Roof Flashing Types: What Each One Does and Where It Goes
Every place a roof meets something has a named flashing detail. Here is the whole map, with a guide for each.
- Intermediate
- 30 minutes to read
- Updated
The short answer
Roof flashing is thin metal or formed membrane that carries water over any joint in the roof. The main types are step flashing along sidewalls, apron flashing at headwalls, counter flashing over masonry, kickouts at wall ends, valley metal, drip edge at eaves and rakes, pipe and vent flashings, and curb or flange flashings at chimneys and skylights. Each laps over the layer below it so water is shed, not sealed out.
A roof is a set of planes that shed water downhill. Flashing handles everything that interrupts those planes: a wall that rises out of the roof, a chimney, a pipe, a skylight, a valley where two planes meet, and the edges where the roof stops. Different interruptions need different shapes, so roofers use a vocabulary of named details. Knowing the names is the fastest way to read an estimate, a home inspection report or a leak diagnosis.
Every one of these details obeys the same rule: the upper layer laps over the lower one, so water is always carried out onto the roof surface and never behind it. If you are new to that idea, start with the lapping rule. It explains why a flashing that looks right can still leak if one piece is lapped backwards.
Below is each type, what it does, where it goes and which guide walks through the install. In the Fraser Valley, where rain arrives sideways against walls and moss holds moisture against metal, flashing is the part of the roof that most often fails first.
Tools
- Binoculars or a phone camera with zoom
- Ladder, used only from the ground or a proper set-up
- Flashlight for attic inspection
- Notebook or phone for photos of each detail
Materials
- Galvanized, prefinished steel, aluminum, copper or lead sheet, depending on the detail
- Ice and water shield membrane
- Rubber or lead pipe flashings
- Manufactured kits for chimneys, skylights and roof mounts
Safety first
- Identify flashing from the ground or from the attic first. Most of it can be assessed without stepping on the roof.
- If you do go up, WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more, and any height where a fall could cause serious injury. Wet moss makes low roofs slippery too.
- Old flashing edges are sharp and lead flashings should be handled with clean hands. Wear cut resistant gloves and wash before eating.
Step by Step
Step 1: Start with the edges: drip edge and eave protection
At the eave, the drip edge goes under the underlayment so water running down the roof passes over the top of it. At the rake it goes over the underlayment, so wind-driven rain cannot get under the membrane edge. Ice and water shield below it, and the starter course above it, finish the detail. The full install is in how to install drip edge.
Where a roof ends above a lower roof or wall, a gutter apron does the same job at the eave when the drip edge has to reach over a gutter.
Step 2: Flash sidewalls with step flashing
Step flashing is the detail for a roof that meets a vertical wall parallel to the slope, such as a dormer cheek, the side of a house addition, or the wall of a taller building next to a garage roof. Pieces are commonly 4 by 4 inches at minimum, and longer than the shingle exposure by at least 3 inches, since the BC Building Code calls for a 75 mm head lap between pieces. See how to install step flashing.
Step flashing belongs only with shingles and similar small-unit roofing. On a metal roof, long continuous wall flashing takes its place. See metal roof wall flashing.
Step 3: Protect the bottom of the wall with a kickout
Kickouts are the smallest flashing on the roof and the one whose absence does the most damage. Without one, all the water from a wall run is dumped against the siding at the roof edge. Sheathing and framing rot out behind the cladding, often with no sign inside the house. See kickout flashing.
Step 4: Flash headwalls with apron flashing
Apron flashing has a roof leg that lies over the shingles and a wall leg that runs up behind the cladding. It is one continuous piece, not stepped, because water runs straight down the wall onto it. The top edge is covered by the wall's water resistive barrier and cladding. See how to flash a headwall.
Step 5: Cover masonry flashing with counter flashing
Masonry cannot have a flashing nailed into it the way a wood wall can, and it moves differently from the roof. Base flashing (step or apron) goes on first, then the counter flashing is set in a cut joint called a reglet or surface mounted and sealed, and laps over the base. See counter flashing and how to cut a reglet.
Different cladding changes the detail. Vinyl, fibre cement, stucco and brick each cover flashing differently. See flashing a roof to a wall by cladding type.
Step 6: Use valley metal where two planes meet
Valleys are the highest-flow channel on the roof, so they get the most generous detail: a membrane underlay, then either an exposed metal channel or shingles woven or cut over the membrane. Each method has rules for width and lap. Start with roof valley types.
Step 7: Flash roof penetrations: pipes, vents, masts and mounts
Anything that passes through the roof needs a flange or boot that sheds water over the penetration. The shapes differ by what the penetration is.
- Plumbing vents: rubber or lead boot. See pipe boots and lead pipe flashing.
- Gas vents and metal chimneys: a clearance-rated flashing with storm collar. See B-vent flashing.
- Electrical service masts: a mast flashing with strict limits on who does what. See electrical mast flashing.
