How To Flash A Chimney With Metal Roofing: The Definitive Installation Guide
Flashing a chimney on a metal roof requires a multi-layered waterproofing system utilizing step flashing, a custom apron, and a cricked counter-flashing assembly to accommodate thermal movement and shed heavy precipitation. Success depends on maintaining continuous water-shedding planes, matching metal expansion coefficients, and utilizing high-temperature underlayments to prevent leaks at the roof-masonry junction.
Pre-Operation & Equipment Checklist
Executing a chimney flashing installation on a standing seam or corrugated metal roof demands rigorous preparation to ensure structural integrity and personal safety. Because metal panels expand and contract significantly with daily and seasonal temperature fluctuations, rigid sealant bonds will eventually fail, making mechanical integration and proper flashing geometry mandatory.
- Tools & Equipment: Heavy-duty snips (aviation snips for left, right, and straight cuts), a brake press for bending sheet metal, an angle grinder with a masonry diamond blade, a heavy-duty caulking gun, a rivet gun, and a seaming tool if working with standing seam profiles.
- Safety & Access Gear: OSHA-compliant harnesses, roof anchors, non-slip footwear, safety glasses, and heavy leather work gloves to protect against razor-sharp metal panel edges.
- Materials: Matching coil stock (aluminum, galvalume, or copper to match the roof panels), high-temperature self-adhering ice and water shield, step flashing pieces, pre-formed cricket components, 1-1/2-inch neoprene washered roofing screws, and exterior-grade polyurethane sealant.
- Prerequisite Knowledge & Standards: Familiarity with local building codes regarding cricket installation (mandatory for chimneys wider than 30 inches on the upslope side) and adherence to SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) architectural sheet metal guidelines.
- Benchmarks: Estimated completion time spans 6 to 10 hours for a standard residential brick chimney; budget approximately $300 to $600 in raw materials depending on metal grade and chimney dimensions.
Step-by-Step Chimney Flashing Execution
Step 1: Surface Preparation and Underlayment Integration
Strip away all existing metal panels, old flashing, and counter-flashing within a two-foot radius of the chimney perimeter down to the bare roof decking. Inspect the plywood or OSB decking for moisture rot and replace compromised wood immediately. Apply high-temperature, self-adhering ice and water shield directly to the exposed decking, running it up the vertical face of the chimney by at least six inches and smoothing it down firmly with a hand roller to eliminate air pockets.
Warning: Never rely solely on roofing sealant to waterproof a chimney penetration; sealant is a secondary line of defense, while overlapping metal geometries must do the primary work of shedding water.
Step 2: Fabricating and Installing the Upslope Cricket
If the chimney width perpendicular to the slope exceeds 30 inches, construct or install a metal cricket (saddle) on the upslope side to divert cascading rainwater and snowmelt around the masonry structure. Fasten the wood-framed or pre-formed metal cricket securely to the roof deck, ensuring its valley slopes match or exceed the primary roof pitch. Apply a continuous sheet of high-temperature underlayment over the cricket surface, lapping it a minimum of 4 inches over the surrounding roof underlayment.
Step 3: Installing the Lower Apron Flashing
Measure, cut, and bend a custom apron flashing piece to span the entire width of the downslope side of the chimney. The lower apron must slide under the upslope edge of the lower metal roof panels while extending vertically up the face of the chimney by at least 4 inches. Secure the horizontal flange to the roof deck using neoprene washered screws spaced every 8 inches, driving them through the flat portions of the metal profiles rather than the ribs.
Step 4: Weaving Step Flashing with Metal Panels
As you install the metal roof panels up the sides of the chimney, integrate individual step flashing pieces into every horizontal rib or corrugation. Each step flashing piece must be custom-bent at a 90-degree angle, resting at least 4 inches flat on the roof deck and rising 4 inches up the chimney wall. Overlap each consecutive piece of step flashing by a minimum of 3 inches, ensuring the upper piece always laps over the lower piece to maintain a positive water-shedding plane. Fasten the roof panels tightly over the horizontal flanges of the step flashings.
