Fabrication: Layout, Cutting, and Bending Sheet-Metal Flashing

Why Fabricated Flashing Matters

Every roof leak begins where water finds a gap. Transitions between roof planes, roof-to-wall junctions, and penetrations (pipes, chimneys, skylights) are the most vulnerable spots on any roof system. Factory-made flashing covers many situations, but field-fabricated pieces are essential when dimensions are non-standard, when a repair must match existing work, or when a high-quality custom fit is required. A roofer who can reliably cut and bend sheet metal to a dimension drawing is more valuable on any crew.

This lesson covers the complete fabrication sequence for three common pieces: a drip edge, a step-flashing unit, and a counter-flashing unit. All three are bent from flat 26-gauge galvanized steel sheet stock using the same basic tools and a sheet-metal hand brake.

Materials and Tools

26-gauge galvanized steel is the most common field-fabrication stock. It is stiff enough to hold shape under wind and foot traffic yet light enough to cut and bend by hand. The galvanized zinc coating resists rust; always deburr cut edges so the coating is not torn back any farther than necessary.

A sheet-metal hand brake (also called a finger brake or hand brake) clamps the sheet and allows you to make precise, repeatable bends. Setting the depth stop correctly before each bend is critical: a stop set 1/16 inch off will produce an out-of-tolerance piece every time.

Aviation snips come in three types. Red-handled snips cut left-curving lines. Green-handled snips cut right-curving lines. Yellow (or straight) snips cut straight lines and gentle curves in either direction. Using the wrong snips distorts the metal and makes it harder to achieve a clean edge.

A scratch awl scribes layout lines directly on the metal. Pencil lines disappear under handling; scribed lines remain visible through the entire fabrication sequence. A steel rule and framing square verify dimensions and confirm that bends are 90 degrees (or the specified angle).

A deburring tool removes the sharp wire edge left by snips. This protects installers and prevents cuts on underlayment and adjacent shingles.

Safety First

Sheet-metal work produces the sharpest edges in the roofing trade. Cut-resistant gloves rated ANSI A4 or higher are required any time you handle sheet stock, operate snips, or carry cut pieces. ANSI A4 gloves resist cuts that would slice through lighter-rated gloves at realistic force levels. Safety glasses are required at all times in the shop: metal chips and snip shavings become projectiles.

Never reach across an open brake jaw. Keep hands and fingers clear of the clamping bar when closing the brake. When carrying sheet stock, grip it with both gloved hands at the sides; never carry a piece with the cut edge leading toward another person. Stack finished pieces flat with a barrier (cardboard or foam) between them to prevent edge contact.

Layout: Measuring and Scribing

Always work from a dimensioned drawing. Read the drawing completely before touching metal. Identify the profile (the cross-sectional shape of bends), the developed length (the total flat length needed before bending), and any angled cuts.

To find developed length, add up all the flat legs of the profile. For a drip edge with a 4-inch face, a 1-inch top flange, and a 3/8-inch hem allowance, developed length = 4 + 1 + 0.375 = 5.375 inches. Mark this on the stock with the scratch awl and a steel rule, scribing firmly in a single pass. Mark bend lines the same way.

Always double-check your scribe marks with the framing square before cutting. A layout error caught before the cut costs seconds; one caught after wastes material.

Cutting: Straight Lines, Angled Lines, and Relief Cuts

For straight cuts parallel to the sheet edge, use straight (yellow) snips and cut just outside the scribe line, leaving the line visible. Maintain steady, overlapping cuts without closing the snips fully at the end of each bite — closing fully leaves a notch in the edge.

For angled cuts (step flashing typically has a 45-degree or roof-pitch angle cut at the upper end), pivot the snips to follow the scribe line. Cut slowly and let the metal curl away rather than forcing it.

Relief cuts are short cuts made into inside corners before bending. Without a relief cut, the metal at an inside corner bunches and cracks. Cut straight to the corner point, then stop. The relief cut allows each leg to bend independently.

Bending at the Brake

Set the brake's depth stop so the bend line aligns exactly with the front edge of the clamping bar. Clamp the sheet firmly. Raise the bending leaf smoothly to the required angle — most flashing bends are 90 degrees. For a hem, bend to 180 degrees (flat). A hem closed fully over a scrap piece of the same gauge first and then removed produces a tighter, cleaner hem than bending blind.

Sequence matters: always make the hem first (the smallest, outermost bend), then work inward toward the larger bends. Reversing the sequence often makes it impossible to fit the piece back into the brake for subsequent bends.

After each bend, check the angle with a framing square and the dimension with a steel rule. If a bend is short (less than 90 degrees), place the piece back in the brake and add pressure. If it is over-bent, carefully back-bend by hand against a flat surface — but over-bending is a sign the stop was set incorrectly, so reset the stop before the next piece.

Checking Your Work

Before presenting a piece for inspection, lay it on a flat surface and measure every leg with a steel rule. Each dimension must be within 1/8 inch of the specification. Check 90-degree bends with a framing square; the gap between square and metal at the heel of the bend must be uniform. Run a gloved finger along every cut edge to confirm deburring is complete. Record your measured dimensions in the fabrication log before calling for inspection.