How to Create a Hem on Aluminum Flashing


A "hem," in the vernacular of the sheet metal trade, refers to a 180-degree bend. Creating a hem on aluminum flashing solves two problems. It stiffens and strengthens the flashing, and at the same time, a hem eliminates the sharp edge of the flashing. Making a hem on flashing, or any other sheet metal, is done with a metal brake. Portable brakes designed for the siding trade can be rented at many rental yards.

Things You'll Need

  • Aluminum coil stock
  • Tin snips
  • Sheet metal brake
  • Cut the coil stock to length. If you need a piece of flashing longer than the throat of your sheet metal brake, you'll have to make it in sections.

  • Mark the depth of the hem on the coil stock. Most hems are between 3/8 inch and 3/4 inch deep, with 1/2 inch being typical.

  • Place the coil stock in the brake, with the hem protruding from the throat. Move the lever to clamp the throat shut.

  • Bend the coil stock beyond 90 degrees to begin the hemming process. Bend as far as possible --- usually about 120 degrees.

  • Remove the work piece from the brake. Place the bend made in the previous step against the brake's hinge. Pull up on the bending handle to complete the hem.

Tips & Warnings

  • Unhemmed edges can be razor sharp; wear gloves when working with aluminum coil stock.

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  • Photo Credit aviation tin snips image by Christopher Dodge from
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