Galvanized Steel Flashing: When It Beats Aluminum

Aluminum gets most of the attention at the home center, yet roofers still reach for galvanized steel flashing on a surprising number of jobs. Steel is stiffer, holds a crisp bend, shrugs off foot traffic and ice, and does not react with fresh mortar or concrete the way bare aluminum does. Its weak point is rust, and how long it lasts depends almost entirely on the zinc coating and what touches it.

Part of our Roofing guide — explore the full guide →

The short answer: choose galvanized steel where the flashing meets masonry, where snow and ice load the roof edge, where the piece must span or resist denting, and where the job calls for soldered seams. Choose aluminum near salt air, against copper or treated lumber you cannot isolate, and on simple step and drip edge work where light weight and easy cutting matter more than stiffness.

Both metals do the same basic job: they steer water away from joints that shingles, siding and brick cannot seal on their own. The difference is how each one ages in your specific spot on the house.

What Galvanized Steel Flashing Is and How It Protects Itself

Galvanized steel flashing is ordinary carbon steel sheet that has been dipped in molten zinc. The zinc forms a metallurgically bonded layer on both faces. When the surface gets scratched, the surrounding zinc corrodes first and protects the exposed steel, a process called sacrificial or cathodic protection. That self-healing behavior is why a nicked galvanized piece does not start rusting at the scratch right away.

Coating weight tells you how much zinc you are buying. Look for a G90 rating on roofing products, which means roughly 0.90 ounces of zinc per square foot of sheet, split between both sides. Lighter coatings such as G60 or G40 show up on cheap ductwork and some bargain rolls, and they rust years sooner on a roof. If the label does not state a coating weight, assume it is light and keep it for interior or temporary use.

Thickness matters too. Roofing flashing typically comes in 26, 28 or 24 gauge, and with steel a lower number is thicker. Twenty-six gauge is the common all-around choice for step flashing, valleys and drip edge. Twenty-four gauge makes sense for counterflashing in brick, long valley runs and any piece that will get walked on or hit by sliding ice.

Galvanized Steel vs Aluminum: Side-by-Side

Neither metal wins everywhere. This comparison covers the points that usually decide the job:

Expansion is another quiet factor. Aluminum moves about twice as much as steel with temperature swings, so long aluminum runs need more allowance at laps and fasteners. Steel stays put better on long drip edge and valley pieces.

Where Steel Outperforms Aluminum

Masonry chimneys and brick walls

Counterflashing that gets ground into a mortar joint sits in damp, alkaline material for decades. Bare aluminum slowly pits in that environment unless it is coated or isolated. Galvanized steel tolerates fresh and cured mortar far better, which is why many masons prefer it for chimney counterflashing, through-wall flashing at brick ledges and cap flashing on block parapets.

Heavy snow and ice country

Ice dams, sliding snow and the occasional roof rake all punish the eave. A steel drip edge or eave flashing keeps its profile when ice shifts against it, while light aluminum can bend down and open a gap at the fascia. The same goes for valley flashing that carries heavy meltwater and occasional ice chunks.

Long, stiff, or walked-on pieces

Wide valley liners, cricket flashing behind chimneys and apron flashing that a roofer will kneel on all benefit from stiffness. Steel also keeps a crisp hem, which helps on drip edges and kick-out details where a sharp bend sheds water cleanly.

Soldered work

Chimney saddles, box gutter liners and custom pans sometimes need soldered seams. Galvanized steel solders well once the surface is cleaned and fluxed; aluminum does not, so it usually ends up caulked and riveted instead.

Rolls, Preformed Pieces and Galvanized Flashing Rolls

You will see galvanized steel roll flashing in widths from about 6 to 20 inches and lengths of 10 to 50 feet. A roll of metal flashing like this is the most flexible way to buy, because you cut and bend exactly what each spot needs. Shoppers often call it tin flashing roll or sheet metal flashing, though modern rolls are zinc-coated steel rather than true tin plate. If a galvanized flashing roll is your plan, a basic hand brake or a pair of locking bending pliers makes clean bends much easier than folding over a board edge.

Preformed pieces save time on repetitive details. Galvanized steel drip edge flashing comes in 10-foot sticks in common profiles, step flashing comes precut in bundles of bent squares, and you can find L-flashing, Z-flashing and valley sections already formed. For a reroof, buy preformed drip edge and step flashing and use a steel flashing roll for chimney, skylight and odd transitions.

