Roof Flashing Installation: A Step-by-Step Guide
- Ryan Michael
- 7 days ago
- 8 min read
Most roof leaks don't start in the middle of a roof plane. They start at the edges, valleys, chimneys, and walls, exactly where two different materials meet. That's the job of flashing, and getting flashing installation roof work wrong is one of the most common reasons Kirkland and Bellevue homeowners end up with water stains on their ceilings months after a new roof goes on.
This guide walks you through roof flashing installation the way our crews actually do it in the field: step flashing along walls, drip edge at the eaves and rakes, and counter flashing or chimney flashing where water flow gets complicated. You'll learn the sequencing that keeps water moving down and off your roof, not sideways into your sheathing.
We'll cover the tools and materials you need, the overlap and fastening rules that separate a durable installation from a callback, and the trouble spots (chimneys, dormers, skylights) where most DIY and even some professional jobs go wrong. If you're planning a repair or a full re-roof in the Pacific Northwest's wet climate, this is the process to get right the first time.
What you need before installing roof flashing
Getting roof flashing installation right starts long before the first piece of metal goes down. You need the correct materials for your climate, the right tools on the roof with you, and a dry, stable deck to work on. Skipping this prep stage is exactly why so many flashing jobs fail within a few wet Northwest winters instead of lasting the 20-plus years good flashing should give you.
Flashing only works as well as the prep work underneath it.
Materials and flashing types
Different parts of the roof call for different flashing profiles, and mixing them up is a common DIY mistake. Aluminum works fine in most Kirkland-area applications, but if your roof sits near salt air or you're pairing flashing with copper gutters, galvanic corrosion becomes a real problem, so match your metals.
Flashing type | Where it's used | Typical material |
|---|---|---|
Drip edge | Eaves and rakes | Aluminum, galvanized steel |
Step flashing | Roof-to-wall junctions | Aluminum, copper |
Kickout flashing | Base of a wall, diverting water to gutter | Aluminum, galvanized steel |
Chimney/counter flashing | Chimneys, dormers | Copper, lead-coated copper |
Valley flashing | Roof valleys | Aluminum, galvanized steel |
Pipe boot flashing | Vent pipes | Rubber/neoprene with metal base |
Roofing cement, self-adhering underlayment (ice-and-water shield), and corrosion-resistant roofing nails or screws round out the shopping list. Never substitute standard nails for the corrosion-resistant fasteners; ordinary steel nails rust and stain within a season in this climate.
Tools you'll need on the roof
Quality tools make the difference between clean, tight flashing seams and a sloppy job that traps water instead of shedding it. Have these on hand before you start:
Tin snips or a metal-cutting shear
Pry bar and roofing hammer
Chalk line and tape measure
Caulking gun with roofing-grade sealant
Utility knife for trimming underlayment
Ladder stabilizer and roof jacks for steep pitches
Safety and weather prep
Washington's roofing window is smaller than homeowners think. Moisture on the deck, even a light morning dew, keeps adhesive underlayment from bonding properly and turns a shingled or metal roof into a slip hazard. Plan your flashing roof installation for a stretch of dry days, check the forecast twice, and never work a roof with standing moisture on it. Personal protective equipment matters just as much as the flashing itself: harness, non-slip footwear, and eye protection for cutting metal. According to OSHA's fall protection standards, residential roofing work above six feet requires fall protection systems, which is a rule worth following even on a modest single-story ranch.
Step 1. Prepare the roof deck and underlayment
Before any flashing touches your roof, the deck underneath has to be sound. Sistering in new plywood where the old decking is soft or delaminated isn't optional. Flashing fastened into rotten sheathing will pull loose within a year or two, no matter how well you cut and bent the metal.
Start by stripping the old roofing material down to bare deck in the work area, then inspect every sheet for soft spots, delamination, or water staining. Tap the surface with a hammer as you go; a dull thud instead of a sharp knock means the wood underneath has already started breaking down. Replace any damaged sections with matching thickness plywood or OSB before moving forward.
A flashing system is only as strong as the deck it's fastened to.
Once the deck is solid, install your underlayment in this order:
Ice-and-water shield along eaves, valleys, and anywhere flashing will overlap (minimum 24 inches up from the eave edge in most Kirkland-area code requirements).
Synthetic or felt underlayment across the remaining field, working from the bottom up.
Overlap each course by at least 4 inches horizontally and 6 inches at end laps.
Extend underlayment past roof edges and wall junctions so flashing has continuous, dry material to fasten into.
This layering matters more in the Pacific Northwest than almost anywhere else in the country. Wind-driven rain works its way under shingles more often here than in drier climates, and ice-and-water shield is your backup layer when that happens. Underlayment that's cut short at a wall or valley leaves a gap flashing can't fix on its own, so check every transition point twice before you set the first piece of metal.
Step 2. Install drip edge flashing at eaves and rakes
Drip edge is the simplest piece of the whole roof flashing installation process, and it's also the piece homeowners most often skip or install backward. Its job is basic but critical: direct water away from the fascia and into the gutter instead of letting it wick back under the shingles or rot the roof deck edge.
