Metal Flashing

Flashing is the engineered metal that seals the joints, edges, and penetrations where a roof is most likely to leak. It seals roof penetrations — vent pipes, exhaust fans, skylights, chimneys — and creates a watertight barrier that redirects water away from vulnerable areas, preventing leaks and damage to the roofing structure, insulation, and interior. The shingles get the attention. The flashing keeps the water out.

Here’s the part most people miss: the majority of roof leaks occur at flashing points. Flashing forms the intersections and terminations of roofing systems and surfaces. Which means the metal type matters less than how it’s layered. Get the install right and a basic galvanized drip edge outlasts the homeowner. Get it wrong and the fanciest copper in the valley still leaks.

This guide covers the main flashing types, the metals worth knowing, how to match the right piece to the job, and the install fundamentals that actually keep water out — especially through a Montana freeze-thaw cycle.

Flashing Types and What Each One Does

Every flashing piece lives at a seam — roof to wall, roof to chimney, panel to penetration. Most of the cracks that require flashing are found where dissimilar materials meet — for example, where a chimney meets roofing, where roofing meets a wall, or where siding meets a window — or at the exposed perimeter of surfaces like roofs and walls.

The common pieces a builder reaches for:

• Drip edge runs the eaves and rakes. Western Building Center carries D-flashing in 10-foot lengths in 28-gauge, in galvanized, black, brown, and white.
• Step flashing handles the roof-to-wall transition. Stock runs 4x4x12 and 4x4x14 in galvanized, black, brown, and white.
• Counterflashing caps the chimney side. Chimney flashing is fitted under the shingles and goes up a predetermined height up the chimney wall.
• Valley flashing for where two slopes meet. Roof valleys are when two sloped roofs meet and the shingles run perpendicular to each other. Flashing is applied along the length of this valley.
• Gutter apron ties the eave to the gutter — stocked in 10-foot black, brown, and white.
• Z-flashing for horizontal siding and trim transitions. WBC carries it in 3/8”, 1/2”, 5/8”, and 5/16” profiles, 10-foot lengths, galvanized and colored.
• Pipe and vent boots seal round penetrations. Vent pipe flashing seals the area around plumbing vent pipes that protrude through the roof. They help prevent water from entering and are often made of flexible materials such as rubber or thermoplastic, allowing flexibility in accommodating various pipe diameters.

On commercial and low-slope work, the list grows. The sheet metal addressed here is a system of formed metal pieces — copings, counterflashings, base flashings, gravel stops, fasciae, gutters, downspouts, scuppers, conductor heads, through-wall flashings, head and sill flashings, expansion-joint covers, and closure trim.

There are specialty pieces too. Solar flashings are specialized components used to integrate solar panels seamlessly into the roof and ensure a watertight seal. And for the service mast, electrical mast flashings are specialized components used to provide a weatherproof seal around the electrical service entrance mast.

Choosing the Metal and Matching It to the Job

Four metals cover nearly everything. A veteran roofer’s rundown from decades back still holds: galvanized steel for drip-edge flashing, aluminum for step flashing on asphalt shingle roofs, copper for most purposes on slate roofs, and lead for chimney counterflashing.

The biggest mistake isn’t picking the wrong metal — it’s mixing metals that shouldn’t touch. Match your flashing to the adjacent metals and fasteners or you’ll get galvanic corrosion eating the connection. For copper sheet specifically, nails should be copper, hardware bronze, or Series 300 stainless steel, 0.109 inch minimum and not less than 7/8 inch long, barbed with large head.

Steel that can rust needs a coating. Materials that can rust must be properly coated. Kynar resin-based coatings are applied to the metal flashing surface prior to roof attachment. These help protect the flashing from harsh elements. On coated steel sheet you’ll see designations like G90 galvanized and AZ50 Galvalume — the coating weight is what’s buying you corrosion life.

Matching the flashing to the application matters as much as the metal:

• Pipe size drives boot size. Knowing the pipe diameter will allow you to choose the flashing size. Multi-size boots cover a range, which helps when you’re not certain of the exact pipe: Oatey offers 1.5-inch, 2-inch, and 3-inch sizes in one flashing and 3-inch and 4-inch in another.
• Metal roofs need a master-flash type. Flashings for metal roofs are designed to work with the unique properties of metal, which can expand and contract with changes in temperature. Oatey Master Flash Roof Flashings are intended for metal-profiled roofs and can be installed on most types of roofing surface.
• Chimney, skylight, and valley each call for a distinct piece. Different roof types require different flashings; for example, a chimney flashing may differ from a skylight flashing, depending on the structure.

Metal	Typical use
Galvanized steel	Drip edge, general flashing (needs coating to resist rust)
Aluminum	Step flashing on asphalt shingle roofs
Copper	Slate roofs, premium long-life work
Stainless steel	Through-wall and demanding-corrosion locations

Installation Basics That Keep Water Out

The rule that governs all of it: water flows downhill, so flashing gets layered shingle-style — each piece overlapping the one below — so runoff sheds over the metal, never behind it. Metal flashing usually depends on gravity and laps for watertightness.

