Aluminum Roof Flashing: Roll Sizes, Gauges and Best Uses

Most roof leaks start at a seam, not in the middle of a field of shingles. Walls, chimneys, skylights and valleys all need a strip of metal that sheds water past the joint, and aluminum roof flashing is the material most homeowners reach for first. It is light, cheap, easy to cut with snips, and it does not rust. Sold in rolls from 6 to 24 inches wide and 10 to 50 feet long, it can be bent into step flashing, counterflashing, L-shaped wall flashing or a simple valley liner on site.

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The catch is that aluminum has limits. It reacts badly with copper, fresh concrete, mortar and some pressure-treated lumber, and thin household rolls dent easily under foot traffic. Choosing the right width, thickness and finish for the spot you are flashing is what separates a repair that lasts 30 years from one that leaks after the second winter.

Here you will find how aluminum flashing rolls are sized, which gauges belong where, when to pay extra for a painted finish, how to bend clean profiles, and which materials it should never touch.

What Aluminum Roof Flashing Is and Where It Works

Aluminum roof flashing is sheet aluminum alloy, usually rolled from a 3000-series alloy that resists corrosion and bends without cracking. You will see it labeled as aluminum flashing roll, aluminium roof flashing in some import listings, or simply flashing coil. All of them describe the same basic product: a flat ribbon of metal wound on a cardboard core or packed in a box.

It works well in these places:

• Step flashing where a shingled roof meets a sidewall, with one bent piece per shingle course.
• Counterflashing that covers the top of step flashing or base flashing along a wall or chimney.
• L-shaped flashing (also called angle or apron flashing) along a headwall where the roof dies into siding.
• Open valley liners on asphalt shingle roofs, using a wide roll.
• Patch work under a lifted shingle or around a small vent where a stock boot is not available.

It is a poor choice against masonry that stays wet, in contact with copper, or on coastal roofs within salt-spray range unless you choose a coated product rated for marine exposure. In those spots, look at copper, stainless or lead-coated options, or isolate the aluminum with a membrane.

Roll Widths and Lengths: Picking the Right Size

Flashing rolls come in a predictable set of widths. Matching width to the job saves you from wasteful cutting and, more importantly, gives you enough metal to lap properly.

Lengths run from 10 feet for small repair packs up to 50 feet for contractor rolls. A 10-foot roll handles one vent or a short sidewall. For a full sidewall on a two-story addition, figure the run length plus about 20 percent for laps and offcuts.

For valleys, roofing practice calls for metal at least 24 inches wide so each side extends well under the shingles. For step flashing, each piece should cover at least 4 inches up the wall and 4 inches onto the roof, which is why 8 to 10 inch stock is common.

Gauges and Thickness: How Heavy Should It Be?

Aluminum thickness is often listed in thousandths of an inch rather than gauge. Big-box rolls are frequently thinner than what professional roofers stock, so read the label carefully.

• 0.0135 to 0.016 inch: Light household rolls. Fine for small patches and step flashing hidden under shingles, but they dent and wrinkle easily.
• 0.019 inch: A common middle grade, and the thickness used for most trim coil. Good for counterflashing, L-shaped flashing and drip edge you bend yourself.
• 0.024 to 0.032 inch: Heavier stock for valleys, chimney aprons and any exposed flashing that will see foot traffic or ice.

As a rule, the more exposed the flashing, the heavier it should be. Pieces tucked entirely under shingles can be thinner because the shingles protect them. Pieces you can see from the ground, such as counterflashing on a chimney, should be at least 0.019 inch so they lie flat and do not oil-can in the sun.

Mill Finish vs Painted Finish

Mill finish aluminum is bare, shiny metal straight from the rolling mill. It costs less and weathers to a dull gray. On a hidden step flashing job or under shingles, mill finish is all you need.

Painted rolls carry a baked-on coating, usually polyester or a tougher fluoropolymer finish. White aluminum flashing and white aluminum flashing roll products are popular because they match white trim, fascia and siding. Brown, black, bronze and gray are also common. The coating adds a little corrosion resistance and a lot of curb appeal on exposed pieces.

A few tips on painted stock:

• Bend with the painted side facing out where it will show, and protect it from scratches with a scrap of cardboard on the brake.
• Touch up cut edges or scratches with matching exterior paint if they face the weather.
• Expect to pay roughly 20 to 50 percent more than mill finish for the same width and thickness.

