the importance of flashing is often minimized and its installation is sometimes ignored especially in poor roofing installations flashing is used to seal and protect joints in a building from water penetration by directing the flow of water away from these otherwise vulnerable areas for flashing to function properly it is important that is installed correctly let's take a few minutes to review proper flashing installation when the roof claims but against vertical walls or chimneys metal flashing must be placed over the end of each course this procedure is called step flashing in this situation step flashing performs better than solid continuous flashing because it ensures a waterproof seal at these critical areas overall step flashing is the most time-tested and proven way to flash a roof to wall transition the metal flashing strips used for step flashing are rectangular typically 10 inches wide and 2 inches longer than the exposed area of the roof shingle for typical 5 inch shingle exposure use a 10 inch by 7 inch piece of flashing GAF has several shingles that are much wider than standard shingles some of these shingles measure 17 inches wide by 40 inches long the non exposed area of these shingles is typically nine and a half inches wide to completely cover these shingles with step flashing use 10 and a half inch wide flashing or simply install two overlapping pieces of a normal step flashing to fully cover this area many times it can be very difficult to install new metal step flashing on a home where there is painted siding in place or masonry siding GAF recommends to only reuse existing step flashing if it is in like-new condition like new means that it is not buckled rusted bent cracked or broken in all cases it is best if possible to install new step flash whether watch or storm guard leak barrier should be in place over the deck protection and up the wall a minimum of 5 inches place the first step flashing strip over the end of the starter course shingle and secure the step flashing to the roof with 2 nails don't nail through the flashing to the wall because the flashing needs to be able to move with any deck and building movement next apply the first course of shingles up to the wall and position the second flashing strip over the shingle the non exposed area of the shingle in the second course should be completely covered by the step flashing continue this process to the top of the roofing area when applying shingles around all vent stacks apply a piece of weather watch or storm guard approximately 2 times the size of the flange onto the deck protectors under the shingles trim the shingle to fit and check shingle alignment apply asphalt cement under the shingles around the vent stacks set the shingles into the cement for more information or to watch more in-depth videos on what you just saw visit GAF com