You can make kickout flashing from a piece of step flashing, but there’s a wrong way and #TheRightWay. The wrong way is easy: Cut the roof-leg of the step flashing at 90 degrees, then slide the lower half under the upper half. Unfortunately, this is guaranteed to leak, even with caulk or adhesive applied to the seam, and especially if you get the overlaps backward.
Unless the seam is properly overlapped and soldered, water will eventually make its way underneath. By the time you discover the leak, the damage to the wall will be done. #TheRightWay is to fold the roof leg instead of cutting it. This creates a seam, but one that water cannot penetrate. To get a good fold, you will need a pair of long needle-nose pliers—long enough to reach all the way from the edge of the step flashing to the inside corner. Use galvanized steel or heavy aluminum (light-gauge aluminum may tear at the corner). The hardest part is finding the extra-long pliers. I don’t have any, so for demonstration purposes I cut the flashing in the photos to match the 2¾-inch throat of the needle-nose pliers I had on hand.
[A] Clamp the roof leg of the step flashing so that the tip of the needle-nose pliers reaches all the way into the corner bend.
[B] Roll the needle-nose pliers “downhill” to create a fold in the roof leg. Here, I’m making a “left-hand” kickout; roll the pliers the other way to create a “right-hand” kickout.
[C] Pinch the wall leg to create a vertical crease, then adjust the width of the fold (with gloved hands or with the pliers) until you get enough angle on the kickout bend.
[D] Flatten both edges of the fold, and crease the wall-leg bend again to create a sharper corner. As with all step flashing, fasten the kickout in place through the wall leg and avoid nailing through the roof leg.
More on Roof Flashing
Learn more tips for leakproof kickout flashing.