Roof Flashing Explained: Where It Goes and Why It Fails

Roof flashing is thin sheet metal fitted wherever the roof surface is interrupted: where a roof meets a wall, around a chimney, at a vent pipe or skylight, in valleys and along the edges. ARMA, the asphalt shingle makers' trade group, describes it as one or more layers of sheet metal inserted between the roofing and a wall or protrusion to direct water toward the gutters and away from the inside of the house, and calls it essential for a leak-free roof ¹.

Shingles shed water across an open slope; at every intersection and penetration, the roof relies on flashing instead ¹. NRCA, the roofing contractors' association, notes that a leak does not necessarily mean the roof needs replacing: leaks can come from flashings that have come loose or from a damaged section ⁶.

The main types of flashing

Step flashing at sidewalls

Where a sloped roof runs alongside a wall, such as a dormer side or a second-story wall, ARMA recommends step flashing: separate bent pieces of metal, one tucked under each course of shingles as it goes on ¹. ARMA's manual sizes each piece at least 10 inches long and at least 2 inches wider than the shingle exposure, so 10 by 7 inches for a 5-inch exposure, bent to run 5 inches onto the deck and 5 inches up the wall ¹. The pieces are nailed to the roof only, never to the wall, because the wall and roof move differently ¹. The siding then comes down over the vertical legs by at least 2 inches and acts as counterflashing ¹.

Building codes say the same. Meriden, Connecticut's handout on its state building code calls for step flashing against all vertical sidewalls ², and Firestone, Colorado's reroofing requirements state that continuous flashing at sidewalls is not acceptable ³.

Headwall (front wall) flashing

Where the top of a roof slope meets a wall, ARMA describes a continuous metal strip bent to run at least 5 inches up the wall and at least 4 inches onto the last course of shingles, set in roof cement and nailed to the roof, not the wall, with siding lapped over it as counterflashing ¹.

Chimney flashing

Chimneys and roofs move independently, so chimney flashing has two parts. ARMA explains that apron flashings are fastened to the roof deck and counterflashings to the masonry; if the chimney moves, the counterflashing slides over the apron without opening a path for water ¹. On the uphill side, a cricket or saddle diverts water around the chimney. ARMA strongly recommends one on any chimney 24 inches or wider and notes the International Residential Code requires one on chimneys wider than 30 inches ¹. NRCA's reroofing guidance also recommends crickets on the upslope side of penetrations 24 inches wide or wider ⁵.

Vent pipe boots

Plumbing vents are sealed with a preformed flange, often called a boot, that slips over the pipe and lies flat on the shingles, with the upper and side shingles lapping over it ¹. ARMA advises checking boots once a year for cracks ¹.

Valleys and drip edge

Valleys, where two roof slopes meet, carry more water than anywhere else. Meriden's code handout calls for a 36-inch-wide strip of ice-and-water barrier in valleys, installed per the maker's instructions ². Along the eaves and rakes, drip edge is the metal that carries water off the roof edge and away from the fascia. ARMA says it should extend at least 2 inches onto the deck and at least 1/4 inch below it, with an outward-bent lower edge ⁴. Meriden's handout sets a 2-inch minimum overlap between pieces and fastening every 12 inches ², while ARMA suggests nailing every 8 to 10 inches ⁴. Firestone requires drip edge on all edges, including rakes ³. The underlayment guide explains how drip edge and underlayment overlap.

Where flashing fails

• Cracked vent boots. ARMA recommends checking boot flanges for cracks once a year ¹.
• Loose or lifted pieces. NRCA lists loose flashings among common leak sources ⁶, and its maintenance checklist includes looking for deteriorated flashings ⁶.
• Too much roof cement. ARMA cautions that excessive asphalt cement may cause blistering ¹.
• Missing counterflashing. ARMA's wall and chimney details depend on siding or counterflashing lapping over the base flashing ¹; ask whether yours does.
• No cricket. A wide chimney without one falls short of ARMA's recommendation and, above 30 inches, the IRC requirement ARMA cites ¹.

Repair or replace the flashing?

A failed boot or a loose piece is often a repair, not a reason for a new roof, in line with NRCA's point that a leak does not necessarily mean replacement ⁶. Fixr reports flashing replacement at $200 to $600, depending on the roof material, vent boot repair or replacement at $75 to $250, and skylight leak repairs, which usually involve new flashing and sealing, at $300 to $800 (updated January 21, 2026) ⁷.

During a full reroof, the question is whether to reuse the old metal. NRCA suggests replacing step flashings and aprons when a new roof is installed, though it recognizes that some regions reuse flashings in good condition ⁵. Firestone's requirements, under the 2018 IRC, say existing flashings that are damaged or deteriorated must be replaced when reroofing ³. Because step flashing sits under each shingle course and behind the siding, replacing it properly is much easier with the old shingles off. See tear-off vs. roof-over.

Questions to ask the roofer

• Will the step flashing, apron and counterflashing be replaced or reused, and why?
• What metal is quoted, and is the drip edge new along every eave and rake?
• Are the vent boots being replaced?
• Does the chimney have a cricket, and does it need one?
• How are skylights and walls being flashed: overlapping metal, or sealant alone?

Flashing problems are one of the signs you need a new roof, or a reason a repair is enough; repair or replace a roof walks through the decision, and types of roofing materials covers the covering itself. When you are ready, you can compare quotes from roofing companies and ask each to list its flashing work line by line.

About these figures

• ¹ Reported: ARMA Residential Asphalt Roofing Manual, chapter 10 preview (2014 edition), on step flashing sizes, headwall flashing, chimney flashing and crickets (24 inches recommended, IRC over 30 inches), vent boots and roof cement, checked November 11, 2026.
• ² Official record: City of Meriden handout under the 2022 Connecticut State Building Code on sidewall step flashing, 36-inch valley strips and drip edge overlap and fastening, checked November 11, 2026.
• ³ Official record: Town of Firestone reroofing requirements under the 2018 IRC on step flashing at sidewalls, drip edge on all edges and replacing deteriorated flashings, checked November 11, 2026.
• ⁴ Reported: ARMA technical bulletin (May 2025) on drip edge dimensions and fastening every 8 to 10 inches, checked November 11, 2026.
• ⁵ Reported: NRCA's Professional Roofing article on reroofing, including crickets at penetrations 24 inches or wider and replacing step flashings and aprons, checked November 11, 2026.
• ⁶ Reported: NRCA consumer answers on leaks from loose flashings and inspecting for deteriorated flashings, checked November 11, 2026.
• ⁷ Reported: Fixr roof repair cost guide, flashing at $200 to $600, vent boots at $75 to $250 and skylight leaks at $300 to $800, updated January 21, 2026, checked November 11, 2026.