Anywhere a sloped roof meets a vertical surface (a brick chimney, a siding wall, or an open perimeter edge), asphalt shingles cannot provide a waterproof seal. This requires precision-bent metal known as Flashing. Budget contractors frequently attempt to re-use old, rusty 20-year-old flashing to hide $400 in material costs, resulting in devastating internal wall leaks. Furthermore, installing metal Drip Edge around the entire roof perimeter is now legally mandated by building codes to prevent water from capillary-wicking into the wood decking.
Roofing is not just about laying flat asphalt down a slope; it is a discipline of custom metalwork. When an inspector looks for points of failure on a roof, they immediately ignore the field of shingles and walk straight to the chimney, the dormer walls, and the eaves.
Water follows the path of least resistance. Without heavily gauged, custom-bent metal properly integrated into the underlayment, water will bypass the shingles and flow directly into the structural framing of your home.
Vertical Intersections: Step & Counter Flashing
Where a roof meets a vertical wall (like a dormer or a chimney), you cannot simply bend a shingle at a 90-degree angle and nail it. It requires a two-part metal defense system.
1. Step Flashing (The Inner Layer)
These are small, 5×7-inch L-shaped pieces of galvanized steel or aluminum. The roofer weaves one piece of metal under every single shingle layer as they work up the wall. If water gets past the siding, it hits the vertical metal wall, drops down to the horizontal metal lip, and safely drains onto the top of the next shingle. This is completely invisible once the wall siding is installed.
2. Counter Flashing (The Outer Shield)
Used primarily on brick chimneys. Because you cannot tuck step flashing behind solid brick, a specialized crew must use a grinder to physically cut a 1-inch deep groove into the masonry mortar. They then insert a specialized piece of metal into that groove and bend it down *over* the step flashing. This prevents water from running down the face of the bricks and getting behind the step flashing.
Capillary Action and the Drip Edge Mandate
At the very edge of the roof (the eaves and rakes), water exhibits a physical property called Capillary Action. Because water molecules are cohesive, water running off the edge of a shingle does not simply fall straight down; it actually clings to the bottom of the shingle and tries to curl backward, traveling “uphill” to soak into the wood fascia board.
To break this surface tension, a piece of L-shaped metal called Drip Edge must be nailed along the entire perimeter. The metal extends outward, forcing the water to physically drop into the gutter.
The Legal Code Update: Prior to 2012, many local codes did not explicitly mandate drip edge, allowing budget roofers to omit it. The International Residential Code (IRC) Section R905.2.8.5 now strictly mandates drip edge installation on all eaves and gables. If a contractor’s estimate does not explicitly itemize Drip Edge, they are bidding an illegal roof replacement.
Custom Metalwork Replacement Estimator
Calculate the hidden cost of properly replacing your roof’s critical metal transition points rather than illegally reusing the old metal.
The #1 reason one roofing bid is thousands of dollars cheaper than another is metalwork reuse. Removing old flashing requires carefully detaching siding or grinding masonry, which takes hours of skilled labor. Replacing it requires expensive custom metal. Budget contractors will do one of two things:
- The Bend-and-Save: They will rip off the old shingles, bend the 20-year-old rusty metal out of the way, install the new shingles, and bend the old metal back down. This fatigues the metal and introduces hundreds of tiny stress fractures.
- The Tar Patch: Instead of installing new metal counter-flashing on a chimney, they will simply smear thick black roofing cement (tar) around the brick. UV rays bake the tar, causing it to crack and turn brittle within 3 years, leaving a gaping hole for water. Never accept roofing cement as a primary flashing method.
Force Contractors to Bid New Metal
Do not allow a contractor to hide old flashing under new shingles. Our 40-Factor Estimator itemizes all perimeter linear footage, generating a comprehensive cost profile that forces contractors to include 100% new metalwork.
Launch the 40-Factor Estimator →Frequently Asked Questions
Does drip edge go over or under the underlayment?
Both, depending on the edge. By code, at the eaves (bottom edge), the drip edge goes directly onto the wood deck, and the underlayment is installed over it so water runs out. At the rakes (sloped sides), the underlayment goes down first, and the drip edge is installed over the underlayment to prevent wind-driven rain from getting underneath.
Can lead flashing be used on chimneys?
Yes, lead is highly favored for chimney and complex masonry flashing because it is extremely malleable and can be precisely hammered into deep mortar joints. It also never rusts. However, due to environmental regulations, some municipalities restrict its use in favor of aluminum or copper.
Engineering & Code References
- International Residential Code (IRC): Section R905.2.8.5 - Explicit mandate requiring a corrosion-resistant drip edge at eaves and gables of shingle roofs.
- Sheet Metal and Air Conditioning Contractors' National Association (SMACNA): Architectural Sheet Metal Manual guidelines on counter-flashing masonry integration and capillary breaks.
- Asphalt Roofing Manufacturers Association (ARMA): Technical directives prohibiting the use of mastic/roofing cement as a substitute for rigid metal flashing components.