What Is Roof Flashing?

What roof flashing does

Shingles work well on the open, flat parts of a roof. Water lands on one shingle and runs down onto the next. The trouble comes at the joints.

Wherever the roof meets a wall, a chimney, a vent pipe, a skylight or another roof plane, there is a seam that shingles cannot close on their own. Flashing is the part that closes those seams.

Most flashing is sheet metal bent to fit the joint. One side lies on the roof under the shingles. The other side turns up the wall or around the opening. Water that reaches the joint hits the metal and is steered back onto the top of the shingles, where it can drain away.

The 2021 International Residential Code puts the goal in one line: flashings must be installed "in a manner that prevents moisture from entering the wall and roof through joints in copings, through moisture permeable materials and at intersections with parapet walls and other penetrations through the roof plane" ¹.

Every flashing type on this page is a different way of meeting that rule at a different spot on the roof.

How flashing keeps water out

Flashing does not rely on glue or caulk to work. It works by lapping, the same way shingles do.

Each piece sits so that the piece above it overlaps it in the direction water flows. Water always runs over a joint, never into it.

Sealant helps at the edges, but the metal and the laps do the real work.

Many joints use two pieces instead of one. The lower piece, called base flashing, sits on the roof and turns up the wall. The upper piece, called counterflashing, comes down from the wall and laps over the top of the base flashing.

A building science guide from the U.S. Department of Energy describes counterflashing at a brick chimney as "embedded into the mortar joints of the brick layer" and overlapping the base and step flashing, "completing the waterproofing transition" ³.

Why two pieces? The house and the chimney or wall move at different rates.

Fine Homebuilding notes that houses and chimneys settle differently, which can tear single-piece flashing, and that counterflashing and base pieces "overlap but aren't physically joined, so they can move independently yet still repel water" ⁴.

Two loose, overlapped parts can slide past each other without opening a gap.

Sources: [3] [4]

Types of roof flashing

Each type below is named for where it goes or what shape it has. Several will get their own page; this table gives the short version.

Trade names overlap. Some roofers call counterflashing "cap flashing," and some call any long piece an "apron." Ask what shape a piece is and where it goes when you compare quotes.

Sources: [5]

The main flashing types in more detail

Step flashing. Where a roof slope runs up beside a wall, each course of shingles gets its own bent piece of metal. One leg lies on the shingle, the other runs up the wall.

The next shingle covers the roof leg, and the next piece of flashing overlaps the one below it.

The code calls for this base flashing to be at least 4 inches high and 4 inches wide, and where there is siding, the wall leg must go behind it ¹.

Counterflashing. On a brick chimney or masonry wall, the top edge of step or base flashing would leave a gap for water to run behind. Counterflashing covers that edge.

It is set into a cut in the mortar or brick and bends down over the lower flashing. On a wall with siding, the siding itself and the house wrap usually play this role.

The code sets cap flashing at the same minimum: corrosion-resistant metal at least 0.019 inch thick ¹.

Apron flashing. On the downhill side of a chimney, a single long piece runs across the face. Fine Homebuilding recommends the bottom flange overlap the shingles by at least 4 inches and the upper flange run at least 12 inches up the chimney face ⁴.

Valley flashing. A valley is where two roof slopes meet and drain into one line.

The code allows an open valley lined with metal at least 24 inches wide, an open valley lined with two layers of mineral-surfaced roll roofing (an 18-inch bottom layer and a 36-inch top layer), or a closed valley lined with smooth roll roofing or self-adhering underlayment under the shingles ¹.

Kickout flashing. Where the bottom edge of a roof stops against a wall that keeps going down, water running off the step flashing can pour straight down the siding.

The code requires flashing at that spot to "divert the water away" from the wall ¹. A kickout is a small angled piece that turns the water out into the gutter.

Pipe flashing. Plumbing vents and other pipes pass straight up through the deck.

The code says flashing around soil stacks, vents and chimneys goes on according to the shingle maker's printed instructions ¹.

Most homes use a ready-made boot that slides over the pipe and lies under the shingles above it and over the shingles below it.

Drip edge. Drip edge is flashing for the roof's outer edges. It is covered in full on our drip edge page. The shingle makers' association recommends it on all shingle roofs, with a kicked-out lower lip so water does not run onto the wood behind ⁶.

Sources: [1] [4] [6]

Worked example: flashing on one simple house

Picture a gable roof with one brick chimney 36 inches wide in the middle of the slope, two plumbing vents, one valley over a front porch, and a side wall where a garage roof meets the house. This is the flashing that roof needs:

Illustrative house. The 36-inch chimney is the detail that changes the job: at 30 inches or less across, the code does not require a cricket. Ask your roofer to list each flashing spot on the quote so you can see none was skipped.

