Therwen 1 Roll Aluminum Flashing Roll Roof Flashing for Sealing Windows Doors Siding Roofing Stains Moss and Mildew Prevention

Why I reached for this roll

A small leak and a rotting window sill kicked off my latest round of exterior upkeep, and I wanted a simple, predictable material I could cut, bend, and fasten without hauling out a brake or special tools. I picked up the Therwen aluminum flashing—14 inches wide by 10 feet long, about 0.008 inches thick—and put it to work across a few projects: an apron flashing under a door threshold, a quick patch on a shed roof penetration, and a makeshift critter guard along a fence base. It handled all three without fuss, and that’s really the headline here: it’s straightforward, versatile sheet aluminum that behaves exactly how it should.

Build and material quality

This flashing is standard aluminum coil stock in a manageable roll size. At roughly 0.0078 inches thick (around 32–34 gauge), it strikes a good balance between flexibility and dent resistance. I could form clean bends by hand over a 2x block, yet once installed, the panels didn’t oil-can excessively. The metal arrives bright silver, smooth, and free from visible mill defects. Edges are sharp—as expected with coil sheet—so gloves matter.

Aluminum is naturally corrosion resistant, and that shows up in practice. For exterior flashing jobs around doors, windows, and siding—especially in areas where galvanized steel can rust or stain—aluminum is a sensible pick. It’s also light enough that handling ladders and long cuts is easier and safer.

A couple of caveats on the material:
- Aluminum and copper don’t play nicely together; avoid direct contact to prevent galvanic corrosion.
- ACQ-treated lumber can attack aluminum over time. If you’re flashing against pressure-treated wood, use a barrier (e.g., acrylic or butyl flashing tape) between the metal and the wood.
- Near coasts, airborne salts can pit aluminum; a good paint system helps if you need longevity in that environment.

Size and handling

Fourteen inches is a sweet spot for a lot of light exterior details. It’s wide enough to create proper laps under siding, build functional sill pans, and do apron flashing beneath shingles, yet narrow enough to cut comfortably with straight snips. The 10-foot length is practical: long enough to span a door head or multiple smaller windows, and short enough to store on a shelf without deforming.

Cutting and forming were easy. I used left- and right-cut snips for curves and a long straight snip for rips. For tight hems, I scored lightly with a utility knife and folded over a 3/8-inch hem by hand; the fold held crisp without cracking. Springback is modest; a second pass with the block squared up folds nicely.

Installation experience

For the door threshold apron, I measured the finished siding reveal, added my upturn and drip, and laid out bends with a speed square. Aluminum at this thickness accepts a 90-degree bend cleanly without a brake if you take your time. I fastened the apron with stainless pan-head screws in the upper leg, keeping fasteners out of the water-shedding surface. A thin bead of polyurethane sealant behind the top flange, plus a bed of butyl tape over the fastener heads, finished the detail. The piece tucked under the siding neatly and shed a test bucket of water without any capillary creep.

On the shed roof patch around a small conduit penetration, I cut a square saddle piece, hemmed the edges, and lapped it 4 inches under and over the shingles. Aluminum pairs well with butyl or asphalt-based flashing tapes; I used butyl, which adheres strongly to the metal and remains flexible. Nail placement was high and dry, and I dressed edges with a swipe of roofing cement. No leaks through the next two storms.

For the fence base critter guard, I ripped the roll into 6-inch strips, hemmed the exposed edges to eliminate sharpness, and screwed the strips along the bottom rail with stainless fasteners—no sealant required. The metal’s stiffness helps it stay flush to irregular wood; a fastener every 10–12 inches was plenty.

Performance and durability

The aluminum sheds water well, which is the point, but it also stands up to sunlight and temperature swings without any notable distortion. Thermal expansion is more pronounced with aluminum than steel, so you don’t want to overconstrain long runs; slotting a few holes or using slightly oversized holes under washers gives the metal the room it needs to move.

