Roofing Flashing: Types and How It Works

Ask any roofer where roofs leak, and they will not point at the shingles; they will point at the metal. Roofing flashing is the sheet metal that seals the joints and transitions where the roof plane meets something else, a wall, a chimney, a valley, a vent pipe. Shingles handle the open field just fine. It is the intersections that fail, and flashing is what keeps them watertight. In my experience, the overwhelming majority of roof leaks trace back to missing, corroded, or badly installed flashing rather than the shingles themselves. Understand flashing and you understand why some roofs stay dry for 30 years while others leak within two.

Here is a working roofer’s breakdown of the types of roofing flashing, the materials, and where each piece belongs.

Why Flashing Exists

Shingles shed water down a continuous slope, but they cannot seal a change in direction or a penetration. Wherever the roof plane is interrupted, water wants to sneak into the gap. Flashing bridges that gap with metal, directing water back onto the shingle surface and away from the seam. The principle is always the same: overlap the pieces so upper metal laps over lower, shingle-style, and never rely on caulk as the primary seal. Sealant is a backup; the metal’s geometry does the real work.

Good flashing is layered into the roof as it is built. You cannot properly add most flashing after the fact by smearing tar over the top; it has to be woven under the shingles and over the underlayment in the right sequence.

Step Flashing

Step flashing seals the joint where a roof slope meets a vertical wall, like where a roof plane runs up against the side of a dormer or a second story. It is a series of small L-shaped metal pieces, typically bent 90 degrees, each about 5 by 7 inches or 4 by 4 inches with a bend in the middle.

The trick is that each shingle course gets its own step piece, woven in so the metal laps up the wall and out onto the shingle below, then the next shingle covers it and the next step piece laps over that. This staircase of overlapping metal is why it is called step flashing. Running one long continuous piece of bent metal instead, which some hacks do, guarantees a leak because there is no overlap to shed water at each course. If you see a solid strip of flashing up a wall covered in tar, it was done wrong.

Counter Flashing

Counter flashing works with step flashing at walls and chimneys. It is the upper piece that laps over the top of the step or base flashing to keep water from getting behind it. On a brick chimney, counter flashing is set into a saw-cut reglet in the mortar joint, bent down over the step flashing below, so water running down the masonry is directed out onto the flashing rather than behind it.

The two-piece system, base or step flashing beneath and counter flashing overlapping on top, lets the roof and the wall move independently without breaking the seal. This is standard practice on any masonry chimney and any wall-to-roof junction that needs to last.

Valley Flashing

A valley is where two roof slopes meet and channel a large volume of water, making it one of the highest-risk spots on the roof. Valley flashing lines that channel. There are two main approaches:

• Open valley: A wide metal channel, often W-shaped or with a center rib, left exposed down the valley with shingles cut back several inches on each side. It handles the most water and is my preference on steep or high-volume valleys.
• Closed valley: Shingles woven or cut across the valley over a metal or membrane liner, hiding the metal for a cleaner look but relying more on the shingle weave and underlayment.

Under either style, ice-and-water shield should run the full length of the valley before the metal goes down, because valleys concentrate water and are prone to ice-dam backup.

Drip Edge, Vent, and Chimney Flashing

Several other flashing types round out a roof:

• Drip edge: L-shaped metal along eaves and rakes that directs runoff into the gutter and keeps water off the fascia. It is required by current building code and goes under the underlayment at the rakes and over it at the eaves.
• Vent pipe flashing: A metal base with a rubber or lead collar (a boot) that seals around plumbing vent stacks. The rubber gasket is the usual failure point, cracking after 10 to 15 years of UV exposure.
• Chimney flashing: A full assembly of base flashing at the front, step flashing on the sides, counter flashing set into the masonry, and a cricket or saddle behind wide chimneys to divert water around them.

