How to Install Step Flashing Where a Roof Meets a Wall
One piece per shingle course, one nail, and the wall cladding lapped over the top. The detail that keeps sidewalls dry.
- Advanced
- 1 to 3 hours per wall
- Updated
The short answer
Step flashing is a row of small L-shaped metal pieces, one per shingle course, woven into the shingles where a sloped roof runs along a wall. Each piece sits on top of a shingle, gets one nail in its upper roof corner, and is covered by the next course. The wall leg runs up behind the siding or wall paper. Every piece laps the one below it by at least 3 inches (75 mm), the minimum head lap in the BC Building Code.
Anywhere a sloped roof runs up alongside a wall, water sheets down the wall and down the roof at the same time and meets in the corner. A single long strip of metal cannot handle that corner, because water that gets under it runs the full length of the strip and into the house. Step flashing solves it with many short pieces, each one tucked under a shingle and lapped over the piece below, so every drop of water is kicked back out onto the roof surface one course at a time.
It is also one of the most common places roofs leak. Missing pieces, a nail through the wall leg, siding pulled tight to the roof, or a missing kickout flashing at the bottom will all let water behind the wall, where it rots sheathing for years before anyone sees a stain. In the Fraser Valley, with months of rain driven sideways against walls, this detail has to be right.
This guide covers sidewalls on shingle roofs. For where the top of a roof dies into a wall, see headwall flashing. For chimneys, see how to step flash a chimney. Metal panel roofs do not use step flashing, see metal roof wall flashing.
Tools
- Roofing hammer or coil nailer
- Hook blade utility knife
- Tin snips (straight cut)
- Hand seamer
- Flat bar for lifting siding
- Tape measure
- Chalk line
Materials
- Pre-bent step flashing, galvanized or prefinished steel, at least 4 by 4 inches and 10 inches long for laminated shingles
- Ice and water shield membrane
- Roofing nails, 1-1/4 inch
- Kickout flashing for the bottom of the wall
- Asphalt roof cement or sealant, only where the shingle manufacturer calls for it
Safety first
- Work tied off. WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more, and sidewalls are often on the high side of a roof.
- Cut sheet metal edges are razor sharp. Wear cut resistant gloves when bending and trimming flashing.
- Lifting siding puts you close to the wall with your back to the eave. Keep your feet on the roof, not on a loose bundle or scrap.
Step by Step
Step 1: Strip the wall edge and check what is behind the cladding
Pull the old shingles and step flashing back to bare deck along the full length of the wall. On a reroof, the old pieces are almost always bent, rusted or nailed wrong, and reusing them to avoid lifting siding is the most common shortcut on sidewalls.
Lift the bottom course of siding with a flat bar, or cut the siding nails with a hacksaw blade if they will not release. You need at least 4 inches of clear wall above the roof deck. Look at the sheathing: dark, soft or stained wood means water has already been getting in, and it has to be repaired before new flashing goes on.
Watch out: If the sheathing crumbles under a screwdriver, stop and assess the framing. New flashing over rotten sheathing traps the rot.
Step 2: Turn the underlayment up the wall
Cut a strip of ice and water shield and fold it lengthwise so part of it runs up the wall and the rest lies flat on the deck. Press it hard into the corner so there is no air pocket for a nail or a foot to puncture later.
Your field underlayment then laps over the deck portion of this strip, following the normal rule that the upper layer always lies over the lower one. The wall building paper or housewrap must end up over the top of the membrane on the wall, not tucked behind it.
Step 3: Size the step flashing to the shingle exposure
The length of each piece has to be the shingle exposure plus at least 3 inches, because the BC Building Code calls for a head lap of at least 75 mm between steps. Some shingle instructions show 2 inches, but in BC use 3. Most laminated shingles have an exposure of about 5-5/8 inches, so pieces must be at least 8-5/8 inches long, and 10 inch pieces are the practical choice. The common 8 inch pieces only give a full 3 inch lap on 3-tab shingles at 5 inches of exposure.
The roof leg should be at least 4 inches wide and the wall leg at least 4 inches tall. Bigger is better on a wall that takes a lot of water, such as a long wall at the bottom of a big roof plane. If you are bending your own, a sheet metal brake turns out consistent pieces quickly.
Tip: Use galvanized or prefinished steel against masonry and mortar. Bare aluminum corrodes in contact with wet mortar and concrete.
Step 4: Start at the eave with the first piece and the kickout
Where the wall continues past the roof edge, water running down the step flashing will pour straight into the wall at the bottom unless it is turned out into the gutter. That is the job of the kickout flashing. It goes on first, over the drip edge and starter course, tight to the wall.
The first step flashing piece sits on top of the starter strip and laps over the kickout, so water moves from one to the next without ever reaching the joint.
Step 5: Lay a shingle course to the wall
Trim the end shingle so it stops just short of the wall. Jamming shingles tight against the wall wicks water up the cut edge and makes the course buckle as the deck moves. A small gap lets water drain down the flashing, which is exactly where you want it.
Some shingle manufacturers call for the shingle end to be set in a bead of asphalt roof cement over the flashing. Check the installation instructions printed on the bundle wrapper and follow them.
Step 6: Place the step flashing piece on top of that shingle
Position the piece so its lower edge sits slightly up from the butt line of the next course. When the next shingle goes on, it covers the flashing completely and the metal never shows. If you can see flashing below the shingle edge, it is set too low.
The wall leg sits tight against the wall sheathing, over the ice and water shield. Check that it laps the piece below by at least 3 inches.