For pipe and vent penetrations on a metal roof, the right method is to cut the pan at the penetration, set the flashing, then continue the pan to the ridge in overlapping shingle fashion. Surface-mounted, face-fastened, caulk-and-pray flashings with up-slope edges are exactly how leaks start.

The Montana failure point is thermal movement. One of the most challenging flashing applications involves thermal expansion and contraction. This is especially so when tying a valley detail into the ridge detail, as movement at these junctures can be severe. Sheet metal standards size for it directly — flashing must allow for thermal movements by preventing buckling, opening of joints, hole elongation, overstressing of components, and sealant failure, using clips that resist rotation. The design temperature change range is 120°F ambient and 180°F at material surfaces. That’s the kind of swing a chinook delivers in an afternoon. Build for movement: oversized or slotted fastener holes, and eliminate through-fasteners where you can.

A field-proven sequence for tough thermal junctions:

1. Determine how the expansion joint detail can move at this location — metal panels and flashing materials expand and contract with temperature change.
2. Determine how water will drain across the flashing. Change of slope and change of plane can force water, snow, or ice in different directions.
3. Eliminate through fasteners.
4. Cover sealants, expandable flashing materials, and underlayment membranes with the metal flashing material. When these components are exposed to ultraviolet rays, they will fail before the metal roof does.

That last point is the one DIYers skip. Sealant and underlayment are backup, not the front line — bury them under metal so the sun never touches them. And the sealants and tapes themselves have to take the abuse: materials need to handle the roof’s expansion and contraction to maintain a watertight seal, and must withstand UV exposure without becoming brittle. If they lose elasticity, they can no longer handle expansion and contraction.

Chimney layout deserves its own note in snow country. A chimney sitting below the ridge dumps water and snow down its back side. One approach is to leave the panels about a foot from the back of the chimney, laying them out so the ribs fall to the outside of the chimney, so water and snow naturally flow around it.

Proper flashing also earns its keep against ice dams — those thick ridges of ice that form along roof edges and can force water under shingles and into your home. In the Flathead and Tobacco valleys, that’s a yearly event, not a freak one.

Codes and Roof-Edge Wind Requirements

This one’s mostly for commercial and low-slope builders. Roof-edge metal is wind-design-critical, and the code treats it that way. The roof-edge subset of this work — copings, gravel stops, fasciae, and drip edges — is wind-design-critical, and the building code requires that low-slope roof-edge metal be tested and labeled per ANSI/SPRI ES-1. That’s not a suggestion: roof-edge metal that does not carry an ES-1 classification meeting the design wind pressure shall not be installed.

Two more wrinkles worth knowing. Where the project’s roof is governed by FM Global insurance, FM Approval (FM 4435) for the roof-edge metal is additionally required. And the rest of the system isn’t off the hook just because it isn’t ES-1 rated: counterflashings, wall flashings, gutters, downspouts, and trim that do not function as roof-edge wind securement are not subject to ES-1 but must comply with the cleat, fastener, and joint provisions in SMACNA for resistance to wind, thermal movement, and water infiltration.

On base flashing dimensions, the sheet-metal standard sizes it at 10 inches to 12 inches, with a maximum allowable of 18 inches, mechanically fastened at the top, with two-piece through-wall counterflashing.

Frequently Asked Questions

Why does most water get in at the flashing instead of the shingles? Because flashing sits at every transition and termination on the roof — the seams between two systems. The majority of roof leaks occur at flashing points; flashing forms the intersections and terminations of roofing systems and surfaces. Open field shingle is easy. Joints are where craftsmanship shows.

Can I mix metals — say, aluminum flashing with steel fasteners? Not without inviting galvanic corrosion. Match the flashing metal to the fasteners and adjacent metals. With copper, that means copper, bronze, or 300-series stainless nails, 0.109 inch minimum and not less than 7/8 inch long, barbed with large head.

What size pipe boot do I need? Measure the pipe. Knowing the pipe diameter will allow you to choose the flashing size. If you’re unsure, a multi-size boot covers a range — one flashing can cover multiple sizes, so an installer doesn’t need to know the exact size; options include 1.5-inch, 2-inch, and 3-inch in one flashing and 3-inch and 4-inch in another.

Does a metal roof use the same flashing as shingles? No. Metal panels move with temperature, so they need a flashing built for it. Because the installation process and compatibility differ, a metal roof requires a master flashing.

Why cover the sealant and underlayment with metal? Because UV destroys them long before it touches the metal. Cover sealants, expandable flashing materials, and underlayment membranes with the metal flashing material — when these components are exposed to the sun’s ultraviolet rays, they will fail before the metal roof. Metal is the shield; everything under it is backup.

Does roof-edge flashing have to be code-rated? For low-slope roofs, yes. IBC Section 1504.5 requires that low-slope roof-edge metal be tested and labeled per ANSI/SPRI ES-1. Field-fabricated edge metal without that listing has to be replaced or engineered and tested for the project before it goes up.