Metal Roofing Calculator: Panels, Ridge Trim & Screws

Metal roofing goes on as long vertical panels that run from the eave up to the ridge, each covering a fixed exposed width. So the panel count is really a question about the eaves, not the whole roof: how many panel widths fit across the bottom edge of each roof plane, and how long does each panel need to be to reach the ridge.

For a simple gable roof there are two planes. The number of panels across one plane is the eave length divided by the panel's exposed width — commonly 36 inches, though 24 and other widths exist. Double that for the two planes. Each panel is cut to the slope length, which is the half-span (half the building width) multiplied by the slope factor for your pitch. A 4/12 roof over a 30-foot-wide building gives a half-span of 15 feet and a slope length of about 15.8 feet per panel.

From there the trim and fasteners follow. Ridge cap runs the full eave length along the peak. Eave and rake trim run the perimeter edges. Exposed-fastener panels take roughly 80 screws per square — that figure already includes the field screws, the side-lap stitch screws, and the trim screws. The calculator adds your waste allowance to the panel count for off-cuts at hips, valleys, and end walls, then rounds up, because panels and trim are bought whole.

This tool assumes a straightforward gable. Hips, valleys, dormers, and porches change the geometry, so treat the result as a solid starting order and confirm with a panel supplier's takeoff for a complex roof.