Where flashing is required

Section R903.2.1 of the 2021 IRC lists the places flashing must go ¹:

• Where the roof meets a wall, such as a dormer side or a second-story wall
• At every change in roof slope or direction, such as a valley or a porch roof tying into the main roof
• Around every roof opening, such as chimneys, skylights and vent pipes
• Where the eave of a sloped roof meets a side wall, to turn water away from the wall (the kickout spot)
• On the uphill side of a chimney or other opening wider than 30 inches, as a cricket or saddle

The diagram shows each joint as its own detail; a quote should name how each one will be flashed.

Benefits of good flashing

• It closes the joints where leaks most often begin, so water stays on top of the roof
• It protects the wall framing and sheathing next to the roof, which you cannot see until damage is done
• Two-piece designs let the roof, walls and chimney move without tearing open
• Metal flashing does not depend on caulk, which dries out and cracks with sun and age
• Correct flashing is a code item, which matters when the inspector signs off a reroof

What flashing cannot do

• It cannot fix a wall that is already wet. Rotten sheathing or framing must be repaired before new flashing goes on.
• It does not replace underlayment. Flashing seals the joints; the underlayment is the backup layer under the whole roof.
• It cannot make up for a bad layout. Pieces lapped the wrong way funnel water in, however much metal is used.
• Sealant is not flashing. A bead of caulk where metal should be is a short-term patch, not a repair.
• It will not stop ice dams. Cold-climate eaves also need an ice barrier and good attic airflow.

What drives the cost of flashing

We do not quote prices here; they depend on your house, your area and the crew. These are the things that move the number. Our roof cost calculator gives a starting range for the whole job.

• Number of joints. Each chimney, skylight, wall and vent adds detail work. A plain gable roof has far less than a roof with dormers and valleys.
• Chimney size and type. A wide brick chimney needs a cricket and counterflashing cut into the mortar, which takes time and skill.
• Metal choice. Galvanized steel, aluminum and copper differ in price and in how they are worked. Copper is often soldered ⁵.
• Replace or reuse. On a reroof, ask whether old flashing is replaced or reused. Bent, rusted or short pieces cannot make proper laps.
• Wall work. Step flashing goes behind siding. If siding must be removed and replaced, that adds cost.
• Hidden damage. Rot found behind old flashing is usually priced as extra once the roof is open.

Sources: [5]

How a roofer flashes a roof during a reroof

This is what you should see happen. It is not a do-it-yourself guide. Roof work is a fall risk, and chimney and wall flashing is skilled work for a licensed pro.

1. The old shingles and, in most cases, the old flashing come off, and the wood at each joint is checked for rot.
2. Drip edge goes on at the eaves, then underlayment, then drip edge at the rakes.
3. Valleys are lined, by one of the methods the code allows, before shingles go near them ¹.
4. The apron goes across the downhill face of the chimney, then step flashing up each side, one piece per shingle course ⁴.
5. On a wide chimney, the cricket is framed on the uphill side and flashed ².
6. Counterflashing is set into the mortar or brick and bent down over the step and base flashing ³.
7. Pipe boots go on as the shingles reach each vent, and a kickout goes at the bottom of each roof-to-wall joint.

What to check as the roof ages

• Look at the chimney from the ground with binoculars: counterflashing should sit tight to the brick with no gaps or loose ends.
• Check rubber collars on pipe boots for cracks; rubber tends to wear out before the metal or the shingles around it.
• Look for rust, holes or lifted edges on any visible flashing, especially in valleys.
• Watch for caulk that has cracked or pulled away. It may be hiding a flashing problem.
• After a big storm, check that no flashing has bent or come loose.
• From the attic, look for water stains around the chimney, vents and skylights after heavy rain.

Warning signs that flashing is failing

• Water stains on the ceiling near a chimney, skylight or outside wall
• Damp or stained wood around the chimney or vents in the attic
• Peeling paint, soft spots or stains on siding below a roof-to-wall joint, a classic sign of a missing kickout
• Loose, bent or rusted metal at the chimney or in a valley
• Thick tar or caulk smeared over a joint, which often means past leaks were patched instead of fixed
• Cracked rubber around a vent pipe

Read each row as a question for the roofer, not as a verdict on the roof.

Why flashing leaks are hard to spot

A flashing leak often does not drip through the ceiling right away. Water that gets behind step flashing or a missing kickout can soak into the wall sheathing and framing for a long time before anything shows inside. By the time paint peels or a stain appears, the wood behind may already be soft.

