NRCA Metal Flashings key terms

A ranking of metals by their electrical potential in a given electrolyte, used to predict which metal will corrode when two different metals are joined in the presence of moisture.

Accelerated deterioration of the less noble of two connected metals exposed to an electrolyte, where current flows from the anodic metal through the moisture to the cathodic metal.

Any moisture film that conducts ions, such as rain, condensation, or acid runoff, and is required for galvanic current to flow between dissimilar metals.

A metal that resists corrosion and tends to act as the cathode in a galvanic couple, such as copper, which will accelerate attack on a less noble metal it contacts.

The metal that gives up ions and corrodes in a galvanic couple; the smaller the anode area relative to the cathode, the faster the attack concentrates.

The metal that receives electrons and is protected in a galvanic couple; a large cathode area facing a small anode produces severe localized pitting of the anode.

Detailing practice of separating incompatible metals with a barrier such as bituminous coating, tape, gasket, or plastic sleeve so no electrolyte path or direct contact exists.

The natural or chemically enhanced chromium-oxide film on stainless steel that provides corrosion resistance; it is damaged by chlorides, iron contamination, and embedded carbon steel particles.

An austenitic stainless alloy with roughly 18 percent chromium and 8 percent nickel, suitable for many flashings but vulnerable to chloride pitting in coastal or deicing-salt exposures.

A stainless alloy containing molybdenum in addition to chromium and nickel, giving better resistance to chlorides and marine atmospheres than Type 304 for flashings and fasteners.

The green-brown surface layer of copper sulfate and carbonate compounds that forms over years of weathering and slows further corrosion of the copper beneath.

Blue-green discoloration of adjacent masonry, concrete, or roofing caused by soluble copper salts washed from copper flashings and gutters by rain.

Copper sheet factory-coated with lead on both faces to provide a dull gray appearance and to suppress the staining that bare copper runoff causes on adjacent surfaces.

Sheet steel coated with a lead-tin alloy, historically common for gutters and flashings; it requires painting and must not contact copper or be installed where runoff is acidic.

Rolled zinc sheet used for roofing and flashings, valued for long life and low maintenance but requiring ventilation and isolation from copper, lead, and steel runoff.

A factory-applied fluoropolymer coating, typically polyvinylidene fluoride, on aluminum or steel edge metal that resists chalking and fading and is specified by color and warranty.

An aluminum-zinc alloy coating applied to steel sheet, identified as AZ50 or AZ55 by coating weight, offering good atmospheric corrosion resistance for edge metal and panels.

A zinc coating applied to steel by hot dipping or continuous line processes and designated by weight such as G90, which protects the steel sacrificially but is consumed over time.

Shop or field bending of sheet metal on a press brake or hand brake using a punch and die to produce straight bends, hems, and return flanges in flashings.

A folded edge of sheet metal that doubles the material back on itself to stiffen the edge, remove a sharp edge, and strengthen a joint before seaming or fastening.

A formed metal strip fastened to the substrate that receives and holds the concealed edge of a flashing or edge metal, allowing movement while resisting wind uplift.

A fastening method in which screws or clips are hidden behind a flashing, cleat, or interlock so the fastener is not exposed to weather or visible on the finished face.

A screw or rivet driven through the face of metal flashing and sealed with a gasketed head or sealant, requiring compatible material to avoid galvanic attack.

Joining metal with a filler that melts below about 840 degrees Fahrenheit, flowing into a capillary gap; copper and stainless require flux and clean surfaces for a sound joint.

Joining metal with a filler that melts above about 840 degrees Fahrenheit but below the base metal melting point, producing stronger joints than soldering but risking heat distortion.

A chemical paste or liquid applied before soldering or brazing to dissolve oxides and prevent new oxidation so filler metal can wet and bond to the base metal.

A vertical interlocking seam formed along the edge of metal roof panels, folded or mechanically crimped to shed water and accommodate thermal movement without exposed fasteners.

Expansion and contraction of metal flashing or panels as temperature changes, which must be accommodated by slip joints, cleats, or expansion provisions to prevent buckling or tearing.

A deliberate break in a gutter, flashing run, or coping that allows each side to move independently, typically formed with a sliding sleeve or overlapping slip joint.

An overlapping metal connection that is fastened on one side only so the other side can slide, used at expansion joints and in gutter and flashing runs to relieve thermal stress.

A channel at the roof edge or in a valley that collects rainwater and directs it to downspouts, sized by roof area and rainfall intensity and sloped toward outlets.

A vertical pipe that carries water from a gutter or roof drain to a storm system or splash block, sized to handle the contributing roof area and outlet capacity.

An opening through a parapet or wall that lets ponded roof water drain to the exterior, often lined with metal and combined with an overflow scupper for emergency drainage.

A secondary drainage opening set above the primary roof drain or scupper elevation to release water when the primary system is blocked, required by many plumbing codes.

A metal box or leader head at the top of a downspout that receives gutter or scupper flow and transitions it into the vertical conductor while controlling splash and debris.

A narrow horizontal groove cut or cast into a masonry or concrete wall that receives the turned-in top edge of counterflashing to shed water away from the wall.

Metal flashing attached to a wall, curb, or penetration above base flashing, lapping over it to shed water and allow the roof flashing to move independently.

The flashing that turns up from the roof membrane or surface at a wall, curb, or penetration and laps into or behind the counterflashing above it.

The metal cap covering the top of a parapet wall, formed with a slope to shed water inward and with a drip edge or continuous cleat on the outer face.

The assembly of base flashing, counterflashing, and coping at a wall extending above the roof, detailed to keep water out of the wall cavity and off the roof edge.

A metal profile at the roof edge with a projecting lip that makes water fall clear of the fascia and wall below instead of running back under the roofing.

Edge metal with a raised vertical flange that retains ballast or gravel on a low-slope roof while providing a finished edge and a drip lip over the wall.

The formed metal perimeter termination of a roof, including drip edges, gravel stops, and fascia profiles, fastened to resist wind uplift and detailed for water shedding.

The detail where a low-slope roof meets a vertical wall, combining base flashing, counterflashing, and often a cant strip or fillet to shed water and allow movement.

A raised frame around a skylight opening that receives base flashing and counterflashing, typically at least 8 inches high so roofing and flashing can terminate above water.

A prefabricated or fabricated metal flashing that seals around a plumbing vent pipe, with a vertical collar and a flanged base sealed to the roof membrane.

A metal pan set around a roof penetration and filled with sealant or pitch to create a watertight seal where the pipe passes through the roof surface.

The main drainage fitting at the low point of a roof that carries design rainfall to the storm system, with secondary or overflow drainage provided separately by code.