Step Flashing vs Counter Flashing: What's the Difference?

When a roof leaks at a chimney, skylight, or along a wall, there’s a good chance the problem traces back to the flashing system — and specifically to a misunderstanding of how its two main components are supposed to work together. Step flashing and counter flashing are not interchangeable terms, not redundant components, and not optional. They are a carefully engineered two-part system, and removing or simplifying either part creates a water pathway straight into the building.

This guide explains exactly how each component works, why the two-part system is necessary, what materials to use, and how to inspect your own flashing from the ground and up close.

What Is Step Flashing?

Step flashing is the primary water-shedding element at the junction between a sloped roof and any vertical surface — most commonly a chimney or a wall rising above the roofline.

Each piece of step flashing is an L-shaped sheet metal piece, typically 5” x 7” (though dimensions vary by application). The two legs of the L are usually approximately equal in width — 4–5 inches up the vertical surface and 4–5 inches across the roof surface.

How Step Flashing Is Installed

The critical feature of step flashing is that it is woven into the shingle courses rather than installed as a single continuous strip:

A shingle course is laid up to the vertical surface.
A single piece of step flashing is placed on top of the shingle, with one leg lying flat on the roof and the other leg turning up against the wall or chimney.
The next shingle course is laid, covering the horizontal leg of the step flashing piece but leaving the vertical leg exposed against the wall.
The next piece of step flashing is placed on top of that shingle course, and the sequence repeats.

The result is an interlocking “staircase” of metal pieces — each overlapping the one below by the same amount as the shingle headlap — running the full length of the vertical surface.

Why this works: For water to penetrate this system, it would need to travel horizontally under a shingle, then upward under the vertical leg of the step flashing, then downward on the inside face. Even in heavy driving rain, this multi-direction path is essentially impossible to force. The system sheds water by gravity at every step.

Why a single continuous piece fails: A single piece of L-metal or a “pan flashing” bent up against the wall does not integrate with the shingles. Water running down the wall surface hits the top edge of the pan, backs up, and eventually finds its way under the shingles or through the joint where the pan meets the wall. Single-piece pan flashing is a code violation in most jurisdictions — and it leaks.

What Is Counter Flashing?

Counter flashing is the second layer of the chimney/wall flashing system. It covers and protects the exposed vertical legs of the step flashing (or base flashing on the front and back of a chimney).

Counter flashing is installed in the mortar joints of the masonry — a groove is cut (or the existing joint is raked out) to receive the bent top edge of the counter flashing, which is then secured with lead wool or roofing sealant. The counter flashing laps down over the step flashing by at least 4–6 inches.

The Key Property of Counter Flashing

Counter flashing is not attached to the roof. It is attached to the masonry or wall only. This is intentional.

Chimneys and roofs move independently. A masonry chimney expands and contracts with temperature changes, settles with the foundation, and can shift slightly in high winds. The roof deck does the same, but not in sync with the chimney. If the counter flashing were mechanically fastened to both the chimney and the step flashing/deck, that differential movement would either pull the counter flashing free of the masonry or lift the step flashing — creating an immediate leak.

The overlap joint between counter flashing and step flashing allows independent movement while maintaining a sealed water path. It’s one of the more elegant details in building construction.

Base Flashing vs. Step Flashing

On the front and back faces of a chimney (the faces that are parallel to the ridge), there is no shingle weaving — the flashing at those faces is continuous. This continuous piece is called base flashing on the lower face and, on the back (uphill) face of a wide chimney, a saddle or cricket diverts water around the chimney.

Step flashing is used on the sides; base flashing and counter flashing are used on the front face. Together they form a complete seal around the entire chimney perimeter.

Why Both Components Are Necessary

Some homeowners and even some contractors ask: can’t you just make one piece do both jobs? The answer is almost always no, for two reasons:

Differential movement — as described above, the chimney and the roof move independently. A single piece attached to both will fail at one attachment point or the other.

Replacement and maintenance — a properly designed two-part system allows counter flashing to be resealed or replaced without disturbing the step flashing below, and allows step flashing to be replaced during a reroof without cutting into the masonry. Single-piece systems force you to disturb everything when any part needs attention.

Common Installation Errors

Using a Single Continuous Pan

The most common and most serious error is substituting a single continuous piece of sheet metal (a “pan”) for properly woven step flashing. This is faster to install — one piece vs. 20 individual pieces — which is why it’s tempting for inexperienced or careless installers. It leaks. Get a second opinion if an installer suggests this approach.

Over-Caulking Instead of Replacing

A legitimate repair method for minor gap sealing is to apply roofing-grade polyurethane caulk to the counter flashing-to-masonry joint. However, applying heavy beads of caulk over failing pan flashing or over step flashing that has lifted is not a repair — it’s a temporary cover that fails within 1–5 years and masks deteriorating conditions underneath.

