the same purpose, so counterflashing isn’t used.

Step 7: Final Inspection and Long-Term Maintenance

Misconception: Once Flashing Is Installed, It’s Maintenance-Free

Reality: Even properly installed flashing can fail over time due to extreme weather and aging materials. Regular inspections are essential to catch issues early and prevent costly repairs.

Inspect the new flashing installation to confirm that all components are properly aligned and secured.
- All overlaps should be tight.
- Document any issues you encounter. This allows you or a roofing professional to have greater insight should future issues arise.
- If you notice any damage or misalignment after installation, promptly contact a professional for input before the problem escalates.

Chimney flashing is an essential element that protects your home from water infiltration. While the steps above can help prepare you for flashing replacement, understanding common misconceptions about proper flashing installation can help you identify shortcuts that some roofers might take. Proper flashing installation means fewer leaks, reduced repairs, and extended roof life.

Key Takeaways

Complete replacement is essential. Repairing ineffective chimney flashing with caulk or roofing cement only masks the problem. For a lasting solution, always replace the entire flashing system.
Chimneys are your roof’s most vulnerable point. Chimneys are the largest penetrations on your roof. Proper flashing installation and use of ice and water shield offers the best protection against water intrusion.
Preparation and inspection are critical. Before replacing anything, thoroughly inspect the decking for signs of moisture damage or delamination, ensuring all underlying issues are addressed.
Siding clearance matters. Siding should be reinstalled with a minimum ¼-inch clearance above the roofline, although ½ inch is preferable. This gap enables proper drainage and prevents easily avoidable moisture damage.
Regular inspections and maintenance are key. Even well-installed flashing can deteriorate over time. Annual inspections and inspections after severe weather allow you to catch loose fasteners, rust, or misalignment before they require costly repairs.
Professional expertise is recommended. Due to the precision required and the potential for costly damage, chimney flashing replacement is best performed by experienced roofing professionals. DIY efforts may jeopardize warranties and result in recurring issues if not executed correctly.