Professional Extruded Flashing on Any Job Site: Building a Sheet Metal Bending Jig

Bending sheet metal flashing on site is one of those tasks that sounds simple enough until you try it. Without the right setup, hand-bent flashing turns into a frustrating exercise in chasing wavy edges, crinkled corners, and pieces that refuse to sit flat against the roofline or wall. The result often looks amateurish and, worse, leaves gaps that defeat the purpose of flashing altogether. Whether you are working on roof flashing installation or window openings, having a reliable method for forming custom profiles on site saves both time and money while delivering professional-grade results.

Extruded flashing is the answer. The term refers not to a plastic extrusion process but to a field technique in which sheet metal is pulled through a shaped plywood die that forces it into the desired cross section. The result is a clean, uniform bend with none of the distortion that comes from hammering or hand-folding. This article walks through how to build the jig, execute the pull, and apply the technique to common flashing scenarios.

Why Hand-Bent Flashing Falls Short

The instinct when fabricating flashing on site is to clamp the sheet metal to a workbench, bend it over the edge with a hammer, and hope for the best. That approach works for simple 90-degree bends in thin material, but it quickly breaks down when you need:

• Multiple bends in a single piece (such as Z-flashing or drip edges)
• Consistent angles across dozens of pieces for a large roof
• Smooth curves without kinking the metal
• Tight radius bends in heavier-gauge material
• Long lengths beyond 8 ft that are impossible to manage on a benchtop

Hand bending produces wavy edges because the metal deforms unevenly along its length. The force applied by a hammer or brake concentrates at the point of impact rather than distributing uniformly across the bend line. Even with a hand seamer or bending brake, small errors multiply over the length of the piece. The result is flashing that rocks on the roof deck, leaves uneven gaps at the edges, and ultimately compromises the weather barrier.

The Extruded Flashing Principle

The extruded flashing jig works on a simple principle. Instead of pushing or hammering the metal into shape, you pull it through a fixed profile that has been cut into a thick plywood form. The profile acts as a die. As the sheet metal passes through, it conforms to the shape of the cut. Because the pulling force is applied evenly across the full width of the strip, the metal bends uniformly without distortion.

The technique was originally published in Fine Homebuilding Issue 9 and has since become a standard field trick among experienced roofers and general contractors. The beauty of the method is its simplicity. The jig takes about 15 minutes to build from scrap materials, requires no special tools beyond a jigsaw, and produces results that rival shop-fabricated flashing.

Building the Plywood Profile Jig

The jig consists of a single piece of plywood with the exact cross section of your desired flashing profile cut through it. This plywood form is then anchored across the studs of an unsheathed wall or partition at chest height, creating a rigid, immovable die.

Materials and Tools

Step-by-Step Jig Construction

1. Draw the profile at full scale. Using a piece of cardboard or heavy paper, trace the exact cross section you need for your flashing. Common profiles include Z-shapes, drip edges, step flashing, and W-shaped valleys. Make the drawing precise to the millimeter because the plywood cut will replicate it exactly.
2. Transfer the profile to plywood. Lay your cardboard template on the 3/4 in. plywood scrap and trace the outline. Make sure the plywood grain runs perpendicular to the bend lines for maximum strength.
3. Cut the profile. Using a jigsaw or keyhole saw with a fine-tooth blade, cut along the traced line. Take your time on corners and tight radii. A smooth cut produces smooth flashing. Sand the cut edges lightly with 80-grit sandpaper to remove splinters that could snag the metal.
4. Mount the jig. Position the plywood across the studs of an unsheathed interior or exterior partition wall at chest height (roughly 4 to 5 ft off the floor). Drive at least four 16d common nails through the plywood and into the studs. The jig must be absolutely rigid because any movement during the pull will distort the bend.
5. Test the fit. Before cutting production pieces, run a short test strip of the same material through the jig to verify the profile produces the angles you expect. Minor adjustments to the cut face can be made with a rasp or sandpaper.

Executing the Pull: Technique for Clean Results

With the jig mounted, the actual extrusion process is straightforward but benefits from good coordination between two workers. This is a two-person operation for lengths longer than 4 ft, though shorter pieces can be handled alone if necessary.

Two-Person Pulling Method

1. Cut the roll flashing to the required length plus 2 in. for waste at each end.
2. One person stands on the output side of the jig (the side the metal will emerge from) and takes hold of the leading edge with gloved hands.
3. The second person stands on the input side, supporting the remaining length of flashing with both hands and keeping it level with the jig opening.
4. The puller applies steady, even tension, drawing the metal through the jig at a slow and consistent speed. No jerking or acceleration.
5. The feeder maintains light pressure, guiding the metal into the die opening without forcing it. The feeder’s role is to prevent the metal from twisting or binding as it enters the profile.
6. Continue the pull until the entire length has passed through the die.