Counter Flashing Not Embedded in Mortar

Counter flashing must be embedded in the mortar joint, not simply surface-mounted with caulk. Surface-mounted counter flashing will work loose as the caulk ages, eventually creating a gap that allows water directly behind the flashing.

Missing Lap at the Counter Flashing Seam

Step flashing and counter flashing must overlap by at least 4 inches (6 inches is better). Insufficient lap allows wind-driven rain to travel up between the layers.

Omitting the Cricket

Building codes require a cricket (also called a saddle) behind any chimney that is 30 inches wide or wider, measured perpendicular to the ridge. A cricket is a small peaked structure that diverts water around the back of the chimney. Without it, water pools in the angle between the chimney and the roof, accelerating deterioration and eventually leaking. Crickets are often missing on older homes.

Valley Flashing Types

Valleys — the angle where two roof planes meet — concentrate significant water flow and require their own flashing approach.

Open Valley Metal Flashing

An open valley uses a pre-formed metal strip (typically W-shaped in cross-section, 24 inches wide) running the full length of the valley. Shingles from each side are cut back several inches from the centerline, leaving the metal visible.

Advantages: High flow capacity, very durable, easy to inspect, compatible with all roofing types.

Disadvantages: The metal is visible from the ground, which some homeowners dislike aesthetically.

Open valleys are generally the most durable and recommended option, particularly for high-flow valleys and roofs in heavy rain or snow climates.

Woven Valley

In a woven valley, shingles from each roof plane are alternately woven together across the valley without any separate metal flashing. The shingles themselves overlap and interlock across the centerline.

Advantages: Aesthetically continuous look; no visible metal.

Disadvantages: Cannot be used with laminated (architectural) shingles — the staggered thickness of laminated shingles makes proper weaving impossible. Has a lower flow capacity than open valley metal. Harder to inspect. If a shingle in the valley fails, repair requires disturbing many surrounding shingles.

Closed-Cut Valley

A closed-cut valley is a hybrid approach: shingles from one roof plane extend through the valley; shingles from the adjacent plane are cut in a straight line a few inches from the valley center. Usually installed over an ice-and-water shield or metal liner.

Advantages: Cleaner appearance than open valley; more compatible with architectural shingles than a woven valley.

Disadvantages: The cut edge must be precisely aligned or it looks poor; still has lower flow capacity than open metal; the shingle courses don’t receive the full lap protection of an open valley.

Drip Edge Flashing

Drip edge is a continuous L-shaped metal strip installed at the eaves and rakes (sloped edges) of the roof. It directs water off the edge of the roof decking and into the gutters or away from the fascia.

At the eaves: Drip edge goes on before the underlayment — so the underlayment laps over the drip edge and water flows onto the metal, not behind it.

At the rakes: Drip edge goes on after the underlayment — so it laps over the underlayment and prevents wind-driven rain from lifting under the edge.

Building codes require drip edge on all new roofing. It is one of the most commonly omitted or improperly installed details on low-quality installations.

Flashing Materials: Galvanized Steel, Aluminum, Copper

Galvanized steel is the most widely used flashing material in residential roofing. It provides good corrosion resistance at moderate cost and is compatible with concrete mortar, asphalt shingles, and most sealants. Typical lifespan: 20–40 years.

Aluminum is lightweight, doesn’t rust, and is easy to form in the field. However, it is not compatible with concrete mortar (the alkalinity of the mortar corrodes aluminum) and should not be used in direct contact with copper (galvanic corrosion). Commonly used for wall flashing, drip edges, and step flashing away from masonry. Lifespan: 20–35 years.

Copper is the premium choice. It is extremely durable (50+ year lifespan), aesthetically distinctive (developing a green patina over time), soft enough to form easily, and compatible with both masonry and sealants. It is also significantly more expensive — a full copper chimney reflash might cost twice as much as galvanized. Best for high-end projects where longevity over replacement cycles is the priority.

How to Inspect Your Own Flashing

From the ground with binoculars, look for:

Visible gaps or separations between counter flashing and masonry
Rust streaks running down the chimney face or onto the roof
Missing chimney cap (a cap should be visible over the flue opening)
Shingles that appear lifted near the chimney, walls, or valleys
Efflorescence (white staining) below the counter flashing line

From a ladder at the eave, look for:

Drip edge that has pulled away from the fascia
Visible daylight at the gutter edge suggesting missing or bent drip edge
Valley flashing that has corroded or is lifting

On the roof (use appropriate fall protection):

Flex the step flashing pieces gently — they should be solid, not corroded through
Run a gloved finger along the counter flashing-to-mortar joint — it should be sealed, not open
Check that the valley centerline has appropriate metal or sealed membrane coverage

If you see any of these warning signs, a professional inspection is warranted before the next rain season — not after.