Hemming Tool

A hem is an edge of sheet metal folded back on itself. In metal roofing, hems remove sharp cut edges, stiffen panel ends, and create the hooks that lock panels onto drip edges, cleats and valley flashings. Hemming tools are the hand and power tools used to form these folds, particularly at the ends of standing seam panels.

Hem types

• Closed hem: folded flat against itself, giving a stiff, weather-resistant exposed edge.
• Open hem: folded with a gap, often a teardrop or U-shape, so another piece of metal can slide in to form a connection.

Typical roofing uses

• Eave hem: the bottom of each panel is notched and hemmed so it hooks over the eave flashing or drip edge, holding the panel down while allowing thermal movement.
• Valley hem: panels meeting a valley are hemmed to engage the valley flashing or a cleat.
• Box or ridge-end turn-ups: the upslope end of a panel is turned up or folded into a box at the ridge or headwall to stop water and wind-driven rain.
• Trim edges on rakes, copings and flashing.

Tools

• Hand hemming tools: wide-jawed folding tools sized for panel widths, often used with a mallet to set the hem.
• Hand seamers for short or awkward hems.
• Portable brakes for long, straight hems on flashing and trim.
• Power hemmers and eave tools that notch and hem panel ends in fewer steps.
• Roll formers, which can produce panels with pre-formed features in the shop or on site.

Many hand hemming tools are rated for particular materials and thicknesses; one supplier notes that many are rated up to 24 gauge steel. Using a tool slightly wider than the panel pan helps keep the bend straight.

Good practice

Mark bend lines accurately, apply even pressure along the length, and avoid cracking the paint at tight radii, especially in cold weather. Notching the seam legs correctly before hemming lets the panel engage the eave cleanly. Installation instructions from the panel manufacturer define the required hem details for warranty compliance.