What Is Step Flashing on a Roof?

What step flashing is

Picture a roof slope that climbs up beside a wall, like the side of a dormer or the wall of a two-story house above a one-story porch. Shingles alone cannot seal that joint. Water runs along the wall, slips under the ends of the shingles and gets into the deck and the wall.

Step flashing fixes that with many small pieces instead of one long one.

The National Roofing Contractors Association describes it this way: where a shingle roof meets a side wall or a penetration such as a chimney or dormer, "individual pieces of metal flashing are installed at the end of each shingle course" ⁶.

Each piece is bent into an L. The flat leg lies on the roof. The upright leg stands against the wall.

It is called step flashing because, seen from the side, the pieces climb the roof in steps, one step for every row of shingles.

How step flashing keeps water out

Each piece of step flashing sits on top of one shingle and under the next one up. The next piece of flashing then sits a row higher and laps over the piece below it.

Fine Homebuilding puts it plainly: one piece is used at every course "so that the flashing laps each course below, in the same fashion as the shingles, to make the joint watertight" ⁷.

This weave is the whole point. If water gets under a shingle near the wall, it lands on a piece of flashing.

That piece sits on top of the shingle below, so the water comes back out onto the roof surface.

The U.S. Department of Energy says the flashing is joined to the roof and wall in "shingle fashion, so that the top layer of the WRB or flashing laps over the bottom layer" ².

WRB means water-resistive barrier, the housewrap or building paper on the wall.

Because the pieces are separate, they can also move a little as the house shifts with heat and moisture, without tearing a long seam.

The order of layers at a roof-to-wall joint

From the inside out, a framed wall with siding should be built in this order. Each layer laps over the one below it ^{2 5}:

The common mistake is step 4 done backwards: wrap tucked behind the flashing. DOE calls that wrong because it "could allow water to get behind the step flashing and into the wall" ⁵.

Kinds of sidewall flashing

The code allows sidewall flashing to be "continuous or step flashing" ¹. Here is how the main options compare:

Step flashing is not the same as apron flashing, which is one piece across the front of a wall or chimney where the roof runs into it head on. The code sends front-wall flashing to the shingle maker's printed instructions ¹.

What step flashing is made of

The 2021 IRC says base flashing must be "corrosion-resistant metal of minimum nominal 0.019-inch (0.5 mm) thickness" or heavy mineral-surfaced roll roofing ¹.

On shingle roofs the step pieces are almost always metal. A DOE guide names galvanized steel at least 0.019 inch thick ⁴. Aluminum and copper are also common in the trade.

Keep metals that react with each other apart. Copper next to aluminum or bare steel can corrode the other metal over time, so a roofer matching new flashing to old gutters or nails should use the same metal or a separator.

Worked example: how many pieces a sidewall needs

A dormer cheek where the roof runs 12 feet up the slope beside the dormer wall, with a shingle that shows 5⅝ inches per row (a common exposure on architectural shingles; check your shingle's label):

Illustrative numbers. The point is that one dormer can need dozens of pieces, each laid by hand with the shingle row it belongs to. That is labor, not just material, and it is why sidewalls add cost to a roof.

Piece size and overlap, from the sources

The code sets the minimum: sidewall base flashing must be "not less than 4 inches (102 mm) in height and 4 inches (102 mm) in width" ¹. DOE repeats it as at least 4 inches up the wall and at least 4 inches out along the roof deck ².

The code does not give a piece length. NRCA "suggests using metal step flashing that is 7 inches (178 mm) long by 8 inches (203 mm) wide for standard-sized asphalt shingles" ⁶.

Length matters because each piece must be longer than the part of the shingle that shows, so the piece above can lap it.

A Fine Homebuilding author writes, "I like to lap all flashing by at least 2 inches" ⁷. That is one builder's habit, not a code number, but it shows the idea.

Your shingle maker's printed instructions may set their own size and nailing. Those instructions are part of the warranty, so the roofer should follow them where they differ.

Where you find step flashing on a house

Look anywhere a sloped roof runs up alongside a wall. Common spots are the sides of dormers, the wall of a second story above a lower roof, the sides of a chimney, and a garage roof that runs along the wall of the house.

DOE says step flashing should go in "any time the roof intersects with a wall, such as a dormer" ².

On a framed wall, you usually cannot see much of it. The roof leg hides under the shingles and the wall leg hides behind the siding. What you can see is the gap between the bottom of the siding and the shingles, and the kickout at the bottom corner.

Counterflashing and siding over the wall leg

The top edge of each step piece is a weak spot. Water running down the wall could slip behind it. Something must cover that edge.

On a wall with siding, the siding and the housewrap do that job.

DOE's guide for existing homes says to "lap the existing house wrap or building paper over the top of the step flashing," then put the siding back with a gap above the shingles ⁵.

That guide calls for "a minimum 1.5-inch reveal between the bottom of the siding and the shingles of the roof" ⁵. A second DOE guide says siding should end "at least one inch above the roof surface" ⁴.

