Whenever a roof meets a wall, skylight, or chimney the need for flashing arises. When the water runs parallel to the wall, step flashing is required. If the wall or chimney is at an angle to the flow of water a top or bottom pan should be used. The reason is that in heavy rains the flashing could be submerged and water, not knowing that it is moving or standing still, will move sideways or upwards, whichever is the path of least resistance. In layman terms, if the water runs into a wall the pan must go under the shingles and if it runs away, on top of them, just like the top and bottom of the chimney.
Step flashing is run like a laced valley. The first step is to place the started shingle on a wall or use the first shingle on the side of the chimney or skylight as the starter. The first step flashing lies on top of the top part of the shingle called the selvage. The selvage of the selvage is called a head lap. It is the very top of the shingle that extends past the course face that lies above the first shingle. When two shingles butt side to side on a row of shingles, there is no leak because the shingle below passes pass that joint.
The fist step of flashing covers the unexposed part of the roofing. Usually we use a 4"x4"x8" step flashing that more than covers the blind or unexposed part of the shingle. On a wood shake with a 10" exposure you'll need 12" steps of flashing to cover the 10" exposure plus some head lap. The nails are placed high on the step flashing so when the next shingle and step flashing are installed the nails are blind.
The flashings are bent at a right angle and should be opened or stretched greater than 90 degrees so the memory keeps them tight against the roof floor and the wall lumber. The nails can be installed on the roof floor near the top or to the wall as high up as you can but never on both. If both the top and bottom are nailed the flashing can't slide when the roof deck and wall migrate with wind and ground caused movement. Settling over time will cause flashings to warp id they are attached to both.
The mail process is put a flashing on the top unexposed part of the shingle covering all of it or more. The step flashing should not be visible except for the first one on the sides of roof penetrations like skylights, chimneys, and cupolas. Nail near the top with two nails or on the wall to keep the flashing from moving. Place the next shingle over it and trim it back more than 1/4" so water can flow and debris won't dam gravity, the theory we use for most of our waterproofing.
Make sure that no end laps or joints are near the wall. Trim some off the roofing so a larger that 8" piece can be used to counterflash the step flashing. If you consider a chimney may block out ten or more feet of surface falling rain and concentrate it on the windward side of the chimney, you now have a waterfall on that side of the chimney. The momentum will cause the water to hit the roof and go sideways. The inertia of the water will cause it to continue sideways and not downward until gravity overcomes it and pulls it down the slope. The great mass of water will also push it sideways until gravity wins. If an end lap is close by the water will be pushed into the lap and might leak. Roofing hopes that the water will flow downward and not sideways. The same holds true for valley. No end laps in or near a valley. Open "W" valleys eliminate that concern a lot but if a roofer is determined to put a small piece next to a valley only his paycheck can prevent him.
Shingle, step flashing, nailing, shingle, step, nail, shingle, nail step....
If you nail low the water coming down the wall that goes under the roofing will find the nail and the nail might be depressed causing a small depression in the flashing. Nail high, flash low.
I highly recommend that an ice and water leak barrier be installed before the flashing is installed in case of snow and ice. Freeze/thaw conditions change the rules and ice dams defy gravity.
Don't use plastic cement because it will crack and any water that gets behind is trapped. Imagine a vase buried up to its top with water entering in the top. It'll take a long time to dry out.
When step flashing meets a top or bottom pan on a skylight some lapping and caulking must be done. Remember that on the top side the water is being pushed sideways and the first course over the top pan should be a full shingle. This leaves room for the water to build up and not meet the nails of the first row above the skylight/chimney obstruction.
The wood siding that counterflashes the step flashing or any other roof to wall nexus must be above the roof plane a reasonable amount. If not it will rot as a result of staying wet. Many carpenters install pre-primered wood or cement board but do not primer the cut angle above the roof.
The fist piece of step flashing on a stuccoed or wood sided wall can be a killer. It will dump the water behind the wall if it is not cut, bent, and caulked on the roof plane so that the water is kicked out and not allowed to flow there. On lap siding the first step flashing is inserted between two pieces of siding and show on the face of the wood.
Roofers work hard and fast. They become good at the mechanics of tearing off, felting, loading, and installing shingles. It's here in the details that lives the roofing devil. They need to slow down and run their flashing right, keep the joints or end laps away, and properly nail. Little things like opening the "L" shaped flashing a little wider will make the roof look nice and flat and keep the flashing from raising. When it raises the water being pushed across it sideways may not just flow across the shingle below it but might return following the metal back towards the wall.
Have you ever heard the mantra"skylights leak?" If they were required to leak nobody would want them.
If the roof is slate the flashing must be copper or stainless.
Stone coated steel tiles and shakes are bent or doubled up a wall and act as their own step flashing but the shingle varieties of these products sometimes aren't.
The straight flashing or "c" curled flashing that many builders use so the siding can be installed before the roof are down right dangerous. Build up of debris or the squashing of the "C" can result in a leak. Heavy duty rains or snow can also leak with this type of flashing. You know that when it pours in Dallas/ Fort Worth it can be blinding. Plus it'll never snow a foot in McKinney or Arlington.
You can't run the step flashing before the roofing unless you are very careful in your measurements and you only nail to the wall. Most wall have no blocking other than intermittent studs to nail to.
This was the most boring thing I ever wrote and I'm not going to read it. I'm sorry for the grammar and punctuation. Step flashing is not very sexy of a topic.
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