Step Flashing Checklist
Cut step flashing pieces 10 inches long and 2 inches wider than the shingle exposure. (For example, cut 10-by-7-inch step flashing for roof shingles with a 5-inch exposure.)
Bend the flashing in half lengthwise so that it extends 5 inches over the roof deck and 5 inches up the wall (see illustration, top right).
Place each step flashing piece about ¼ inch short of the bottom edge of the shingle that will overlap it — just enough so that the flashing piece is not visible when the overlapping shingle is in place.
Because the flashing is 2 inches wider than the exposure of the shingles, each step flashing piece will overlap the one on the course below by 2 inches.
Nail the flashing to the roof deck only. Do not nail it to the wall.
Bring wood siding down over the vertical sections of the step flashing to serve as counterflashing. Do not nail siding through the step flashing.
Because sidewall flashing must handle a large volume of water, one of the most critical flashing details occurs where the roof-wall junction terminates. To deflect water from the siding, install a kick-out in the corner (see illustration, bottom right).
Though not always possible in the construction sequence, it's best to install a full 36-inch-wide piece of rubberized asphalt on the wall before nailing subfascia and trim boards in place, then come back to install the kick-out.
This article is adapted from the JLC Field Guide to Residential Construction, Vol. 1, www.jlconline.com.