L-shaped roof flashing is usually referred to as step flashing. Step flashing is installed where the roof and the side wall meet to defend against water penetration. Step flashing can be purchase pre-molded into the L-shape or flat. If you purchase the flashing flat, you will have to mold each piece yourself. Completing a professional grade step flashing installation is important. Because the flashing will be covered by shingles, building paper and siding, you must make sure that it is done right. If your step flashing job is poor, you will have to disrupt large portions of your roof to investigate or repair. It is important that you avoid nailing into the roof structure. All step flashing should be nailed only to the sidewall. This avoids making holes in the roof as well as allows for the flashing to move with your house.
Step 1—Set Custom Corner Piece
Before you can begin installing the step flashing, you need to make sure that the shingles below the sidewall have been laid. The first piece of step flashing should overlay the last row of shingles. To cover the corner, you will need to cut a custom piece of flashing. If you are using a pre-formed flashing, you are going to make a cut at 45 degrees in toward the midline. Once the cut is made, you will set the flashing on the corner and nail it in place. Next, run a bead of caulk down the seam of the corner. You will attach the next piece of flashing here.
Step 2—Add First Piece of Step Flashing
This first piece of flashing will lie in the seam along the roof and the side wall. You will need to make a small bend on the top corner of this piece so that it will roll over the corner and attach to the caulk you laid in the last step. Nail the flashing to the sidewall.
Step 3—Begin Alternating Shingles with Flashing
You are not going to add the step flashing all at once. Each piece of flashing will be overlapped with shingles. Now that the first piece is laid, add a row of shingles. Add the next piece of flashing, nail it in place, and add another row of shingles. Continue like this until you have reached the top of the roof.
Step 4—Attach Custom Piece at Top
If the roof comes to a peak at the top of the sidewall, you will have to use your tin snips to cut 2 more custom pieces. You will have to cut a piece of flashing along the midline and bend the outside half down. The bent half will roll over the peak of the roof. Nail the flashing in place like normal. Lay a bead of caulk on the seam at the top of the roof and cut an opposite piece. Place the opposite piece in place so that it creates a seal with the caulk and nail it in place. Add the last row of shingles.
When you are finished, the metal flashing will be seen only on the sidewall. The pieces that laid flat on the roof should be covered by shingles.
That’s it. With a little bit of patience, you can lay L-shaped step flashing on your roof just like the pros.