Technical Foundation
What Is a Roof Flashing?
Roof flashing is formed sheet metal installed at roof penetrations, transitions and edges to direct water away from joints that shingles or membrane alone cannot seal. Step flashing is installed in overlapping pieces up a sloped roof-to-wall junction, one piece per shingle course, so water is shed onto the shingle below rather than running behind the wall siding; valley flashing runs the full length of a roof valley to carry converging water flow from two roof planes without leaking at the seam.
Material choice governs service life and galvanic compatibility: galvanized steel is the economical general-purpose choice, aluminum resists corrosion but must not contact dissimilar metals like copper without an isolating underlayment, and copper offers the longest service life and is standard on high-end and historic roofing. Gauge or weight (for copper, expressed in ounces per square foot) is selected for the flashing's exposure and the loads it must resist, such as foot traffic on a chimney cricket.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Profile | Step, valley, drip edge, counter, pipe boot | Each profile addresses a specific water path — matching the wrong profile to a detail invites leaks |
| Material | Galvanized steel, aluminum, copper | Governs corrosion resistance, service life and galvanic compatibility with adjacent metals |
| Gauge / Weight | 26-24 gauge steel; 16-20 oz copper | Heavier gauge resists denting and foot traffic on high-exposure details like chimney crickets |
| Finish | Mill, painted, pre-weathered copper | Painted finishes match roof color; pre-weathered copper avoids a bright patina transition period |
| Length/Size | 10 ft stock lengths, formed step pieces 5x7 in. | Stock lengths are field-cut and lapped; step pieces are sized per shingle exposure |
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Applications
Where Roof Flashing Get Used
Roof flashing is specified anywhere two roof planes meet, or a roof surface transitions to a wall, chimney, vent or edge.
Residential Roofing & Re-Roofing
Step and drip edge flashing installed at roof-to-wall junctions and eaves during new roof and re-roof projects.
Chimney & Skylight Waterproofing
Counter flashing and cricket flashing kits sealing the transition between chimney or skylight curbs and surrounding roofing.
Commercial Low-Slope Roofing
Edge metal and transition flashing integrated with membrane roofing systems at parapets and roof-to-wall details.
Historic Restoration
Copper flashing specified to match original materials and achieve multi-decade service life on restoration projects.
General Contracting & Roof Repair
Individual step and pipe boot flashing pieces stocked for spot repairs and roof penetration retrofits.
FAQ
Roof Flashing Questions, Answered
What's the difference between step flashing and valley flashing?
Step flashing is installed in individual overlapping pieces up a sloped roof-to-wall junction, one piece integrated with each course of shingles, so water sheds down and away from the wall. Valley flashing is a continuous or lapped run of metal down the full length of a roof valley, carrying the combined water flow from two converging roof planes to the eave without leaking along the seam.
Can I mix aluminum flashing with copper gutters or fasteners?
No — aluminum and copper in direct contact with moisture present will corrode galvanically, with the aluminum sacrificing itself rapidly. Use an isolating underlayment between dissimilar metals, or choose flashing and fasteners in a single compatible material system, particularly at any point where runoff from copper will contact aluminum flashing downstream.
What gauge or weight of flashing should I use?
For steel, 26 gauge is standard for most step and valley applications, with 24 gauge or heavier used on high-traffic or high-exposure details like chimney crickets. For copper, 16 oz is common general use and 20 oz is specified where greater durability or foot traffic resistance is needed. Match gauge to the specific detail's exposure, not just the rest of the roof's material.
Do I need counter flashing in addition to step flashing at a chimney?
Yes — step flashing handles the water path along the roof slope, while counter flashing is set into a reglet cut into the chimney masonry and laps down over the step flashing, preventing water from getting behind the step flashing where it meets the vertical chimney face. Skipping counter flashing is one of the most common causes of chimney leaks.
How long does roof flashing typically last?
Galvanized steel flashing typically lasts 20-30 years depending on coating and exposure, aluminum performs similarly with better corrosion resistance in most environments, and copper flashing routinely exceeds 50-100 years of service, which is why it remains standard on historic restoration work despite higher material cost.
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