Installing step flashing on a roof valley is an important step when trying to prevent water damage to your roof. This is an easy process if done correctly, but if it is not done properly it will cause major roof damage, which will eventually lead to water dripping inside of the house, ruining the interior walls and ceilings as well. This process, as with all other aspects of roofing, requires you to begin at the bottom of the run and work your way up the valley, placing the step flashing, and the roofing shingles, in an exact order.
Step 1: First Step Flashing
Begin at the lowest level in the valley. Position your first piece of step flashing, and secure it down with nails. If you are also roofing during this time, lay the next row of shingles down. This allows the step flashing to be directly covered with the roofing shingles, causing the flashing to be sealed more tightly and have less chances for water to back up underneath the installed pieces.
If you are not roofing at the same time, then make sure that you follow this same pattern. Pry up the existing shingles, and slide the row of step flashing underneath them. Then secure the shingles back down into place.
Step 2: Finish Step Flashing the Roof Valley
Continue placing the pieces of flashing from the lowest point in the valley to the highest point, making sure that each piece of flashing overlaps onto the proceeding one. This means that by the time that you reach the top of the run, the water can flow straight down the step flashing without having any joints in the way to interfere with the natural flow.
This gives you the appearance of metal steps going up the valley from the bottom to the top. Be sure that you place the step flashing, then place the shingles, and then place the next piece of flashing all the way to the top of the valley. This forms a sort of funnel in which water and moisture can run down off of the roof. When you reach the ridge cap make sure that the flashing is placed underneath it.
Step 3: Caulk the Edges
Run a bead of roofing caulk all along the outer edges of the step flashing and shingles, ensuring that a watertight barrier is formed. Even though the step flashing, if the steps above were followed, have been installed to allow the water to flow directly off the roof, it is still possible for the moisture to back up into the joints. The roofing caulk will stop this from happening at all, making the entire area weatherproofed for years.