Roof flashing is the corrosion-resistant sheet metal that seals a roof's transitions and penetrations, the chimneys, walls, valleys, skylights, and vent stacks where a continuous shingle field cannot shed water on its own. The roofing industry estimates roughly 90 to 95 percent of roof leaks originate at flashing details, an estimate attributed to the NRCA.
Knowing the flashing types, the codes that govern them, and how to verify a correct installation helps a homeowner judge both a repair and a full re-roof.
How Does Roof Flashing Work?
Roof flashing works by lapping corrosion-resistant metal across every joint a shingle field cannot cover, shedding water at chimneys, sidewalls, valleys, skylights, and vent stacks rather than relying on sealant alone. The roofing industry estimates roughly 90 to 95 percent of roof leaks originate at these flashing details and only 5 to 10 percent at the open shingle field, an estimate attributed to the NRCA, which is why the metal at the transitions carries the waterproofing burden.
Eight flashing types each seal a different detail: step, counter (cap), valley, apron or head, drip edge, kickout or diverter, vent-pipe boot, and chimney flashing. Step flashing weaves one separate metal piece per shingle course against a sidewall or chimney, and counter (cap) flashing caps those step pieces and sets into the masonry, so a chimney transition is a two-part base-and-counter system, per InterNACHI and shingle-manufacturer guidance. A kickout or diverter flashing redirects water away from the wall cladding where a sloped-roof eave meets a vertical sidewall; a missing kickout sends water behind the siding into the wall cavity, the cause of hidden rot and mold, per IRC R903.2.1 and InterNACHI.
Properly lapped metal sheds water without depending on caulk, while a sealant-only repair is temporary because sealant dries and cracks within a few years, per GAF. Flashing fails through corrosion and rust, lifting and bending by wind, short laps, and those drying sealant laps. A self-adhered ice-and-water shield, specified under ASTM D1970, runs beneath valley, eave, and penetration flashing and self-seals around fasteners, adding a sealed secondary barrier under the metal at the most leak-prone details.
What Codes Govern Flashing in New Jersey?
New Jersey flashing work follows the International Residential Code as adopted in the state, which sets the drip-edge specification, requires flashing at roof-wall intersections, and defines where an ice barrier goes. Drip edge extends at least 2 inches onto the deck and at least 1/4 inch below the deck or fascia, fastened no more than 12 inches on center with at least 2-inch end laps, required at both eaves and rakes, per IRC R905.2.8.5.
IRC R903.2.1 requires flashing at roof-wall intersections, including a kickout or diverter flashing where a sloped-roof eave meets a vertical sidewall, so a code-correct roof routes water out of the wall line rather than behind the cladding. An ice barrier extends from the eave to at least 24 inches inside the exterior wall line, and at least 36 inches along the slope on roofs 8:12 or steeper, per IRC R905.1.2 under the 2021 IRC as adopted in New Jersey. That membrane resists meltwater backup at the eave, but ice-and-water shield is specified under valleys, eaves, and penetrations per ASTM D1970, not legally required at every flashing location.
Ice dams form from attic heat loss and air leakage from the living space below, per Building Science Digest 135 and University of Minnesota Extension, which melt snow that refreezes at the cold eave. The eave ice barrier under IRC R905.1.2 resists the resulting backup, yet it is the secondary defense, not the cause or cure; controlling the dam itself starts with air-sealing and insulating the attic. On a detached one- and two-family dwelling, repair or replacement of the roof covering counts as ordinary maintenance and requires no construction permit, per N.J.A.C. 5:23-2.7, while a commercial building repairing more than 25 percent of its total roof area in a 12-month period requires a permit.
When Do You Repair Flashing Versus Replace It?
Repair flashing when a leak stays localized to a single transition and the surrounding covering holds; replace the flashing system when damage exceeds 25 to 30 percent of the roof area or one repair approaches 50 percent of replacement cost. Those are contractor-consensus thresholds, not a code or NRCA statistic, and they frame the decision rather than dictate it.
A localized repair addresses one chimney, valley, or sidewall detail by removing and resetting the flashing and surrounding shingles, which suits a roof whose covering otherwise sheds water. A caulk-only patch buys only a few years before the sealant cracks, per GAF, so a durable repair relapses the metal rather than relying on a fresh bead. A full-roof flashing replacement during a re-roof installs drip edge, valley, step, and penetration flashing to IRC R905.2.8.5 and R903.2.1, and that scope costs more than an isolated transition repair.
Verifying a contractor's flashing work comes down to a few checks: confirm woven step flashing, one piece per shingle course rather than a continuous one-piece strip, which marks a defective installation per InterNACHI; confirm a kickout where the eave meets a sidewall; confirm a two-part base-and-counter chimney system set into the masonry; and confirm the contractor holds registration as a New Jersey Home Improvement Contractor. A continuous strip against a wall or chimney signals work that does not weave the metal as a correct installation requires.
Flashing seals the transitions where most roof leaks begin, governed by IRC R905.2.8.5, R903.2.1, R905.1.2, and ASTM D1970, so a homeowner who recognizes woven step flashing, a kickout, and a two-part chimney system can judge whether a repair or a full re-roof is the sounder choice.