Apron Flashing for a Roof: Complete Guide

Apron flashing on a roof is the L-shaped metal that seals the horizontal joint where a sloped roof plane runs into a vertical wall, the front of a chimney, or the base of a dormer. Get it wrong and you get a leak that shows up ten feet away on a ceiling, with nobody able to figure out where the water is coming from.

An apron flashing roof detail is one of the most common failure points I chase down on service calls, right up there with pipe boots. This guide covers what it is, how it differs from step flashing, sizing, install steps, and what it costs to do right.

What Apron Flashing Is

Picture where a low porch roof butts head-on into the side of a two-story house, or where the downhill face of a chimney meets the shingles. That’s a horizontal transition — the roof edge runs level across the wall. Apron flashing is a single continuous piece of bent metal with one leg lying on the roof under the shingles and the other leg standing up the wall, shaped like an apron hanging down over the roof.

The horizontal leg carries water coming down the roof out over the top of the shingles below. The vertical leg gets tucked behind the wall cladding or counterflashing so water running down the wall lands on top of the apron, not behind it.

Apron Flashing vs. Step Flashing

People mix these up constantly, so here’s the clean distinction:

• Apron flashing handles horizontal roof-to-wall joints — where the roof edge runs level across the wall. One continuous piece.
• Step flashing handles the sloped sides of that same intersection — where the roof climbs the wall. It’s a series of small bent squares woven one per shingle course.

A chimney needs all of it: apron across the downhill front, step flashing up both sides, and a cricket or headwall flashing across the uphill back. The apron and the step flashing overlap at the corners. Miss that lap and the corner leaks every time.

Materials and Sizing

Standard apron stock:

• Aluminum, 0.019 in. to 0.032 in.: Most common, rust-proof, comes pre-finished in white, brown, and black. Fine for wood, vinyl, and fiber-cement walls.
• Galvanized steel, 26 or 24 gauge: Stiffer for long runs, but avoid it against masonry mortar, which is corrosive.
• Copper, 16 oz.: For masonry chimneys and premium work — mortar-friendly and lasts 75-plus years.

Size the legs generously. I want at least a 4-inch roof leg lapping over the shingle below and a 4-inch to 5-inch wall leg going up behind the siding. On a wide run, like a 6-foot porch-to-wall joint, use a single bent piece if you can get it, or overlap sections at least 4 inches, sealed and lapped downhill.

How to Install Apron Flashing at a Headwall

Here’s the sequence for a roof-meets-wall (headwall) joint:

1. Dry-fit the apron. Bend or order it so the wall leg lands flat against the sheathing and the roof leg lies tight on the deck. A slightly over-90-degree bend springs tight to both surfaces.
2. Run ice-and-water shield up the wall and onto the roof deck first, lapping the corner. This is your backup layer.
3. Shingle up to the wall. The last course runs under the apron’s roof leg.
4. Set the apron so its roof leg laps over the top of that last shingle course. Fasten only the wall leg — nails high on the vertical leg, never through the horizontal roof face where water runs.
5. Tuck the wall leg behind the siding, or under counterflashing on masonry. On a brick chimney, cut a reglet into a mortar joint, insert counterflashing, and seal with polyurethane sealant.
6. Extend the apron 2 to 3 inches past each side and lap it over the top piece of step flashing at the corners.

The Counterflashing Detail on Masonry

On a chimney or brick wall, the apron’s vertical leg should never be surface-sealed to the brick and left at that — caulk fails in a few seasons. Cut a groove (a reglet) about 1 inch deep into a horizontal mortar joint, bend a separate counterflashing piece, insert its top edge into the groove, and repoint or seal with a masonry-grade polyurethane. The counterflashing laps down over the apron’s wall leg so water sheds from brick to counter to apron to shingle in one continuous downhill path.

Common Failure Points

Face-nailing the roof leg is the number one killer — every nail is a hole in the water path. Caulk-only terminations on masonry that dry out and crack. Roof legs lapped too short so wind-driven water backs up past them. And skipping the corner lap between apron and step flashing, which leaves an open triangle right where two water paths converge. When a headwall leaks and the flashing looks fine from the ground, it’s almost always one of these four.

What It Costs

Apron flashing itself is inexpensive; the labor and the surrounding work drive the price:

• Aluminum apron stock: $1.50 to $3 per linear foot, material only.
• New apron on a headwall during a re-roof: $8 to $15 per linear foot installed.
• Full chimney reflash (apron, step, counterflashing, sealant): $300 to $700 for a typical chimney, more for tall two-story or wide masonry stacks.
• Copper upgrade: roughly 2 to 3 times the aluminum labor-and-material figure.

A leaking chimney reflash is one of the highest-return roof repairs you can buy — a few hundred dollars of metal and labor versus thousands in drywall, insulation, and framing repairs if the water keeps running.

Apron Flashing at a Dormer or Porch

A dormer’s front wall meets the main roof in the same horizontal joint a chimney front does, so it gets an apron across the face and step flashing up the cheeks. The move people miss is the corner where the apron dies into the first step shingle — the apron’s end must fold around the corner and lap over the top of that step piece, not butt into it. On a porch roof tying into the house wall, the apron runs the full width of the porch’s top edge and tucks up behind the wall’s siding or a course of house wrap, so water off the wall lands on the porch shingles.

Sealant: Where It Belongs and Where It Doesn’t

Good flashing sheds water by geometry — laps and overlaps — not by caulk. Sealant is a backup at terminations, not the primary defense. Use a quality polyurethane or a tripolymer sealant rated for roofing, never plain silicone or painter’s latex caulk, which won’t bond long-term to metal and masonry. The one place sealant earns its keep is sealing the counterflashing into the reglet and dressing the top edge of the wall leg. Anywhere you’re tempted to run a fat bead of caulk to “fix” a flashing gap, the flashing itself is wrong and the caulk will fail in a couple of seasons.

Signs Your Apron Flashing Is Failing

Inside, a headwall leak usually shows as a stain on the ceiling near an exterior wall, or dampness at the top of a wall below a porch or dormer roof. Outside, look for lifted or corroded metal at the roof-wall joint, cracked dried caulk at the wall leg, rust bleed on the shingles below, and step flashing pieces that have slipped out of line. If your chimney has never been reflashed and the roof is 15-plus years old, the flashing is often the oldest, most tired thing up there — worth a look before it leaks.

DIY or Hire It Out

A short, low porch-to-wall apron on a walkable pitch is doable if you’re handy with tin snips and understand the lap-under/lap-over sequence. A chimney is a different animal: you’re cutting reglets into masonry, weaving step flashing, and working at height on a steep field. That job goes to a roofer. On any apron flashing roof detail, the rule that keeps you dry is simple — every upper layer laps over the layer below, and water only ever travels downhill across metal, never behind it.