The key variable is speed. Pull too fast and the metal may skip or chatter against the cut edges, producing scratches or uneven bends. Pull too slowly and the friction can cause the metal to gall, especially with softer materials such as copper or aluminum.

Working with Different Metal Gauges

• Galvanized steel (26 ga to 28 ga): The most forgiving material for this technique. The steel has enough stiffness to hold the bent shape without spring-back, and the zinc coating reduces friction against the plywood die.
• Aluminum (.019 in. to .032 in.): Softer than steel and more prone to scratching. Lubricate the jig cut edges with paraffin wax or a dry lubricant before each pull. Aluminum also tends to work-harden if the pull is too slow, so maintain a steady, moderate pace.
• Copper (16 oz to 20 oz): Copper pulls beautifully through the jig but requires the most care with surface finish. The metal marks easily, and finger oils can cause discoloration over time. Wear clean cotton gloves over the work gloves for architectural projects. For tips on working with copper, see the guide on bending custom copper flashing for valley applications.
• Lead-coated copper: Same technique as bare copper but with extra care at the cut edges. The lead coating can flake if the jig surface is rough.

Applications and Integration with Other Flashing Systems

Common Flashing Profiles to Produce

• Z-flashing (drip cap): Used above windows, doors, and horizontal trim boards to shed water away from the wall plane. The extruded jig produces consistently parallel bends that keep the cap tight against the trim.
• Drip edge for roofs: The angled profile along roof eaves that directs runoff into the gutter. A consistent 3/4 in. drip edge over a 40 ft run requires uniform bends that only a jig can guarantee.
• Step flashing: The L-shaped pieces interleaved with shingles along roof-to-wall intersections. While step flashing is typically cut from flat stock, the jig can pre-bend the vertical leg for faster installation.
• W-shaped valley flashing: Open valleys require a complex double-bend profile that is difficult to hand-form. The jig approach is ideal for these long, repetitive pieces.
• Apron flashing: Used where a roof meets a vertical wall below a window or chimney. The single bent profile is fast to produce in quantity.

Integrating with the Weather Barrier

Field-bent flashing does not exist in isolation. It must integrate with the larger water management system of the building. At every seam and joint where extruded flashing meets the building wrap or adjacent flashing pieces, a secondary seal is essential. This is where proper flashing tape application becomes critical. The tape bridges the gap between the rigid metal flashing and the flexible weather barrier, creating a continuous drainage plane.

A few rules for integrating field-bent flashing into the weather barrier:

• Always install flashing in shingle-lap fashion: upper pieces overlap lower pieces so water flows over, not behind, the joints.
• Apply self-adhered flashing tape to the substrate before installing metal flashing at vulnerable transitions such as inside corners and pipe penetrations.
• Nail flashing at the top edge only. Fasteners through the bottom edge create entry points for water.
• Extend flashing a minimum of 4 in. beyond both sides of any opening to ensure complete coverage at the corners.

Roof-to-Wall Intersections

The most demanding application for extruded flashing is the roof-to-wall intersection, particularly on sloped roofs where concentrated runoff can overwhelm a poorly detailed joint. For these conditions, the extruded jig can produce long, continuous counterflashing pieces that tuck into the masonry or siding and extend down over the base flashing. At the bottom of the slope, where water volume is highest, installing kickout flashing diverters prevents moisture from traveling behind the siding and causing concealed rot.

Quality Control on Site

One advantage of the extruded flashing jig is the ability to inspect each piece as it emerges from the die. Run your hand along the length of the bent piece immediately after pulling. Any waves, kinks, or irregularities are visible and feelable at this stage. Set aside pieces that do not meet the standard and adjust the jig or the pulling technique before continuing. Reject rates with the extruded method are typically under 5%, compared to 20% or higher with hand bending.

Store completed pieces vertically in a dedicated rack or lean-to, not stacked flat. Stacking allows the bends to settle and deform under their own weight, especially in softer metals such as copper and aluminum. Vertical storage preserves the angle of the bend until the piece is ready for installation.

For homeowners and builders tackling their own weatherproofing work, investing 15 minutes in building an extruded flashing jig pays dividends in the quality of every subsequent flashing piece. The technique turns what is often a frustrating compromise into a reliable, repeatable process that produces professional results with materials that cost next to nothing.