How to Use Hand Seamers and Tongs for Flashing and Metal Trim

Step by Step

1. 1Step 1: Choose the seamer or tongs for the job

   Hand seamers come in widths from about 3 inches to 6 inches. A 3 inch seamer gives good leverage in tight spots, and a 6 inch seamer covers more of the bend at once. Tongs, sometimes called bending tongs, are 12 to 18 inches wide or more, and they are used with two hands on longer pieces. Some seamers have a bent jaw for getting into a corner or around a panel rib.

   Check that the jaws are clean, aligned and have no nicks or burrs, which leave marks on prefinished metal.

2. 2Step 2: Mark and measure the bend line

   Measure from the cut edge, and mark on the back or the face where it will not show. A common hem is about 1/2 inch to 1 inch, and a typical flashing flange is 2 to 4 inches, depending on the detail. Transfer the same measurement at several points so the line stays straight. A crooked mark gives a crooked bend.

   Use a steady hand and a clean straightedge, or a sheet metal square.

3. 3Step 3: Set the jaws on the line

   Line up the lower jaw with the line, with the metal fully inserted so it does not slip. The jaw should be square, not tilted, since a tilted jaw creates a bend that drifts off the line. Take your time on the first bend of a piece, because it sets the direction for the rest.

4. 4Step 4: Squeeze and lift in short steps

   Begin at one end. Squeeze firmly, lift the handles slightly, and the metal folds over the jaw. Release, slide the seamer along by about the jaw width less a little overlap, and repeat. Keep the same angle of the handles each time so the bend is even. Do not try to make the final bend in one pass. Work the whole length to a small angle first, then go again to the full angle.

   For a longer bend, use tongs with a second person or work your way along with a steady rhythm.

5. 5Step 5: Finish the bend to the final angle

   A 90 degree flange is checked against a square. A hem is folded over itself and pressed flat with the seamer, then the closed edge is checked for evenness. Do not over-bend, because metal springs back a little and repeated flexing work-hardens and cracks it. If you over-bend, do not bend back and forth on the same line.

6. 6Step 6: Form end dams, kinks and small returns on panels

   On standing seam and ribbed panels, an end dam is a small fold at the bottom of a panel, made to stop water running off the end or to seal the rib. Clamp the seamer across the rib and bend in small steps, keeping it square and without crushing the rib. The panel manufacturer's instructions show the dimensions. See how to hem and bend metal panels and how to cut metal roof panels.

7. 7Step 7: Clamp and lock trim and seams

   Pieces of trim and flashing often slot together with a folded edge and a cleat. Crimp the joint to close it firmly without damaging the paint. A few small crimps along a seam holds it until it is fastened. Use the seamer gently on prefinished metal, since hard pressure marks the finish. Wrap a scrap of cloth around the jaws if you are worried about marking.

8. 8Step 8: Clean up and look after the tools

   Metal filings on a roof rust and stain, so sweep them up. Clean the jaws with a rag and a drop of oil on the pivot. Nicked jaws mark every piece of metal they touch, so dress them with a fine file or replace them.

Grip Technique

When a Brake Does It Better

Common Mistakes

• Skewed jaws. The bend drifts off the line and shows as a wave.
• Trying to make the full bend in one pass. Bend in small steps, working along the line.
• Bending back and forth on the same line, which cracks the metal.
• Marking prefinished metal with dirty or nicked jaws.
• No gloves. Cut edges of sheet metal slice skin easily.
• Leaving metal filings on the roof, where they rust and stain.