What Is Step Flashing and Counter Flashing?

Two separate pieces of metal doing two separate jobs wherever a roof meets a vertical wall or a chimney. Step flashing is a series of small L-shaped pieces, one woven in with every course of shingles as the roof is laid, each one lapping the one below like steps. Counter flashing is the piece above it, let into or fixed onto the wall, covering the top edge of the step flashing so water cannot get behind it. You need both. One without the other is the single most common reason a chimney leaks, and it is why the code requires flashings to be replaced rather than reused when a roof is torn off. Arrowhead Roofing has replaced flashings on every re-roof in northeast Oklahoma since 1987.

Note: this is a general guide to how the detail works. Code requirements and manufacturer instructions both apply, and your installer should verify what is required in your jurisdiction on the date the work is done.

Four pieces of metal turn up at a wall or a chimney and people use the names interchangeably. They are not interchangeable.

The Short Answer

Water running down a wall has to get back out onto the roof surface, and it has to do it without going behind anything. That is the entire problem the detail solves.

Step flashing solves it by being layered into the roof as the roof is built. Each L-shaped piece sits on top of the shingle course below and under the course above, so water landing on it can only go one way, which is out and down.

Counter flashing solves the other half. Without it, the top edge of the step flashing is an open seam against the wall, and water running down the masonry goes straight behind the metal and into the building.

Why It Cannot Be Added Later, and What People Do Instead

This is the part that explains most of what you will see on an older roof.

Step flashing is interleaved with the shingles. To install it correctly you have to be laying the roof at the same time. On a finished roof you cannot slide it in without lifting every course above it.

So when somebody finds a leak at a wall and does not want to open the roof, the usual answer is sealant. A bead of caulk or a smear of roof cement along the joint. It works, for a while, and then it does not, because sealant is a consumable and the joint it is covering moves with every temperature change.

A roof with a heavily sealed wall joint is telling you two things: it has leaked before, and the flashing underneath was not corrected.

The Code Requirement Everybody Skips

Flashings are the item most commonly reused on a re-roof, and the code is direct about it.

The reroofing provisions adopted at OAC 748:20 require that existing flashings, edgings, outlets, vents or similar devices forming part of the assembly shall be replaced where rusted, damaged or deteriorated. A separate provision requires flashings to be reconstructed in accordance with the manufacturer’s installation instructions.

Read that as a homeowner reading an estimate. New flashing is not an upgrade somebody is selling you. Old flashing that is rusted, damaged or deteriorated is not permitted to go back on under a new roof.

We change step and base flashing on every re-roof rather than assessing each piece for deterioration and reusing what passes. It is a decision rather than a requirement, and it is the one place on this page where we are doing more than the code obliges.

Flashing Going On, in Sequence

Watch this short video for how the pieces go on in order, which is the part that is hard to picture from a written description.

Where It Goes Wrong

Four failures, and you can spot three of them from a ladder.

No counter flashing at all. Step flashing installed, top edge left open against the masonry, sometimes with a bead of sealant standing in for it.

One long piece instead of many small ones. Sometimes called continuous or apron flashing used where step flashing belongs. It cannot shed water course by course, so anything that gets behind it stays behind it.

Counter flashing surface-mounted and sealed rather than let into the mortar joint. A reglet cut into the joint holds the metal mechanically. Sealant alone is holding it until the sealant goes.

Reused flashing. Old metal, old fastener holes, and a new roof laid around it.

What Changes the Answer

Three things change what the detail needs on a particular house.

What the wall is made of

Brick and stone take a reglet cut into the mortar joint. Stucco and siding are different details with different sequences, and stucco in particular is where we find the most improvised work, because the flashing has to be integrated with the wall’s own drainage plane rather than stuck to its face.

How wide the chimney is

A wide chimney on a slope needs a cricket behind it. Without one, water and debris collect on the uphill side and sit against the masonry, which is a slow leak with a long lead time.

Whether the roof is being replaced anyway

If the roof is coming off, the flashing question answers itself, because the detail can be built correctly from the deck up. If it is not, correcting flashing means opening the courses around it. That is the repair-or-replace calculation in miniature: should you repair or replace your roof?

Related Questions

What else on the estimate is required rather than chosen?

More than most homeowners expect, and the code decides several of them. What building code applies to roofs in Oklahoma? covers the four state amendments.

Why is leak barrier in the valley on my estimate?

Because Oklahoma requires it there unconditionally, whether or not the old roof had any. Is ice and water shield required in Oklahoma? covers why.

My metal roof leaks and the panels look fine.

Same principle, different material: it is nearly always a detail rather than the field. Why is my metal roof leaking if the panels look fine? covers where.

See Also

The two answers next to this one are what the code decides for you, and why the flashing line matters on an estimate.