Flashing Materials and Gauges

Flashing metal is chosen for durability and compatibility with the roof and adjacent materials:

• Galvanized steel: Common and economical, typically 26 or 24 gauge; paint or coat it to slow rust
• Aluminum: Rust-proof and easy to bend, common for step and drip edge, but do not use it in direct contact with masonry mortar, which corrodes it
• Copper: Premium, long-lasting 50-plus years, self-weathering, ideal for masonry and high-end work, but expensive
• Lead: Traditional for pipe boots and moldable chimney work, though less common now

Match the flashing to the environment. Copper or coated steel at chimneys, aluminum for general wall and edge work, and never mix incompatible metals that cause galvanic corrosion.

Where Flashing Fails and How to Prevent It

Most flashing leaks come from a handful of causes: reusing old flashing on a new roof instead of replacing it, relying on caulk instead of proper overlap, continuous “apron” flashing where step flashing belongs, cracked rubber pipe boots, and counter flashing that was surface-mounted with sealant instead of set into a mortar reglet.

When you reshingle, insist that all flashing be replaced, not reused. New shingles over 20-year-old rusted flashing is the classic false economy; the flashing outlives its sealant and starts leaking while the new shingles still look great. Replace pipe boots as a matter of course, cut fresh reglets for counter flashing, and weave every step piece by hand. Flashing is cheap metal and a few extra hours of labor, and it is the difference between a roof that sheds water for decades and one that stains your ceilings after the first hard storm.

Kickout Flashing: The Piece Everyone Forgets

One small flashing piece causes an outsized share of hidden water damage: the kickout flashing. It sits at the bottom of a roof-to-wall junction, right where the step flashing ends and the roof edge meets the wall, and its job is to kick water away from the wall and into the gutter instead of letting it run down behind the siding.

When a kickout is missing, water sheets down the wall junction, slips behind the siding, and soaks the wall sheathing and framing, often rotting them for years before anyone notices a stain inside. It is one of the most common causes of concealed wall rot on homes. A proper kickout is a small angled diverter installed at the base of the wall termination, and any competent roofer installs one at every such junction. If you have unexplained interior water stains near a wall that meets a roof, a missing kickout is a prime suspect.

Cricket and Saddle Flashing on Chimneys

Wide chimneys need more than perimeter flashing, they need a cricket, also called a saddle. This is a small peaked structure built on the up-slope side of the chimney that divides and diverts water around it, preventing the pile-up of water, snow, ice, and debris that would otherwise dam against the back of the chimney.

Code generally requires a cricket on any chimney wider than 30 inches measured across the slope. Without one, the flat back wall of the chimney acts like a dam, trapping water and debris until it eventually finds a way through the flashing. The cricket is framed, decked, and flashed like a miniature roof, then integrated with the chimney’s step and counter flashing. On a wide masonry chimney, skipping the cricket is a guaranteed future leak.

Inspecting and Maintaining Flashing

Because flashing outlives its sealant, it needs periodic attention even on a sound roof. Twice a year and after major storms, look for these warning signs from the ground with binoculars or up close if you can safely access the roof:

• Rust streaks or corroded, thinning metal at chimneys and walls
• Cracked, dried, or peeling caulk at counter flashing and pipe boots
• Split or hardened rubber collars on plumbing vent boots, a very common leak source after 10 to 15 years
• Lifted, loose, or missing flashing pieces after high wind
• Interior stains on ceilings and walls near chimneys, skylights, and roof-wall junctions

Re-sealing a reglet joint or replacing a $15 pipe boot is a cheap fix that prevents a major leak. Catching flashing problems early is the whole game.

The Bottom Line on Flashing

Flashing is the least glamorous and most important waterproofing on your roof. Shingles handle the easy open field; the metal handles the hard intersections where roofs actually leak. When you reshingle, replace every piece rather than reusing old, corroded flashing, install kickouts and crickets where they belong, weave step flashing by hand, set counter flashing into masonry reglets, and rely on overlap geometry rather than caulk to shed water. Spend the extra hours and dollars on the metal, and your roof stays dry for decades. Ignore it, and you will be chasing ceiling stains long before the shingles ever wear out.