Step 5: Cutting Reglets and Installing Counter-Flashing
Using an angle grinder equipped with a diamond masonry blade, cut a horizontal groove (reglet) approximately 1 inch deep into the mortar joints around the perimeter of the chimney, positioned roughly 2 inches above the top edge of the step and apron flashings. Insert the top edge of the metal counter-flashing into the reglet, bending it downward to mirror the slope of the roof and cover the vertical legs of the step flashing. Pack the reglet tightly with high-grade structural polyurethane or silicone masonry sealant to lock the counter-flashing in place and prevent water infiltration behind the metal.
How To Install Chimney Pipe Flashing at Ian Milligan blog
Metal Flashing Material Properties Comparison
| Material | Thermal Expansion Rate | Average Lifespan | Formability & Workability | Corrosivity Risk on Steel Roofs |
|---|---|---|---|---|
| Galvalume Steel | Low | 40-50 Years | Moderate (requires robust snips) | Low (matches steel roof panels) |
| Aluminum | High | 35-45 Years | High (very easy to bend/shape) | Medium (requires barrier coating) |
| Copper | Moderate | 75+ Years | Excellent (soft and malleable) | High (causes galvanic reaction if touching bare steel) |
| Lead-Coated Copper | Moderate | 70+ Years | Excellent | Low (inert outer coating) |
Common Site Failures & Field Fixes
- Failure: Water pooling behind the upslope side of the chimney, leading to interior ceiling leaks directly beneath the penetration.
- Root Cause: Absence of a roof cricket, allowing water and heavy snow pack to accumulate against the flat masonry wall.
- Actionable Fix: Retrofit a custom-fabricated metal cricket with a minimum 4:12 pitch to actively channel water around both sides of the chimney structure.
- Failure: Premature tearing or cracking of sealant lines along the top edge of the counter-flashing.
- Root Cause: Thermal expansion mismatch between the metal flashing and the rigid masonry wall, causing shear stress on surface-applied beads of caulk.
- Actionable Fix: Cut a proper reglet into the mortar joint rather than surface-mounting the flashing, secure the metal lip with lead wedges, and seal with architectural elastomeric joint sealant.
- Failure: Galvanic corrosion holes developing where the chimney flashing meets the roof panels.
- Root Cause: Direct physical contact between dissimilar metals, such as an aluminum flashing piece touching a galvalume steel roof panel in the presence of moisture.
- Actionable Fix: Replace the compromised flashing with a metal material that matches the roof panels (or install a non-conductive bituminous separation membrane between dissimilar metals).
Frequently Asked Questions
Do I need a cricket for a small chimney on a metal roof?
You generally only require a cricket if the chimney width facing the upslope direction exceeds 30 inches. For narrower chimneys, a standard angled upslope flashing diverts water efficiently without requiring a raised saddle structure, though local building codes should always be verified prior to installation.
Can I use roofing caulk instead of step flashing on a metal roof?
No. Caulking alone is a temporary patch that will degrade under UV exposure and thermal cycling within one to two seasons. Permanent weatherproofing relies entirely on overlapping metal geometries, such as step flashing and counter-flashing, with sealant serving merely as an auxiliary weather seal inside reglet joints.
How high should step flashing extend up a chimney wall?
Step flashing and counter-flashing must extend a minimum of 4 inches vertically up the masonry wall, though regions experiencing severe wind-driven rain or heavy snow drifts should use 6 inches of vertical rise. The counter-flashing must overlap the top of the step flashing by at least 2 inches.
What is the best metal type to use for a chimney on a galvalume roof?
Galvalume steel or pre-finished steel coil stock matching the exact gauge and color of your roof panels is the ideal choice. Using matching metal eliminates galvanic corrosion risks and ensures uniform thermal expansion rates across the entire assembly.
Master Your Metal Roofing Projects Today
Ensure your structural penetrations remain completely watertight by applying professional-grade sheet metal techniques and durable material selections tailored to your specific roof profile. Equip yourself with the right tools, follow strict flashing overlaps, and safeguard your building against costly moisture intrusion for decades to come.