The gap keeps the siding out of the water on the roof and lets it dry. Ask your siding maker which number they require.

On brick, stone or stucco, there is no siding to lap over the flashing. A separate counterflashing is set into the wall and bent down over the step pieces. Our flashing guide covers counterflashing and chimney flashing in more detail.

The kickout at the bottom of the run

Step flashing carries water down the roof along the wall. At the bottom, where the roof edge stops and the wall keeps going down, that water has to go somewhere. Without help it pours straight down the face of the wall and soaks into it.

The 2021 IRC covers this spot in R903.2.1: "A flashing shall be installed to divert the water away from where the eave of a sloped roof intersects a vertical sidewall" ¹. That piece is the kickout, also called a diverter.

DOE says kickouts "should be installed at the eaves of all roof-wall intersections" and that they must be "right sized and integrated with the water-resistant barrier, or housewrap. Otherwise, they can allow water inside wall cavities" ³.

DOE also warns that kickouts bent on site from flat sheet metal "can be undersized and may have unsealed seams that allow leakage," and points to one-piece molded plastic or welded metal parts instead ³.

Another DOE guide says to fasten the diverter to the roof deck and starter strip, not to the sidewall ⁴.

Benefits of step flashing

• Water that gets under one shingle is sent back out on top of the next, so one small gap does not become a leak
• The separate pieces move with the house without tearing a long seam
• It matches how shingles are laid, so it goes in row by row as the roof goes on
• A single damaged piece can, in some cases, be reached and replaced without redoing the whole wall
• It meets the 2021 IRC sidewall rule, which inspectors check at final

What step flashing cannot do

• It cannot work alone. If the housewrap is behind it or the siding is missing, water gets behind the wall leg.
• It cannot handle the bottom corner by itself. A kickout is needed where the roof edge meets the wall.
• It is hard to inspect. Most of it is hidden under shingles and siding.
• Caulk is not a substitute. Sealant on top of bad flashing wears out and hides the real problem.
• It does not fix rot that is already in the wall or deck. Damaged wood must be repaired first.

What drives the cost of step flashing

• Length of the wall joints. More dormers and more roof-to-wall runs mean more pieces and more hand work.
• Siding removal. To replace step flashing, the lowest rows of siding usually come off. DOE's retrofit guide says to expose at least 8 inches of the wall ⁵. Some siding comes off cleanly; some breaks and must be replaced.
• Wall type. Brick or stucco walls need counterflashing cut into the wall, which is slower work.
• Metal. Copper costs more than aluminum or galvanized steel.
• Hidden damage. Rot found behind old flashing adds wall or deck repair.
• For a full-roof budget, use our roof cost calculator or the roofing cost guide.

How a crew installs step flashing

This is what you should see on a reroof. It is not a do-it-yourself guide. Roof work is a fall risk, and a licensed roofer should do it.

1. The lower siding along the wall is removed, and the old flashing comes off with the tear-off. The crew checks the wall and deck for rot ⁵.
2. Underlayment goes on the deck and turns up the wall a little.
3. The kickout goes in first at the bottom corner, fastened to the deck and starter strip, not the wall ⁴.
4. The first shingle row goes on. A step piece goes on top of its end, roof leg flat, wall leg up.
5. The next shingle row covers the roof leg. The next step piece goes on top and laps the one below. This repeats to the top of the wall ⁷.
6. Each piece is held with one nail, high on the piece, in the roof or the wall but not both, so it can move ⁴.
7. The housewrap is lapped down over the wall legs, and the siding goes back on with a gap above the shingles ⁵.

Where the nail goes, and why sources differ

Sources agree on one nail per piece, placed high, and never through both legs.

One DOE guide says to fasten with "one nail placed at the top corner" on "the roof or the house wall, but not both, to allow for movement" ⁴.

If a nail pins both legs, the house moving can tear the metal.

Which leg gets the nail is a trade habit. One Fine Homebuilding author nails "to the sidewall only, where both the next piece of flashing and the siding will cover the nail head" ⁷.

Others nail the roof leg so the pieces come out easily at the next reroof. Both can work if the nail is covered. Follow the shingle maker's instructions.

Reuse or replace on a reroof

This is the big question on most reroofs, and there is no code rule that forces one answer. Here is how to think about it.

Old step flashing is woven into the old shingles. Taking the shingles off without bending the pieces is hard.

The wall leg is behind siding, so pulling it out cleanly usually means taking off the lower siding.

DOE's guide for existing homes takes the replace approach: "Remove the siding and any existing flashing to expose at least 8 inches of the wall" ⁵.

Reusing makes the most sense when the metal is sound, the pieces come out flat, the wall leg was properly lapped by the wrap, and removing the siding would cause more harm than good.

Replacing makes more sense if the pieces are rusted, bent, too short for the new shingle, or if there are signs of water in the wall below. If you want to learn more about the full job, see our reroofing guide.

Either way, ask the roofer to say in writing which they plan to do, and to photograph the wall leg and the wrap before the siding goes back.

