How to Cut Metal Roof Panels Without Damaging the Finish

Step by Step

1. 1Step 1: Mark the cut on the back of the panel

   Mark on the underside where possible, or with masking tape on the face, so no pencil or marker shows on the roof. Use a straight edge or a chalk line. For angled cuts at hips and valleys, use a sliding bevel to transfer the angle and cut a test piece from an offcut first.

   Check which side is the finished face and which edge is the top. Cutting a panel upside down or left-handed is an expensive mistake, since panels are cut to length.

2. 2Step 2: Choose the right cutting tool

   Electric shears make long straight and curved cuts quickly and leave a clean edge with a small curl. A nibbler takes out a small slug of metal with each stroke, so it cuts ribbed and standing seam panels without distorting them. Aviation snips suit short cuts and trimming. Hand shears bend thin steel less than a saw.

   Avoid angle grinders, abrasive chop saws and circular saws with abrasive blades. The spark shower melts the paint, burns off the zinc coating at the edge and scatters hot steel chips that embed and rust. Some installers use carbide-tipped cold saws with a dry-cut blade made for metal roofing, but manufacturers often still prefer shears.

   Watch out: A grinder on a prepainted roof can void the paint warranty, and the rust stains that follow are hard to remove.

3. 3Step 3: Support the panel and make the cut

   Lay the panel face up on padded sawhorses, or face down if the tool cuts better that way, with the cut line just past the edge of the support. A flexing panel buckles and leaves a rough edge. Let the tool do the work. Do not push shears hard enough to stall them.

   Cut slightly outside the line and trim back if needed. With shears or nibblers, the offcut curls away from the tool. With snips, cut in short strokes without closing the blades all the way, which leaves a clean edge instead of a notch.

4. 4Step 4: Cut ribs and seams with care

   Shearing through a rib with heavy shears crushes it. Cut through the flat of the panel, then use snips or a nibbler on the rib, making relief cuts at the edges of the rib so it folds neatly. On a standing seam, notch the seam leg to the marked line and avoid tearing into the flat.

   See hemming and bending panel ends for how to fold a notched end into a dam or hem.

5. 5Step 5: Deburr and protect the cut edge

   A light pass with a fine file removes sharp burrs without scratching the paint. Dab matching touch-up paint on the bare edge with a small brush. Galvalume and galvanized coatings protect a cut edge for a time through sacrificial action, but the paint and a hem make it last.

   The best protection is a hem or fold, which hides the raw edge under itself. Where the cut edge will be covered by trim, a touch-up is usually enough.

6. 6Step 6: Sweep up every shaving and offcut

   Steel shavings are the main source of rust stains on a new metal roof. They sit on the panel, get wet and rust, leaving an orange mark that bleeds into the paint. Sweep them off every day, not at the end of the job. A magnet sweeper on a pole picks up the small chips.

   Do not use a wire brush or scrub pad on the paint. Flush the roof with water at the end and check gutters for chips that can rust in place.

7. 7Step 7: Check the cut pieces against the roof before final fit

   Lay the cut panel in place and check the angle against the valley or hip, the gap to the wall and the overhang at the eave. Making a small adjustment now is easy. Once the panel is fastened or seamed, it is not.

   Keep the offcut for the next cut on the same line as a template. On hips and valleys, panels typically repeat a similar angle with changing lengths.

Why Abrasive Cutting Causes Rust

Choosing Between Shears and Nibblers

Common Mistakes

• Using an angle grinder or abrasive saw, which scorches paint and leaves rust-forming particles.
• Leaving shavings on the roof overnight.
• Cutting without support, so the panel flexes and the edge is ragged.
• Not touching up the cut edge.
• Crushing ribs with heavy shears instead of relief-cutting them.
• Marking the finished face with a pencil or marker that cannot be removed.
• Cutting a panel to size before a dry fit.