Flashing is a thin sheet metal often used in construction for weatherproofing roof edges, windows, doors and other areas. It is commonly made of aluminum or galvanized steel and can be challenging to bend without cracking or breaking. There are some techniques that can help bend flashing smoothly and prevent damage.
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Why Does Flashing Break When Bent?
Flashing breaks when bent because it is a thin and rigid material. The metal has little flexibility and stretching capacity before the material’s structure fails and cracks open. There are a few reasons why flashing is prone to breaking when bent:
- Flashing is very thin, usually between 0.25-0.32mm thick for aluminum and 0.4-0.5mm for galvanized steel. The thin profile makes it brittle and inflexible.
- Flashing is made of rigid metals like aluminum and galvanized steel which have a crystalline structure. They lack ductility so they snap when bent instead of flexing.
- Flashing does not stretch – when bent, the interior surface becomes compressed while the exterior surface stretches and tears.
- Tight bend radii create concentrated stress forces that can fracture the metal.
Understanding why flashing breaks helps inform ways to approach bending it without cracks or tears. The key is to use techniques that avoid concentrating force in small areas and that allow the material to flex gradually.
How to Bend Flashing Without Breaking It
Here are some recommended techniques and tips for bending flashing without causing it to break or crack:
Bend Flashing Along Its Length
Flashing has a grain or roll direction which is the long way the sheet metal passes through the rolling mill. This makes the material a little more flexible along its length. Try to orient any bends parallel to the length of the flashing to take advantage of this increased flexibility.
Bend a Wide Section
Avoid making tight bends on narrow sections of flashing. The localized stress and compression is likely to kink and fracture the metal. Make bends across wider sections of flashing whenever possible. Use at least a 4-6 inch wide section for bending.
Make Small Incremental Bends
Rather than trying to bend flashing in one motion, make many small incremental bends to achieve the desired angle. Bend a small 5-10 degree angle, flatten, then repeat. This gradual bending allows the metal to flex without concentrating force in one area.
Use Bending Tools
Specialized HVAC bending tools like a folding tool, seamers and bending brakes can help properly bend flashing. These tools apply even pressure across a wide area to create controlled bends without kinking or cracking the flashing.
Use a Hard Surface
Bend flashing over a sturdy table edge, workbench or other hard, smooth surface. The hard edge helps make a crisp bend line and prevent pinching and deformation. Avoid bending over your knee or freehand which can wrinkle the metal.
Bend With Your Hands
For narrow flashing, bending by hand can work well. Hold the flashing firmly with both hands spaced apart. Apply force gradually by feel and inspect frequently for any tears. Stop bending at the first sign of cracks.
Soften With Heat
For difficult bends, heating the flashing can help soften and relax the metal to prevent fractures. Use a propane torch or heat gun to briefly heat a wide section along the bend line before bending. Avoid overheating which can damage protective coatings.
Use a Plastic Mallet
Light, controlled tapping with a plastic mallet can help shape flashing and make bends. The mallet applies low force over a wide area to gradually form the metal. Always tap down onto a hard surface.
Apply Sealant
For challenging bends, applying a sealant like silicone along the interior bend surface can add strength and flexibility. The sealant helps bridge any microscopic cracks and fractures.
Example Bending Scenarios
Bending Flashing for a Roof Edge
Bending flashing to run along a roof edge or parapet wall cap requires long, smooth bends. It’s best to bend the flashing in multiple small increments using sheet metal brakes. Apply pressure across at least a 4 inch wide section. Bend 5-10 degrees at a time, occasionally inspecting for cracks, and make relief cuts if needed. Heating the bend line first can also help. Finish by aligning the flashing and firmly creasing the bend with a rubber mallet against a block of wood.
Bending Step Flashing
Step flashing is bent in short increments to follow a roof line up a chimney or wall. Slowly hand bending or stepping on the flashing can work for the short bends. For neater results, use a sheet metal brake and make several small bends in succession across the width of the flashing. Maintaining at least a 2 inch bend radius is key to prevent kinking the thin flashing.
Bending Flashing Around a 90 Degree Corner
Flashing around a 90 degree corner, like a window or door, requires two 45 degree angle bends. Mark the flashing at the correct width to wrap around the corner. Using a bending brake, make two 45 degree bends by applying force across a wide section in 10 degree increments, inspecting between bends. Heat can be used to soften the flashing and Sealant will also help avoid cracks. Go slowly and gently when finishing the final bend.
Bending Decorative Flashing
Decorative flashing for fascia and trim is thinner and needs extra care when bending. Slow, incremental hand bending works best. Make relief cuts on the interior side first if needed. Bend over a hard, rounded edge like a pipe to avoid kinking. Always bend in the direction of the grain and stop at any sign of cracks. Gentle tapping with a plastic mallet can help shape the final bends.
Tips for Working With Flashing
Follow these additional tips when working with flashing to avoid cracks and breaks:
- When straightening flashing, bend back and forth gently to relieve tension. Sharp creasing can fracture the metal.
- Trim flashing with metal snips or shears. Cutting with tin snips takes advantage of the metal’s grain and avoids cracks.
- Use heat to anneal and soften flashing before making complex bends. Apply heat until the metal takes on a uniform glow and allow it to cool slowly.
- Make small slits on the interior side of bends to relieve tension. The slits allow the exterior side to stretch without cracking.
- Use a sheet metal screw or bolts with washers to fasten flashing. Avoid nailing and creating holes that can tear.
- Apply caulk, mastic or primer to any cracks for extra waterproofing. Thoroughly clean and sand the area first.
- When bending near edges, make the bend at least 1 inch from the edge to avoid stress cracks.
- Keep the grain or roll direction of the flashing aligned through the entire project.
Troubleshooting Cracked Flashing
If cracks and breaks do occur when working with flashing, here are some troubleshooting tips:
- For small cracks and tears, sand the area smooth and apply sealant or a flashing repair compound.
- Use pop rivets or screws with rubber washers to close gaps or rejoin separated flashing seams.
- Remove any kinked sections and replace the damaged flashing with new material.
- For longer cracks, cover with a overlapping patch piece secured with flashing sealant and fasteners.
- If cracking is frequent, try changing bending technique, use different tools or switch to a thicker gauge flashing.
- Improve work surface – use a smooth sturdy table or bench and avoid bending over knees or freehand.
- Adjust heat application – under or overheating can make flashing crack more easily when bent.
Best Materials for Bending Flashing
While all flashing can crack when bent, some types are more flexible and ductile than others. The best materials for bending include:
- Soft temper aluminum – More malleable so it can flex without fracturing. Easy to bend by hand.
- Thicker gauge low temper aluminum – Up to 0.040 inch thick for more durability and resistance to cracks.
- Copper – Very flexible and ductile, less likely to develop cracks during bending.
- Lead flashing – Extremely easy to bend by hand without cracking due to high flexibility.
- Stainless steel – More ductile than galvanized steel and can handle tight radii.
- Pre-coated aluminum flashing – vinyl, Kynar and enamel coatings add strength and flexibility.
The most crack-resistant metals for flashing are softer, thicker and more ductile. Copper and lead are easiest to bend smoothly. For galvanized or aluminum, go with thicker flashing and gentler bending techniques.
Conclusion
Bending flashing without causing cracks or breaks takes care, the right techniques and quality materials. The keys are bending in multiple small increments instead of sharp angles, using proper sheet metal tools, and handling the flashing gently. When issues do arise, there are methods to troubleshoot and repair any cracked sections. With some finesse and the right approach, flashing can be successfully bent and shaped without damage for waterproof building projects.