Tools and Materials

• Aluminum flashing roll in the right width and thickness for the job
• Aviation snips (left, right and straight cut)
• Hand seamer for short bends
• Portable sheet metal brake for long, straight bends (rentable)
• Tape measure, speed square and a fine-tip marker or scribe
• Aluminum or stainless roofing nails (never copper or plain steel)
• Roofing sealant rated for metal, or butyl tape
• Self-adhered membrane strips for isolating the metal from incompatible materials
• Work gloves, safety glasses and a roof harness for steep or high work

How to Cut and Bend Aluminum Flashing

1. Measure the joint. Note the roof-side leg and wall-side leg you need. For step flashing, 4 inches on each leg is a common minimum; for headwall L-shaped flashing, 4 to 6 inches up the wall and 4 to 6 inches onto the roof.
2. Unroll and flatten. Let the coil relax on a flat surface. A slight curve will remain; bending usually takes it out.
3. Mark the cut and bend lines. Use a scribe or marker and a straightedge. Mark bend lines on the face that will be inside the bend.
4. Cut to length. Use straight snips for long cuts. For many identical step flashing squares, cut a strip first, then crosscut it into pieces.
5. Bend on a brake or with a seamer. A brake gives crisp, straight bends over long lengths. For short pieces, clamp the metal between two straight boards and fold it by hand, then sharpen the crease with a seamer.
6. Add a hem where edges are exposed. Folding a 1/2 inch hem along an exposed edge stiffens the piece, hides the sharp cut and helps shed water.
7. Dry fit before fastening. Check that legs sit flat and that each piece laps the next piece downslope by at least 2 inches in the direction water flows.

Fasten flashing high on the wall leg or high on the roof leg so the nail is covered by the next course or piece. Nails through the exposed lower portion are a common source of leaks.

Galvanic Corrosion and Incompatible Materials

Aluminum is an active metal. When it touches a more noble metal in the presence of water, the aluminum corrodes first. That process is called galvanic corrosion, and it can eat pinholes into thin flashing within a few years.

Keep aluminum roof flashing away from:

• Copper pipes, copper flashing or water running off a copper roof or gutter.
• Pressure-treated lumber treated with copper-based preservatives, which are standard for decks and sill plates today. Put a membrane or a layer of self-adhered underlayment between them.
• Fresh mortar, concrete and stucco, which are alkaline and can etch aluminum. Use a membrane or a coated metal, and set counterflashing into a reglet only after the masonry has cured.
• Plain steel or galvanized nails, which can corrode against aluminum. Use aluminum or stainless fasteners.

Where aluminum must meet an incompatible material, a strip of self-adhered membrane or a coat of bituminous paint on the aluminum creates a barrier that stops the reaction.

Roof Safety When Installing Flashing

Flashing work puts you at roof edges, around chimneys and on steep slopes. Set the ladder on firm, level ground at a 4-to-1 angle (1 foot out for every 4 feet up), extend it at least 3 feet past the roof edge, and tie it off at the top. Keep three points of contact while climbing and never carry rolls of metal up the ladder by hand; hoist them with a rope instead.

Wear soft-soled shoes, avoid wet, frosty or windy days, and use a harness tied to a roof anchor on any slope steeper than about 6 in 12. Cut aluminum edges are razor sharp, so gloves are not optional. On two-story homes or steep roofs, a roofing contractor with fall protection gear is the safer choice.

Troubleshooting Common Flashing Problems

• Leak at a sidewall: Usually a single continuous strip used instead of step flashing, or missing counterflashing. Replace with individual step pieces lapped with each course.
• White powder or pitting: A sign of corrosion from contact with copper, treated wood or masonry. Replace the section and add a separation layer.
• Wavy exposed flashing: Metal too thin for the span. Swap in 0.019 inch or heavier stock and hem the edges.
• Sealant cracking: Sealant should be a backup, not the main defense. Rework laps so water sheds by gravity, then reseal.
• Water behind headwall flashing: Siding or housewrap not lapped over the wall leg. The wall leg must sit behind the weather barrier, not on top of it.

Cost of Aluminum Flashing Rolls and Installation

Material costs are modest. A 10-foot roll of light mill finish flashing in a 6 to 10 inch width usually runs from the low teens to around 30 dollars. Wider 20 to 24 inch rolls, heavier gauges and painted finishes cost more, often 40 to 150 dollars depending on length. A pack of precut step flashing pieces typically costs 15 to 40 dollars.

Professional flashing repair around a chimney or along a sidewall generally runs a few hundred dollars for simple jobs and climbs past 1,000 dollars when shingles, siding or a cricket must be rebuilt. Valley replacement is often priced per linear foot along with shingle work.

When to Call a Roofing Pro

DIY flashing makes sense for a short sidewall, a small patch or a vent you can reach safely from a single-story roof. Call a roofing contractor when the roof is steep or two stories up, when water has already damaged sheathing or framing, when a chimney needs a new cricket or reglet cut into masonry, or when multiple layers of shingles must come off to reach the old flashing. A pro can also check for hidden rot and confirm the flashing ties into your housewrap and siding correctly.