How to Bend Flashing and Trim on a Portable Sheet Metal Brake
Mark it, clamp it, bend it in order. A brake turns flat coil into drip edge, step flashing and trim with straight, repeatable bends.
- Intermediate
- 30 minutes to set up and learn
- Updated
The short answer
A portable sheet metal brake bends flat metal along a clamped line. Set the brake level and solid, cut the metal to width, mark the bend lines, clamp with the line at the bending edge, lift the apron to the angle in one smooth pull, and bend in an order that keeps each new fold clear of the tool. Use a slitter to cut coil, and protect prefinished metal from marks.
A portable sheet metal brake is the tool that turns flat prefinished coil into the trim and flashing every roof needs: drip edge, rake trim, eave flashing, valley metal, wall flashing, cap flashing and custom pieces that no store stocks. It clamps the metal along a straight line and lets you fold it to a precise angle. For a roofer, it makes the difference between paying for every flashing shape and making the exact piece for the job.
It is a simple machine, but the results depend on setup, marking and bend order. A brake that is not level or tight will give crooked bends, and a bend made in the wrong order leaves a fold that cannot be finished. This guide covers the setup, the bend sequence, hems, and cutting coil to width with a slitter. For smaller bends on the roof, see how to use hand seamers, and for cutting, how to choose and use tin snips. The bends are used in step flashing and metal eave trim.
Tools
- Portable sheet metal brake, 10 feet or longer
- Coil slitter or shears
- Tape measure and straightedge
- Permanent marker
- Tin snips
- Square or angle gauge
- Cut-resistant gloves
Materials
- Prefinished steel or aluminum coil or sheet
- Scrap metal for test bends
- Protective film or a soft cloth for prefinished metal
- Bench or sawhorses rated for the brake
Safety first
- Keep fingers out of the clamp area and away from the bending apron. Hands get pinched between the bed and the moving bar.
- Wear cut-resistant gloves and safety glasses. Coil metal springs as it is cut, and a cut edge can slice through skin.
- Set the brake on a level, stable bench and lock or clamp it down. A brake that moves during a bend can cause injury.
- Lift the brake with help and keep your back straight. Long brakes are heavy and awkward.
Step by Step
Step 1: Set up the brake on a level, secure surface
A brake that rocks or twists gives crooked bends. Use the stand or bench the manufacturer supplies, and lock the legs or clamps. Check the clamping pressure with a scrap piece. Many brakes have adjusters for the gap, which must match the metal thickness. A gap that is too tight crushes the metal, and one that is too loose lets the metal slip.
Wipe the bed and the bar clean. A speck of grit leaves a mark on every piece you bend.
Step 2: Measure and cut the metal to width
Add up the widths of each leg and hem, for example a 1 inch hem, a 3 inch leg and a 4 inch leg make a flat width of 8 inches, plus a little for the bends. A test piece shows how much the bends take up, which varies with the metal and the radius. A coil slitter cuts coil cleanly along its length. For short pieces, use shears or snips and keep the edge straight.
Cut a few extra pieces at the start of the job for test bends and mistakes.
Tip: Keep a cutting list with all pieces and their lengths so you can cut from coil with as little waste as possible.
Step 3: Mark the bend lines
Marking on the back of prefinished metal keeps the face clean. Number the bends so you do not get lost, and mark the direction of each one, since some go toward you and others away. Double check the measurements before you bend, because a wrong bend cannot be reversed without a mark.
Step 4: Clamp the metal with the line at the edge of the bar
Line up the marker line with the edge of the bar. Check at both ends and in the middle. Clamp firmly but without crushing the metal or marking prefinished paint. For long pieces, support the free end so the weight does not pull the metal out of the clamp.
Step 5: Bend with one smooth pull
Stand at the centre for short pieces, or use two people for long ones. A steady pull gives a clean, even bend. A jerky lift leaves ripples. For a 90 degree bend, go slightly past 90 and let the metal spring back. Measure the angle with a square or a gauge, and adjust the next bend if needed. Brakes usually have a stop that sets a repeatable angle.
Bend the first piece, compare it to the plan or a template, and only then bend the rest.