What to check as the roof ages

• The bottom of the siding sits above the shingles, not buried in them
• A kickout is present at the bottom of every roof-to-wall run
• No rusted or lifted flashing edges showing between the siding and the shingles
• No thick caulk patches along the wall line, which often hide a flashing problem
• No stains, soft spots or peeling paint on the wall below the roof joint
• From the attic, no dark or wet wood along the wall line under the joint

Common step flashing failures

• No kickout. Water from the step flashing runs down the wall face and into it at the bottom corner.
• Wrap behind the flashing. DOE calls this out: water can get behind the step flashing and into the wall ⁵.
• One long piece instead of steps. Water that gets under a shingle can run along a continuous strip and out at the bottom, behind the next layer.
• Pieces too short. If a piece is not longer than the shingle exposure, the next piece cannot lap it.
• Nailed through both legs. The metal cannot move and can tear ⁴.
• Siding touching the roof. It wicks water and stays wet; DOE guides call for a gap ^{4 5}.
• Caulk instead of flashing. Sealant dries, cracks and lets go.

Why sidewall leaks are often found late

A bad step flashing joint rarely drips through a ceiling right away. The water runs into the wall, not the attic. It soaks the sheathing and framing behind the siding. By the time paint peels or a stain shows inside, the wood may be soft.

That is why the bottom corner of a roof-to-wall joint deserves a close look before you buy a house or sign a reroof contract. A roof inspection that includes the sidewalls is worth asking for.

What the code requires

These rules come from the 2021 International Residential Code, Chapter 9, as adopted in Maryland and read verbatim ¹. States and cities adopt the IRC on their own schedule and can amend it.

Sources: [1]

Step flashing vs related flashing

Common misconceptions

Myth One long piece of flashing is better than many small ones.

Reality On shingles, the small pieces lap each row, so water under a shingle comes back out. The code allows both, but step flashing is the usual shingle detail.

Myth Caulk along the wall line is enough.

Reality Sealant is not flashing. It cracks and hides problems.

Myth The siding should sit right on the shingles to look neat.

Reality DOE guides call for a gap above the roof so the siding stays dry.

Myth Step flashing alone protects the whole joint.

Reality The bottom corner needs a kickout, which the 2021 IRC requires.

Myth Old step flashing is always reused.

Reality Many reroofs replace it, because bent or rusty pieces cannot lap properly and the wall leg sits behind siding.

Local code and step flashing

The rules on this page come from the 2021 IRC. Your state or city may use an older or newer edition, or amend it. The kickout sentence in R903.2.1 was not in every older code, so an older house may never have had one.

A reroof is usually inspected under the code your building department uses today. Ask your roofer which edition applies, and whether the inspector looks at sidewalls and kickouts. Where the wall is brick or stucco, ask how counterflashing will be handled, since local wall types vary.

What to ask your roofer about step flashing

• Will you replace the step flashing or reuse it, and why?
• Will you remove the lower siding to do it, and who puts it back?
• Will you add or replace a kickout at the bottom of every roof-to-wall run?
• Will the housewrap lap over the flashing, and what gap will the siding have above the roof?
• What metal and piece size will you use, and does it match the shingle maker's instructions?
• If you find rot behind the old flashing, how will you price the repair?

Keep going

• Roof repairFixing leaks at walls and flashing
• Roof replacementStep flashing is replaced or reused as part of a reroof
• Repair or replaceDecide if a flashing leak means a new roof
• Roof underlaymentThe layer under the shingles and flashing

Related terms

Roof Flashing

Roof flashing is thin, water-shedding material, usually bent metal, installed at the joints of a roof where shingles alone cannot keep water out: along walls, around chimneys and pipes, in valleys and at the roof edges.

Kickout Flashing

Kickout flashing is a small angled piece of metal or molded plastic set at the bottom of a roof-to-wall joint, where the roof edge stops but the wall keeps going down, that turns the water running along the wall out into the gutter instead of letting it pour down and into the wall.

Counterflashing

Metal flashing set into or over a wall or chimney that laps down over the base or step flashing, so water cannot get behind it.

Chimney Flashing

Chimney flashing is the set of metal pieces that seal the joint where a chimney passes through a roof: an apron on the downhill side, step flashing up both sides, a cricket or back pan on the uphill side, and counterflashing set into the masonry that laps over all of them.

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Asphalt Shingles

Asphalt shingles are roofing pieces made from a fiberglass mat coated with asphalt and topped with mineral granules, laid in overlapping rows so water runs off the roof.

Reroofing

Reroofing is the work of putting a new roof covering on a house that already has one, either by tearing the old covering off down to the deck (a replacement) or by laying the new covering over the old one (a recover, often called an overlay).

Drip Edge

Drip edge is a strip of bent metal flashing installed along a roof's eaves and rake edges that carries water off the edge of the roof and away from the fascia and the deck beneath.

Also referenced from: Drip Edge, Roof Flashing, Valley Flashing, Chimney Flashing, Kickout Flashing, Pipe Flashing