- Satellite dishes, solar and antennas: flashed standoffs. See roof mount flashing.
Step 8: Flash big penetrations: chimneys and skylights
Both follow the same logic as a wall, applied on four sides, with the highest side getting a diverter so water goes around it. Chimneys add counter flashing into the masonry. See chimney flashing parts and how to flash a skylight.
Step 9: Flash slope changes and dormers
A pitch change such as a mansard lower slope meeting a flatter upper slope needs a transition flashing and lapped underlayment. See flashing a pitch change. A dormer combines an apron at the front, step flashing on the cheeks and valleys at the back. See flashing a dormer.
Step 10: Know where sealant fits
Sealant belongs at laps the manufacturer specifies, at exposed nail heads and at storm collars. It does not belong as the only thing keeping water out of a joint. See roofing sealants for what goes where.
Base Flashing vs Counter Flashing
Base flashing is the piece that sits on the roof and is lapped with the roofing: step flashing, apron flashing and valley metal are all base flashing. Counter flashing is the second piece that covers the top edge of the base flashing and is attached to the wall, chimney or curb, not to the roof. It lets the roof and the structure move independently without tearing the joint.
Flashing Materials and Where They Fit
- Galvanized steel: the common choice for step, apron and valley flashing, usually 26 gauge. Paint cut edges if the product calls for it.
- Prefinished steel and aluminum: used where colour matters, such as against siding. Keep bare aluminum away from wet mortar and concrete.
- Copper and lead: long lasting, used on higher-end homes, chimneys and heritage work. Do not mix copper with aluminum or galvanized steel where water can run between them.
- Rubber and thermoplastic boots: used for pipes. They are the part of the roof most likely to wear out first, because UV cracks the collar.
Fraser Valley Notes
Wind-driven rain is the real test of any roof to wall joint in this region, so counter flashing and cladding laps matter more than they would in a dry climate. Moss and needle debris held against flashing keep it permanently wet, which shortens the life of pipe boots and unpainted metal. After a windstorm or a heavy fall of debris, check the valleys and the wall joints first.
Common Mistakes
- Treating every flashing as caulking. Sealant applied over a missing lap fails within a few seasons and hides the leak in the meantime.
- Reusing old flashing on a new roof. Pieces rusted at the bend or nailed through the wall leg carry their leaks onto the new roof.
- Leaving out the kickout. Water runs down behind the siding and rots the wall without any ceiling stain.
- Using the wrong type for the joint. Step flashing on a headwall, or one long strip along a shingle sidewall, both leave paths straight into the house.
- Mixing metals. Copper touching aluminum or galvanized steel corrodes the lesser metal within a few seasons.
- Ignoring the wall above. The best flashing fails if the building paper or siding above it sheds water behind instead of over it.
When to Call a Professional
If you can see rust, open laps, missing kickouts or gaps filled with caulking, have a roofer look at the roof rather than adding more sealant. A pro can also check how the flashing ties into the wall's water resistive barrier, which is invisible from the ground.
Anything involving a chimney, a skylight, a B-vent, a service mast or a wall taller than one storey is worth professional attention. Those details combine several flashings in one place and fail in ways that are slow and expensive.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
Roof Flashing Types and Where Each Goes: Questions
What are the different types of roof flashing?
The main types are step flashing for sidewalls, apron or headwall flashing at the top of a roof plane, counter flashing over masonry, kickouts at the bottom of a wall, valley flashing, drip edge at eaves and rakes, and penetration flashings for pipes, vents, masts, skylights and chimneys. Each is a differently shaped piece lapped so water is shed over every joint.
What is the difference between step flashing and counter flashing?
Step flashing is base flashing: small L-shaped pieces woven into shingles along a wall. Counter flashing is a second piece of metal, set into or attached to a wall or chimney, that covers the top of the base flashing. Step flashing sheds water on the roof surface, while counter flashing keeps water from getting behind the step flashing.
Where is flashing required on a roof?
Flashing is needed wherever the roof surface is interrupted or ends: along walls, at headwalls, in valleys, around chimneys, skylights, pipes and vents, at dormers, at slope changes, and at the eaves and rakes. The building code and every roofing manufacturer require it at these points, and installation instructions specify the type and size.
What is the most important flashing on a roof?
There is no single answer, but the most commonly overlooked one is the kickout at the bottom of a sidewall. It is small and cheap, yet without it water runs behind the cladding and rots the wall. Valleys and chimney flashings carry the most water and cause the most dramatic leaks when they fail.
How long does roof flashing last?
It varies by material and exposure. Galvanized and prefinished steel flashing commonly lasts as long as a shingle roof, while rubber pipe boots often crack and fail sooner, sometimes in 10 to 15 years. Copper and lead last much longer. Flashing is the part of a roof most worth inspecting after storms and before every reroof.
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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