Step Flashing is metal laid with the shingles to seal along with a straight vertical obstruction such as a wall, brick chimney, or skylight. All step flashing is laid basically the same way. You can buy step flashing in mill finish (unpainted) aluminum, enameled aluminum (black, brown, and white), copper, or galvanized metal (but don’t use the galvanized).
Modem step flashing is manufactured from 5″ x 7″ rectangles. The rectangles are bent 90° so that a 2-inch leg goes up the wall and the 3-inch leg goes over the top of each shingle as you lay it. Always lay each piece of step flashing on each succeeding course at the same location on the shingle. Each piece of step flashing or step is 7 inches long.
The courses of shingle are only 5 inches. When you nail each succeeding piece of step at the same location on each succeeding shingle, the upper piece of step overlaps the lower piece by 2 inches. I always set my piece of step with the bottom of the step just above the self-sealing strip. Keeping the strip exposed lets the shingle seal all the way across to the vertical leg of the step.
If you have trouble visualizing this, lay down a shingle and rest a piece of step along the edge of the shingle with the bottom of the step just above the self-sealing strip. Place another shingle 5 inches up and rest another piece of step in the same location.
Repeat the same process with a third and fourth shingles. Now grab the 2-inch vertical legs of the four pieces of step and pull them out together. The water is going downhill and all the step has a 2-inch downhill lap. It can’t leak.
Nail: Some contractors don’t lay their step as they lay each course. They leave the ends of the shingles loose along the wall and come back later to lay the step flashing. It’s quicker for them to do the step all at once. The problem is, the roofer can get called away to help with something else, or take a coffee break, or just plain get careless. At any rate, it’s easy to skip a piece of step this way. When a piece is left out, the wall has a 3-inch unprotected gap instead of an unbroken series of step flashing with 2-inch laps all the way down. You may get lucky and the felt will carry you for a while, but eventually, the felt will weaken, and it’s going to leak. In a severe storm, it’s going to leak a lot.
Some roofers nail their step high, thinking that will help keep it from leaking. This practice tends to raise the bottom of each individual piece of step, which tends to lift the tab of the shingle laid above it. This makes the shingles look ragged, and the loose edges are subject to wind damage.
I nailed my step near the bottom. The nail head is down flush, and the metal in the step, shingle, top of the next lower step, and felt all seal around the shaft of the nail. It’s not going to leak.
Nail: We have covered this once, but it is worth repeating. Too many contractors mix dissimilar metals. They use the light .019 aluminum and nail it with galvanized roofing nails. The galvanic action between the nails and pieces of step could leave a series of corroded holes in the entire length of the step flashing. If you have the contractor lay twenty-five- or thirty-year shingles, the step flashing may end up leaking like a trickier hose before the end of the useful life of the roof.
If a keyway hits close to the step flashing, you don’t need to nail above that key. When you nail the step in place, you lock the shingle down, too.
Water comes around the edges of the shingles at the wall (or obstruction). The water gets on the 3-inch leg of step beneath the shingle. The 3-inch leg drops the water straight down the roof and out on top of the shingle to which the step is nailed. The water is now on top of the shingles and continues flowing down the roof on top of the shingles. If a little water runs around the side of another shingle farther down the roof, the step at that spot carries it down and diverts it back up on top of the shingles.
If by chance a little moisture runs off the 3-inch leg of a piece of step, the water flows over the shingle the step is nailed to until it runs over the exposed surface of the shingle and from there, down the roof.
Avoid having a butt joint right at the step flashing. Cut off the last tab of the last whole shingle you lay. Then lay and trim a whole shingle so you have a full tab plus a part of a tab going into the wall. If you do leave the joint near the step, the water will roll off the 3-inch leg of the step and down through the joint in the shingles to the felt. The felt might carry it for a while, but twenty-plus years is asking too much of the felt.
Nail: I can’t tell you how many experienced roofers came to work for me and looked at me like I was crazy when I stopped them from putting a construction joint right beside the 3-inch leg of the step.
They just never realized how to step flashing really works. You know how they had been doing step flashing before. The upper leg of the step flashing can go behind the wood, aluminum, or vinyl siding. It can also go beneath a skirt flashing that is tied into the wall or side of the chimney. The skirt flashing is tied into the wall and caulked along its top edge. The siding or skirt flashing keeps any water from infiltrating the top of the step.
When a wall rises above an up-and-over roof, you need to step flash the ridge. I ran my pieces of a step up each side until the top of the last piece of step on each side of the roof stopped right at the ridge. I then cut down the center of the 2-inch leg of a piece of step and bent the 3-inch leg in the middle to the shape of the ridge. I usually had to go to one side or the other of the up-and-over roof and slide this piece up under the siding. Then I moved the piece up and bent the 3 inches over the step on the other side of the roof. This bent piece went over the straight pieces to continue the downhill lap from the ridge on down both sides of the roof.
I would then try to slide a small rectangular piece of metal in front of the cut that opened to a “V” notch when I bent the piece of step over the ridge. I caulked the notch, kneading the caulk back in bed-hind the notch in the 2-inch side of the step. The caulk not only sealed the notch but would hold the small rectangular patch in place in front of the V-notch.
Nail: I saw a lot of older roofs where the contractor had just run his step-up both sides of the roof to the ridge and gunked the tops of the straight pieces together with mastic. The roofer had then jammed mastic back in against the wall too. It works and is OK until the mastic cracks. I saw a couple of roofs where the step flashing just stopped near the ridge, and there was no mastic or anything protecting the wall at the ridge. It’s true that water accumulates down the roof, but the ridge has to have some protection. There was some damage to the sheathing at these unprotected ridges.
On low slope roofs, the shingles are reduced from 5-inch to 4-inch courses. This means the step automatically goes from a 2-inch to a 3-inch downhill overlap. In addition, you can buy oversized pieces of step.
The oversized pieces, such as the 9″ x 12″, cost more than the standard 5″ x 7″ step, but if you have a particularly trouble-prone location on your home, consider the larger sized step. In a pinch, you can make step flashing yourself, but the manufactured step is cheap enough that it isn’t worth your time.