Step 6: Bend in the right order
For a simple Z shape or a drip edge, the order is usually obvious. For a piece with several bends, check that the finished fold can still clear the bar and the clamp. Some shapes need a bend made in the opposite direction first, or a piece flipped over. Test on scrap if the piece is complicated. Hems are best done first, and small returns last.
Step 7: Make hems and returns
A hem folds the edge back on itself to make a smooth safe edge and add stiffness. Bend the edge 180 degrees tight if the brake allows, or bend to about 135 degrees and finish flat with a mallet or seamers. Hems should lie flat with no gap. See how to hem and bend metal panels for panel work.
Step 8: Check, label and protect the finished pieces
Hold the piece up to the job or a template. Check that the flanges are straight and the ends square. Stack pieces without scratching them, and keep them flat and dry. Pick up all scraps and offcuts from the work area, since they are sharp and rust. Clean the brake, oil moving parts and cover it when not in use.
Cutting Coil With a Slitter
A coil slitter is a hand tool with wheels that cuts a straight line in coil as it is pulled along a guide. Set the guide at the width you want, pull the slitter the full length of the coil with steady pressure, and support the cut strip. Wear gloves. The cut edges are sharp and may curl. A slitter is faster and straighter than snips, and waste is lower when you cut to width.
Avoiding Marks on Prefinished Metal
Keep the bed and bar clean, check for grit and use a protective film or a soft cloth between the tool and the paint. Do not drag metal across the bed. Test the clamp pressure with scrap. If the brake leaves marks, reduce the pressure, wipe the surfaces and try again. Bends that are too tight for the paint to stretch will crack the coating, so check the manufacturer's minimum bend radius.
Common Mistakes
- An unlevel or loose brake. Bends come out crooked and inconsistent.
- Measuring from different edges. Pieces end up with legs of different widths.
- Forgetting springback. The angle opens a few degrees after the bend.
- Wrong bend order. The finished fold will not clear the tool and the piece is ruined.
- Grit on the bed. It marks prefinished metal on every bend.
- Bending too tight for the paint, which cracks the coating.
When to Call a Professional
If you need only a few pieces of flashing, a sheet metal shop can make them to your drawing, often at a cost lower than buying a brake. For complicated shapes, long pieces and copper or stainless, professional fabrication is worth the money.
Roofers who build custom flashing every day have a good feel for bend allowances and order. If your first pieces do not fit, ask a roofer or a shop to show you on the first one.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Bend Flashing on a Sheet Metal Brake: Questions
What is a sheet metal brake used for in roofing?
A sheet metal brake bends flat coil or sheet metal into flashing and trim: drip edge, rake and eave trim, step flashing, valley metal, cap flashing and custom pieces. The metal is clamped along a marked line, and an apron is lifted to fold it to a precise angle. Portable brakes are used on site or from a trailer.
How do you bend metal flashing without a brake?
For short, simple bends you can clamp the metal between two straight boards or angle iron and bend it by hand or with a mallet, or use hand seamers along a marked line. Results are less consistent and the finish can be marked. For long bends or many pieces, a brake is faster and gives cleaner results.
How do you bend a hem on a sheet metal brake?
Bend the edge to a tight fold of about 135 degrees or more, open the fold a little, then flatten it against itself with a hemming attachment, a mallet or hand seamers. The aim is a flat hem with no gap that leaves a safe, stiff edge. Test on scrap and check the hem width matches the plan.
How do you cut coil for a sheet metal brake?
Use a coil slitter, which cuts a straight line as it is pulled along a guide set to the width you want, or cut with power shears or snips against a straightedge. Measure the flat width from the legs plus the bend allowances, cut a test piece, and wear cut-resistant gloves because coil edges are sharp.
Why does my metal crack when I bend it on the brake?
The paint or the metal is being bent tighter than it can stretch. Prefinished coatings have a minimum bend radius, and cold weather makes it worse. Reduce the pressure, use a slightly larger radius, bend gently and check the manufacturer's guidance. A dirty or worn bar can also cause cracking at the fold.
Related Guides
All Roofing Tools guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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