Metal Roof Flashing: The Most Common Failure Point
If a metal roof leaks, the panels almost never are the cause. The panels are the simplest part of the system — large sheets of waterproof metal that overlap or lock together. Water can't get through a properly connected panel.
Leaks happen at the transitions — where the metal panels meet something else. A chimney. A wall. A vent pipe. A valley. A skylight. These transition points are where flashing does its job, and it's where the vast majority of metal roof problems originate.
What Flashing Is and Why It's Critical
Flashing is sheet metal fabricated into specific shapes that bridge the gap between the roof panels and anything that penetrates or interrupts the roof surface. At a chimney, for example, flashing wraps around the base of the chimney and tucks beneath the metal panels, creating a watertight channel that directs water around the chimney and back onto the roof surface.
Every roof has multiple flashing locations: chimney flashing (the most complex), plumbing vent boots, HVAC exhaust vents, wall-to-roof transitions (where a wall meets the roof surface), valley flashing (internal angles where two roof planes meet), skylight flashing, ridge flashing, and eave and rake trim.
Each location requires a different flashing configuration, and each must accommodate the thermal expansion and contraction that metal panels undergo with temperature changes. This is where skill matters.
What Good Flashing Looks Like
Properly executed flashing has several observable characteristics.
Material match. Flashing should be the same metal as the panels (or a compatible metal) to prevent galvanic corrosion. Steel panels get steel flashing. Aluminum panels get aluminum flashing. Mixing metals at contact points causes accelerated corrosion.
Continuous sealing. Flashing joints should be lapped (overlapping) and sealed with butyl tape or appropriate sealant rated for metal roofing. Silicone-based sealants are typical. The sealant is a backup — the primary water barrier is the physical overlap of the flashing pieces.
Thermal accommodation. Flashing details should include expansion joints or flexible connections that allow the metal to move without breaking the seal. A rigid flashing connection on a metal roof will eventually crack or separate as the panels expand and contract.
Clean integration. Flashing should sit flush against walls and penetrations without gaps, buckles, or visible daylight. The transition from panel to flashing should be smooth and intentional, not cobbled together.
What Bad Flashing Looks Like
From the ground, bad flashing is sometimes visible if you know what to look for.
Visible gaps between flashing and walls or chimneys. Excessive or messy sealant (caulk used to fill gaps that proper flashing would have eliminated — a sign the installer is relying on sealant instead of craftsmanship). Flashing that doesn't extend far enough up walls (it should extend at least 4 inches above the roof surface). Dissimilar metals visibly in contact (copper flashing on steel panels, for example). And step flashing at walls that doesn't integrate cleanly with each panel course.
If you see any of these during installation, ask questions before the crew moves on. These details are much easier and cheaper to fix during installation than afterward.
The Chimney Challenge
Chimney flashing is the most complex and failure-prone flashing detail on any roof. The chimney penetrates the roof on all four sides, each requiring a different flashing approach: headwall flashing above the chimney, step flashing along the sides, cricket or saddle construction behind the chimney (to divert water around it), and base flashing where the chimney meets the roof below.
A properly flashed chimney on a metal roof involves multiple custom-fabricated pieces that interlock and layer to create a comprehensive water barrier. It's time-consuming and requires experience. A contractor who rushes chimney flashing or relies heavily on sealant instead of proper metalwork is a contractor to be concerned about.
Maintaining Flashing Over Time
Even properly installed flashing may need attention over the decades your metal roof is in service. Sealant compounds have a shorter lifespan than the metal panels — typically 15 to 25 years before they begin to lose adhesion. During your annual roof inspection, check flashing areas for any sealant that's cracking, separating, or pulling away. Re-sealing a flashing joint is a simple, inexpensive maintenance task that prevents leaks.
Butyl tape at flashing joints may also need refreshing at the 20 to 25 year mark. Again, this is routine maintenance that's far less expensive than dealing with water damage from a failed seal.
The Bottom Line
Flashing is where installation quality shows itself most clearly. The panels can be perfect, but if the flashing at a single chimney or wall transition fails, you've got a leak. This is why contractor selection matters so much for metal roofing — the detail work at transitions separates experienced metal installers from general roofers trying their hand at metal.
When evaluating contractors, ask specifically about their flashing approach. How do they handle chimneys? What sealant products do they use? Do they custom-fabricate flashing on-site? The answers reveal their level of metal-specific experience more than any other question.
For the complete installation process, read our installation guide. For contractor vetting guidance, see our contractor selection guide.