Drip edge does one job, but skip it and water finds the fascia every time.
At the eaves, install drip edge directly on the deck, under the ice-and-water shield. This order matters. Water that sneaks past the shingles hits the underlayment first and sheds onto the drip edge, not the bare wood underneath it. Fasten the metal every 12 inches with corrosion-resistant roofing nails, keeping fasteners close to the inner edge so wind can't lift the outer lip.
At the rakes, the sequence flips. Drip edge goes on top of the underlayment here, since the water flow direction is different along the gable ends. Overlap each piece by at least 2 inches, and always run the upper piece over the lower one, following the same downhill logic you used with the underlayment courses.
Sizing and overlap for your climate
Use drip edge with at least a 2-inch vertical face and a 3-inch horizontal flange for anything on Kirkland, Bothell, or Bellevue homes. Wider flanges give wind-driven rain, common off Lake Washington, less chance to travel sideways.
Overlap sections by 2 to 3 inches
Nail every 12 inches, staggered slightly to avoid splitting the metal
Bend the drip edge slightly outward at corners so water doesn't pool at the miter joints
Get this step wrong and every layer above it, including your step flashing, is working against a compromised edge.
Step 3. Install step flashing and kickout flashing along walls
Wherever a roof plane meets a sidewall, dormer, or chimney, you need individual step flashing pieces, not one long strip of metal. Each piece measures roughly 8x8 inches, bent at a 90-degree angle, and installed in sequence with every shingle course as you work up the slope. This is the section of installation of roof flashing where most leaks actually originate, because a single continuous strip can't flex with the roof and always leaves a gap somewhere along the run.
Step flashing works because it moves with every course, not despite it.
Work from the bottom of the wall upward. Slide each step flashing piece under the shingle course above it and over the course below, then fasten it to the roof deck only, never through the siding or house wrap. Overlap each piece by at least 2 inches with the one before it, following the same shingle-lap logic as your underlayment. Skipping a piece or trying to stretch coverage with fewer, larger pieces is the shortcut that causes wall-line leaks two or three winters down the road.
Where kickout flashing matters most
Kickout flashing goes at the bottom of any roof-to-wall run, right where the step flashing ends and the gutter begins. Without it, water running down the wall junction shoots behind the gutter and straight into your siding and sheathing, a problem that shows up as rot inside exterior walls long before you see it from outside.
Install kickout flashing at a minimum 110-degree angle to divert water outward
Position it directly above the gutter's edge, never behind it
Seal the top edge with roofing cement where it meets the last step flashing piece
Check it every few years during gutter cleaning, since debris buildup can redirect water back toward the wall
Step 4. Flash chimneys, skylights, and roof penetrations
Chimneys and skylights need a two-part flashing system: base flashing that ties into the roof deck, and counter flashing that overlaps it from above. Base flashing gets installed the same way as step flashing, one piece per shingle course, running up both sides and across the front (apron) and back (saddle or cricket) of the chimney. Counter flashing then tucks into a reglet cut roughly 1.5 inches into the mortar joint, bent down over the base flashing so water sheds outside the seam instead of behind it.
Counter flashing exists to cover the seam base flashing can't hide on its own.
Back chimneys wider than 30 inches need a cricket, a small peaked structure built to divert water around the chimney instead of letting it pool against the uphill side. Skip the cricket on a wide chimney and you're guaranteed standing water and ice damming there every winter.
Skylights use a manufacturer-specific flashing kit almost every time, and that's not a place to improvise. Follow the kit's step-by-step order exactly:
Install the head flashing at the top of the skylight opening first.
Add step flashing pieces down both sides, integrated with each shingle course.
Install the sill flashing at the bottom last, overlapping the underlayment beneath it.
Vent pipes and other small penetrations get a rubber or neoprene pipe boot with a metal base, slid over the pipe and fastened flat to the deck, then shingled over on the upper courses and left exposed on the lower ones. Reseal every boot you find with cracked rubber; UV exposure breaks down neoprene faster than most homeowners expect, usually within 10 to 15 years, and a cracked boot is a direct path for water straight into your attic.
Knowing when to call in a roofing professional
Handling flashing installation roof work yourself is realistic for a simple drip edge repair on a single-story ranch. It's a different story once you're dealing with chimneys, crickets, or multiple wall intersections at once. Steep pitches, cut mortar joints for counter flashing, and skylight kits all carry real risk of a callback, or worse, a fall, if you're working solo without roof jacks and fall protection.
Mistakes here don't show up immediately. They show up as a stain on your ceiling two winters later, after the water's already found its way into your sheathing and insulation. If any part of this job involves a chimney, a valley, or a wall junction taller than you can safely reach from a ladder, bring in a professional crew instead of guessing.
Legacy Exteriors has installed flashing on hundreds of Kirkland-area roofs, and every quote we write is locked in before work starts, no surprises at the end. Request your free roofing inspection and get a straight answer on what your roof actually needs.