After a few weeks of exposure and a couple of heavy rains, there’s no staining or edge oxidation. Paint adhesion is acceptable with prep; I scuffed a test piece with 220-grit, wiped with solvent, and sprayed a self-etching primer followed by exterior enamel—no issues with adhesion or fish-eye.

Where it excels

• Door and window flashing: Sill pans, head flashings, drip edges, and small claddings. The 14-inch width gives you generous laps under WRB or siding layers.
• Roofing details: Apron flashing against vertical walls, small chimney saddles, pipe/vent surrounds (not as a code-compliant boot), and patchwork under shingles.
• Siding transitions: Kick-out flashings and custom diverters to route water away from trim.
• Light-duty barriers: Rodent/animal guards at the base of fences or sheds and splash shields behind faucets.
• Shop and farm odds-and-ends: Guards, shims, and thermal shields (at safe distances) where a thin, formable metal is handy.

Limitations and what it’s not

• Not a self-adhesive flashing: You’ll need fasteners and/or compatible sealants and tapes. For window installs that rely on peel-and-stick sequencing, this is a complement, not a substitute.
• Not for masonry step flashing where code or best practice calls for heavier gauge or copper. Aluminum in contact with lime-rich mortar can corrode without proper separation.
• Not a high-heat solution: Don’t use it as a flue or stove-pipe component; keep to code-required clearances and rated materials.
• Not structural: It’s sheet metal, not a load-bearing element.

Practical tips from my installs

• Hem exposed edges: A 3/8-inch hem stiffens the piece, eliminates the worst of the sharpness, and looks finished.
• Sequence matters: Always lap from bottom to top and shingle your layers so water never sees an upslope seam.
• Fasteners: Use aluminum or stainless to avoid galvanic staining. Keep fastener lines high on vertical legs, out of water paths.
• Sealants: Polyurethane or high-quality hybrid sealants bond well to aluminum. Butyl tape is excellent between laps; avoid cheap asphalt mastics where heat could cause bleed-out.
• Barrier layers: Against pressure-treated wood, add a strip of acrylic/butyl flashing tape or a paintable barrier to protect the aluminum.
• Paint prep: Scuff, degrease, etch-prime, then topcoat if the silver finish won’t match your project.

Value and packaging

As a compact 10-foot roll, this is a useful size for DIYers and pros tackling scattered details without committing to a full 50-foot coil. The cost per foot is reasonable compared to big-box coil stock, especially if you don’t need color-matched finishes. My roll arrived round, not kinked, with only the expected coil set; a quick reverse curl tamed the leading edge before layout.

Alternatives to consider

• Thicker aluminum (0.010–0.019 inch) for exposed areas that see impacts or where you want very crisp bends with minimal oil-canning.
• Painted coil stock if you need a finished look without painting on site.
• Peel-and-stick butyl or acrylic flashing membranes for window WRB integration, used alongside metal drip/head elements.
• Copper or stainless where longevity in harsh conditions or masonry contact is paramount, budget allowing.

The bottom line

The Therwen aluminum flashing delivers exactly what I want from a light-gauge roll: it’s easy to cut and form, resistant to corrosion, forgiving to install, and versatile across a lot of small exterior jobs. The 14-inch width and 10-foot length are practical, and the material quality is consistent. It’s not a cure-all—use the right tapes and sealants, mind dissimilar metals, and follow good drainage details—but as a dependable, general-purpose flashing sheet for roofs, windows, doors, siding, and small repairs, it performs well.

Recommendation: I recommend this flashing for homeowners and pros who need a manageable, light-gauge aluminum for everyday waterproofing details and small projects. It hits the sweet spot of workability and durability, offers good value in a compact roll, and integrates cleanly with common roofing and siding practices when installed correctly. If your projects demand heavier gauge or code-specific materials (masonry, high-impact areas, or high-heat applications), choose accordingly; otherwise, this roll should earn a spot on the shelf.