Roof Flashing: Types, Locations, and How Each One Fails

Roof flashing is the sheet metal that turns water at every joint where the roof meets a wall, chimney, valley, edge or pipe, and each type fails its own way: at its laps, its fasteners or its metal. On a re-roof, decide piece by piece what gets replaced and what is reused, and write that list into the contract.

Flashing is mostly hidden under shingles and siding, which is why it shows up in quotes as a single vague line. Knowing the types and where each one sits among the other roof components lets you read that line, and ask for the version that names every piece.

How Flashing Keeps Water Out: Laps, Not Glue

Flashing works the way shingles do: by overlapping. Each piece sits so that the piece above it laps over its top edge, and water running downhill is always handed to the outside of the next layer. Nothing in that arrangement depends on glue. Sealant can back up a joint, but the laps are what keep water out.

The code states the goal in one line. Section R903.2 of the International Residential Code (IRC) puts it this way in Seattle’s residential code, Chapter 9: “Flashings shall be installed in a manner that prevents moisture from entering the wall and roof.” A joint that only stays dry because of a bead of caulk does not meet that goal for long, whichever edition of that model code your town enforces.

Every Flashing Location on a Typical Roof

The same code section sets where flashing goes. Section R903.2.1 requires it “at wall and roof intersections, wherever there is a change in roof slope or direction and around roof openings.” On a typical house, that produces this map:

Step flashing is the part of this map that belongs to this page. Each piece is bent to an L, with one leg on the roof under a shingle and the other up the wall, and a new piece goes in with each course. The code sets a floor for it: under section R905.2.8.3, base flashing against a vertical sidewall “shall be continuous or step flashing and shall be not less than 4 inches (102 mm) in height and 4 inches (102 mm) in width.” The siding or the wall’s weather barrier then laps over the vertical legs.

Apron flashing runs along a wall where the roof slopes away from it, such as the front of a dormer. It laps down over the top shingle course and up the wall, under the siding.

Chimneys combine step flashing, an apron, a back flashing or cricket, and counterflashing set into the masonry. Why chimneys leak and which trade fixes each part is covered in chimney flashing and masonry.

Kickouts sit at the bottom of a roof-wall joint. DOE’s Building America guidance on step and kick-out flashing at roof-wall intersections says to install kick-out flashing “at the end of a roof-wall intersection to divert water away from the wall and into gutters,” and section R903.2.1 requires a flashing to divert water where a sloped eave meets a sidewall. Checking for them from the ground is covered in kickouts at roof-wall ends.

Edges get drip edge along the eaves and rakes. Whether code requires it where you live, and how it laps with underlayment, is covered in edge metal at eaves and rakes.

Valleys carry the runoff of two slopes at once. Open metal, closed-cut and woven methods are compared in valley methods compared.

Penetrations for plumbing vents and other pipes use boots or collars, and why those fail is covered in pipe boots and penetrations.

How Each Type Fails

Every flashing failure comes down to one of three things: the laps, the fasteners or the metal. HUD’s Residential Rehabilitation Inspection Guide lists flashing that has deteriorated or come loose as an inspection item, a description wide enough to cover all three.

Laps. A lap that runs the wrong way sends water behind the flashing instead of over it. So does a missing piece, a step flashing replaced with one long strip, or an apron that stops short of the shingles. These are installation errors, and they can leak as soon as the roof sees wind-driven rain.

Sealant standing in for a lap. Caulk or roofing cement smeared over an open joint can look like a repair. It is not flashing, and once it cracks, water goes straight in. Why sealant cannot replace flashing, and where it does belong, is covered in why sealant is not flashing.

Fasteners. A nail through exposed flashing is a hole in the water path. Nails also back out as wood moves, lifting the metal and opening the lap.

Metal. Corrosion thins metal until it holes through, and some exposures speed it up:

• Salt air. Near the coast, metal choice matters. FEMA’s Coastal Building Materials, Technical Fact Sheet No. 1.7, from December 2010, covers corrosion resistance among its material recommendations.
• Moss treatments. Oregon State University Extension’s guide to controlling roof moss, reviewed in 2025, warns against zinc sulfate products on roofs with copper gutters, downspouts or flashing because “zinc corrodes copper.”
• Dissimilar metals. When two different metals touch with water present, one can corrode faster, so patches and fasteners should match or be compatible with the flashing metal.

Physical damage shows up at the laps too: flashing bent by ladders, ice or tree limbs no longer laps the way it was formed.

Reuse or Replace During a Re-Roof

A tear-off removes the shingles that step flashing, aprons and valleys are lapped with, so every piece gets a decision whether or not anyone writes it down. Some pieces lend themselves to reuse and some do not:

• Step flashing is woven in with the old shingle courses. Reusing it means fitting new shingles around bent, nail-holed metal, so replacement is the cleaner scope. Because the wall’s siding or weather barrier laps over the vertical legs, replacing step flashing means loosening the lowest siding courses along that wall, so the quote should say who does that.
• Counterflashing in masonry can be in good shape and reset, or it can be loose in eroded mortar. Its condition decides, and resetting may need a mason.
• Valley metal, drip edge and apron flashing are judged on corrosion, bends and fit with the new roof’s layers.
• Pipe boots have rubber or plastic collars that age on their own schedule, so treat them as their own line.

Old roofs bring one more check. EPA’s page Learn About Asbestos lists roofing shingles among building materials that may contain asbestos and says exposure “may occur only when the asbestos-containing material is disturbed or damaged.” On an older roof, ask whether old flashing cement or mastic will be tested before it is scraped off.

Use this checklist for each piece:

1. Is it bent, dented or creased so it no longer laps correctly?
2. Is there rust, pitting or holes?
3. Does the joint depend on sealant to stay dry?
4. Is it a different metal from what the new roof will use next to it?
5. Can it be removed and reinstalled without damage?

A yes to any of the first four, or a no to the fifth, means replace.

Which Flashing Should Your Quote Replace?

Have the contract list each flashing location by name with “replace” or “reuse” next to it: step flashing at each wall, apron flashing, chimney flashing and counterflashing, kickouts, valleys, drip edge, and each penetration. Bent, corroded or sealant-dependent pieces get replaced. Anything marked reuse should come with the reason, and every new piece should name its metal so you can check it against the metals, gutters and fasteners next to it.

Here is the five-question checklist applied to one invented house, the way the finished list could look in a contract. The answers are examples; the crew’s tear-off photos supply the real ones.

One yes in any of the first four columns, or a no in the fifth, turns the decision to replace; only the counterflashing passes every test. Written this way, the contract shows which pieces you paid to renew, and the tear-off photos show whether that is what happened.

Ask the crew to photograph flashing during tear-off, before and after each piece goes in. That record shows what was reused and what was new, and it is the only way to check laps once the shingles cover them.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

Sources

• 2021 Seattle Residential Code, Chapter 9: Roof Assemblies, City of Seattle, Department of Construction and Inspections
• Step and Kick-Out Flashing at Roof-Wall Intersections, U.S. Department of Energy Building America Solution Center
• Residential Rehabilitation Inspection Guide, U.S. Department of Housing and Urban Development, Office of Policy Development and Research
• Coastal Building Materials, FEMA Technical Fact Sheet No. 1.7, Federal Emergency Management Agency (listing on DOE Building America Solution Center)
• How to control roof moss and prevent long-term damage, Oregon State University Extension Service
• Learn About Asbestos, U.S. Environmental Protection Agency