A painted finish is an option on many steel products. The paint adds a second barrier on top of the zinc and helps it blend with trim colors, but the cut edges still need touch-up.

Fastener and Metal Compatibility Rules

Most early rust on galvanized steel flashing comes from what it touches, not the metal itself. Keep these rules in mind:

• Use hot-dipped galvanized nails or screws. Electro-galvanized fasteners carry a thin zinc layer and rust out first. Stainless steel fasteners also work and are a good upgrade near the coast.
• Never use copper nails or let copper drip onto it. Runoff from copper roofing or copper caps eats zinc quickly and leaves rust streaks within a few seasons.
• Isolate it from pressure-treated lumber. Modern copper-based wood treatments attack zinc. Put a strip of self-adhered membrane or flashing tape between treated wood and the steel, or use a vinyl or coated flashing at ledger boards.
• Do not mix with aluminum in direct contact. Dissimilar metals in a wet joint create galvanic corrosion. Separate them with membrane or choose one metal for the whole detail.
• Treat cut edges. Snipped edges expose bare steel. A zinc-rich cold galvanizing paint on visible cuts slows edge rust.

How to Install Galvanized Step and Counterflashing at a Wall

Here is the typical sequence where a sloped roof meets a sidewall or chimney:

1. Strip old flashing and check the sheathing and wall framing for soft wood. Replace damaged decking before going further.
2. Run underlayment and, in cold climates, a self-adhered membrane turned up the wall at least 4 inches.
3. Install the first shingle course, then set a step flashing piece (typically 5 by 7 or 8 by 8 inches, bent 90 degrees) on top of it, tight to the wall.
4. Nail the step only to the roof deck, high on the piece, with one fastener. Never nail into the wall leg, so the roof and wall can move independently.
5. Lay the next shingle course over the step, then the next step overlapping the one below by at least 2 inches. Repeat up the wall.
6. On siding walls, lap housewrap and siding over the vertical legs. On masonry, cut a reglet into a mortar joint about 1 inch deep and insert counterflashing that laps down over the steps.
7. Secure counterflashing with wedges or masonry fasteners and seal the reglet with a polyurethane or butyl sealant rated for metal and masonry.

Galvanized steel flashing also pairs well with kickout flashing at the bottom of a sidewall, the piece that throws water into the gutter instead of down the wall.

Troubleshooting Rust and Leaks

• Rust streaks at the cut edge only: normal edge exposure. Wire brush lightly and coat with cold galvanizing paint.
• Rust spreading across the face after a few years: likely a light coating or copper runoff. Plan replacement with G90 or switch to a better-suited metal.
• White powdery residue: zinc oxidation, often from rolls stored wet and stacked. It is mostly cosmetic if the surface beneath is intact.
• Leaks at a chimney despite good metal: check the counterflashing reglet sealant, the cricket on the upslope side and whether the steps overlap in the right direction.
• Rust where the flashing touches a deck ledger: treated lumber contact. Add a membrane separation layer when you replace it.

Cost Ranges

Material prices vary by region and steel markets, but general ranges help with budgeting. Galvanized steel roll flashing usually runs a little less per foot than comparable aluminum in plain finish and about the same in painted finish. Preformed galvanized drip edge sticks are an inexpensive line item on a reroof. Step flashing bundles are modest in cost.

Labor drives the real expense. A roofer replacing chimney flashing with new steps, counterflashing and a cricket commonly charges several hundred to a couple thousand dollars depending on chimney size and roof pitch. Wall flashing repairs that require removing and reinstalling siding can run higher because of the carpentry involved.

Safety and When to Call a Pro

Flashing work happens at the roof edge, so treat fall protection as part of the job. Set extension ladders on firm, level ground at a 4-to-1 angle, extend them at least 3 feet past the eave, and tie them off at the top. On steep or slick roofs, wear soft-soled shoes and use roof brackets or a harness anchored to the framing. Freshly cut sheet steel edges are razor sharp, so wear cut-resistant gloves and eye protection while snipping.

Hire a roofing contractor for anything two stories or higher, any roof steeper than about 6 in 12, and any chimney that needs reglet cutting or a new cricket. A pro is also the safer choice when you find rotted decking or wall framing, since structural repair and proper sequencing with underlayment matter as much as the metal itself.