Step 7: Nail once, in the upper roof corner
One nail, high on the roof leg, near the top edge of the piece, where the next shingle and the next piece will cover it. That holds the piece in place without making a hole anywhere water flows.
Never nail the wall leg. The roof and the wall move independently as framing dries and settles. Nailing both legs ties them together and the flashing tears or pulls open. Also keep shingle nails clear of the flashing's bend so they do not pierce the part of the metal that carries water.
Watch out: A nail through the wall leg or through the bend is the most common hidden sidewall leak. It sits under the siding where no one looks.
Step 8: Repeat course by course to the top
Work up the roof one course at a time so every piece is woven in, never installed as a group afterwards. At the top of a wall that ends below the ridge, the last piece folds around the corner of the wall, and the joint is covered with membrane and the headwall or siding detail above it.
Check every few courses that the pieces are still lapping properly and that none have slid down under foot traffic.
Step 9: Close the wall: building paper and cladding over the flashing
The water resistive barrier on the wall must shingle over the wall legs of the step flashing, so water running down behind the siding lands on the outside of the metal. Tape or patch the paper if it was torn while lifting the siding.
Reinstall the siding or trim with a clearance above the roof surface, typically 1/2 inch to 2 inches depending on the cladding. Vinyl siding and fibre cement manufacturers both publish a minimum clearance above roofs. Siding that sits on the shingles wicks water, traps debris and rots its bottom edge. On stucco or masonry, the wall legs are covered with counter flashing instead.
Step Flashing by Wall Type
The roof side of step flashing is the same on every house. What changes is how the wall covers the wall leg. The full breakdown is in how to flash a roof to wall by cladding type.
- Vinyl siding: lift the bottom courses, lap the housewrap over the flashing, and keep the J-channel or siding edge clear of the roof.
- Fibre cement: the lowest board must stay off the roof surface by the manufacturer's clearance, and cut ends need paint.
- Stucco: the wall legs usually go behind the stucco with a separate counter flashing or a two-piece receiver so the stucco does not have to be broken out at the next reroof.
- Brick, stone and chimneys: step flashing is covered by counter flashing set into a reglet cut into the mortar joints.
Why Not One Long Strip?
A continuous L-shaped strip looks tidier, and it works on metal roofs where the panels are designed for it. On shingles it fails, because water that gets under the strip at any point has a straight path down the whole wall, and the shingles cannot lap over it the way they lap over individual pieces. Step flashing gives every shingle course its own exit, which is why every shingle manufacturer specifies it for sidewalls.
Common Mistakes
- Reusing old step flashing to avoid lifting the siding. Old pieces are often rusted through at the bend or were never lapped properly, and the new roof inherits the leak.
- Nailing the wall leg. It locks the roof to the wall, tears the flashing as the house moves, and puts a hole where water runs.
- Pieces too short for the exposure. With laminated shingles at 5-5/8 inches of exposure, a 5 inch piece leaves a gap between pieces and water runs straight through.
- Leaving out the kickout at the bottom of the wall. Water off the last piece dumps into the wall cavity, the most common cause of rotten sheathing below a roof to wall joint.
- Caulking instead of flashing. Sealant along the wall line cracks within a few seasons of sun and cold. It is not a substitute for lapped metal. See roofing sealants.
- Siding tight to the shingles. Cladding touching the roof wicks water up into the wall and holds wet needles and moss against the flashing.
When to Call a Professional
If lifting the siding shows dark or soft sheathing, if the wall is stucco or masonry, or if the wall sits above a steep or high roof you cannot safely tie off on, this is a job for a roofer. Sidewall leaks often travel a long way inside the wall before they show, so the visible stain may be well below the real problem.
A roofer will also check the kickout, the headwall at the top and the cladding clearance in the same visit, which is usually where a sidewall leak actually starts.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Install Step Flashing: Questions
What size step flashing do I need for architectural shingles?
Use pieces at least 4 inches on the roof leg, at least 4 inches up the wall, and at least 3 inches longer than the shingle exposure, because the BC Building Code calls for a 75 mm head lap between pieces. Most laminated architectural shingles have about 5-5/8 inches of exposure, so 10 inch long pieces are the practical choice. They lap each other by more than 3 inches and stay hidden under the next course.
Where do you nail step flashing?
One nail per piece, in the upper corner of the roof leg, into the roof deck. The next shingle and the next piece of flashing cover that nail. Never nail the wall leg, because the roof and the wall move independently and a nail there tears the flashing and puts a hole where water runs down the wall.
Can you reuse step flashing when replacing a roof?
It is possible when the old pieces are in perfect condition, correctly sized and still properly lapped, but that is rare. Most old step flashing is rusted at the bend, too short for modern shingle exposure, or was nailed through the wall leg. Replacing it means lifting the siding, which is the right time to check the wall sheathing for hidden water damage.
Does step flashing go under or over the shingles?
Both. Each piece sits on top of one shingle course and underneath the next one. That weaving is what makes it work: water that runs off a piece lands on top of the shingle below it and drains down the roof, and the shingle above hides the metal. The wall leg goes behind the siding and the wall building paper.
How far should siding be from the roof?
Most cladding manufacturers call for a gap between the bottom of the siding and the roof surface, commonly between 1/2 inch and 2 inches depending on the product. Fibre cement siding usually needs more clearance than vinyl. The gap stops the siding from wicking water, keeps debris from packing against the flashing, and lets the step flashing drain freely.
Related Guides
All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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