That is why roofers check flashing first when they look for a leak. If you see signs on this page, have a roofer check the flashing before the damage spreads. Ask the inspector to photograph each flashing joint, so you have a record for repairs, a future insurance claim or a home sale.

What the code requires

These rules come from the 2021 International Residential Code, read as adopted in Maryland. Most states adopt the IRC, often with changes, so your building department's edition is the one that applies.

Sources: [1] [2]

Chimney cricket height by roof pitch

R1003.20 sets how tall a cricket must be, as a share of its width (W), based on the pitch of the roof ²:

Example: a 36-inch-wide cricket on a 6/12 roof must rise about 9 inches (¼ of 36).

Flashing vs the layers around it

Common misconceptions

Myth A bead of caulk is as good as flashing.

Reality Caulk dries out and cracks. Flashing works by lapping metal so water runs over joints. Sealant only helps at the edges.

Myth Old flashing can always be reused.

Reality Bent, rusted or short pieces cannot make proper laps. Most reroofs replace it, and the quote should say so.

Myth One big piece is stronger than many small ones.

Reality Along walls and chimneys, separate step pieces and loose counterflashing let parts move without tearing ⁴.

Myth Only chimneys need flashing.

Reality The code requires it at walls, valleys, changes in slope, vents and roof edges too ¹.

Myth Counterflashing is optional trim.

Reality On a chimney or masonry wall, it is the piece that keeps water from running behind the base flashing.

Local code and flashing

The rules on this page come from the 2021 IRC. States and cities adopt the code on their own schedule and may change it, and an older house may have been built before some of these rules, such as the kickout requirement, applied.

A reroof is usually inspected under the code your building department uses today.

Climate changes what matters most. In snowy areas, valleys and the uphill side of chimneys hold snow and meltwater, so the cricket and valley liner work hard.

Near the coast, salt air can wear on some metals faster, so ask which metal your roofer uses.

Ask your building department or roofer which code edition applies where you live, and whether the inspector checks flashing details.

What to ask your roofer about flashing

• Will all flashing be replaced, or will any be reused? Which pieces?
• Will the chimney get new counterflashing set into the mortar, or will the old pieces stay?
• Does my chimney need a cricket, and will you build one?
• Will you add kickout flashing where a roof edge meets a wall?
• What metal and thickness will you use? Is it at least 0.019 inch?
• If you find rot behind old flashing, how will you price the repair?

Keep going

• Roof repairFixing failed flashing and the leaks it causes
• Roof inspectionHave every flashing joint checked and photographed
• Roofing squareHow roofers measure the rest of the job
• Repair or replaceDecide whether a flashing leak means a new roof

Related terms

Step Flashing

Step flashing is a set of small, L-shaped pieces of metal, one for each row of shingles, woven into the roof where a sloped roof runs up beside a wall, a dormer or a chimney side, so water on the roof is sent back onto the shingles and away from the wall.

Counterflashing

Metal flashing set into or over a wall or chimney that laps down over the base or step flashing, so water cannot get behind it.

Chimney Flashing

Chimney flashing is the set of metal pieces that seal the joint where a chimney passes through a roof: an apron on the downhill side, step flashing up both sides, a cricket or back pan on the uphill side, and counterflashing set into the masonry that laps over all of them.

Valley Flashing

Valley flashing is the lining installed in a roof valley, the inside corner where two roof slopes meet, so the water both slopes send into that one line runs down to the eave instead of into the deck.

Kickout Flashing

Kickout flashing is a small angled piece of metal or molded plastic set at the bottom of a roof-to-wall joint, where the roof edge stops but the wall keeps going down, that turns the water running along the wall out into the gutter instead of letting it pour down and into the wall.

Pipe Flashing

Pipe flashing, often called a roof boot, is a flat flashing plate with a raised collar that slides over a plumbing vent pipe where it comes through the roof, so the plate sheds water onto the shingles and the collar seals tight around the pipe.

Drip Edge

Drip edge is a strip of bent metal flashing installed along a roof's eaves and rake edges that carries water off the edge of the roof and away from the fascia and the deck beneath.

Ice and Water Shield

Ice and water shield is a self-adhering, rubberized asphalt membrane laid directly on the roof deck under the roof covering, where it seals around nails and blocks water that ice dams or wind-driven rain push back under the shingles.

Roof Leak

Water getting past the roof covering into the deck, attic or house. Most leaks start at flashing, penetrations or damaged shingles, not open field.

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Also referenced from: Roofing System, Low-Slope Roof, Metal Roofing, Standing Seam Metal Roof, Solar Shingles, Drip Edge, Step Flashing, Valley Flashing, Chimney Flashing, Kickout Flashing, Pipe Flashing, Roof Inspection Report, Reroofing