What Is Chimney Flashing? Step, Counter, Apron and the Cricket

What chimney flashing is

A chimney is a big box that sticks up through the roof. Water running down the roof hits the uphill side of that box, flows around both sides, and leaves past the downhill side. Shingles cannot seal any of those four joints by themselves. Chimney flashing is the metal that does.

The Department of Energy's building science guide splits the job into three kinds of metal: "base flashing, step flashing, and counterflashing" ³.

The base pieces sit on the roof and turn up the face of the chimney. The step pieces weave in with the shingles along each side.

The counterflashing comes down from the brick and covers the top edges of the other two.

This page goes deep on chimneys. For flashing at walls, valleys, pipes and roof edges, see our roof flashing guide.

How the pieces keep water out

Every flashing piece works by lapping, the same way shingles do. The uphill piece always lies over the downhill one, so water running down the roof always lands on top of the next layer and never gets behind it.

The trick at a chimney is that the roof and the chimney move on their own.

Fine Homebuilding notes that houses and chimneys settle at different rates, and that counterflashing and base pieces "overlap but aren't physically joined, so they can move independently yet still repel water" ⁴.

If one piece were nailed to both the roof and the brick, the movement would tear it. Two loose pieces that slide past each other stay sealed.

So the base and step flashing are fastened to the roof deck only. The counterflashing is fastened to the chimney only. Where they meet, one simply covers the other by a few inches. Fine Homebuilding gives 4 inches as the overlap of counterflashing over base flashing ⁴.

The point of the stack is the gap in fastening: nothing is nailed to both the roof and the brick.

The parts of a chimney flashing set

Some suppliers sell a ready-made apron with corner "ears" that wrap the front corners of the chimney ⁴. Corners are where most chimney leaks start, so ask how they will be made.

Sources: [3] [4]

Apron and step flashing: the base layer

The apron. On the downhill face, one piece of metal runs the full width of the chimney. Its bottom flange lies on top of the shingles, and its top flange turns up the brick.

Fine Homebuilding recommends the bottom flange lap the shingles by at least 4 inches and the upper flange run at least 12 inches up the chimney face ⁴.

Step flashing. Along each side, the roofer lays one shingle, then one bent piece of step flashing, then the next shingle, all the way up.

Each piece laps the one below it, so water steps down from one to the next.

The DOE guide says step flashing is "integrated in a layered manner with the asphalt shingle roofing" ³. Common pieces are about 5 by 7 inches.

One roofer in Fine Homebuilding uses 6 by 8 inch pieces to better resist wind-driven rain and heavy snow ⁵.

For sidewalls, the code sets base flashing at "not less than 4 inches (102 mm) in height and 4 inches (102 mm) in width" ².

For chimneys, it says flashing "shall be applied in accordance with the asphalt shingle manufacturer's printed instructions" ². That means the shingle maker's details are part of the code on a shingle roof.

Counterflashing and reglets cut into the mortar

Counterflashing is the top layer. The DOE guide describes it as "embedded into the mortar joints of the brick layer" where it "overlaps the base flashing and the step flashing, completing the waterproofing transition" ³.

To set it, the installer cuts a slot, called a reglet, into a mortar joint. The upper edge of the counterflashing bends into that slot, and the rest hangs down over the base flashing.

Fine Homebuilding's traditional method sets the metal about 1½ inches into the joint ⁴.

Another method in the same magazine cuts a groove about 1¾ inches deep, roughly 6 inches above the roof deck, all the way around the chimney, so the counterflashing can run in one long piece with fewer seams ⁵.

On the sides of the chimney, the mortar lines are level but the roof is sloped. So side counterflashing is often cut in steps that follow the brick courses, or set into a single level groove with a wider piece.

The metal is held in the slot with wedges or fasteners and then sealed. Fine Homebuilding's method uses folded copper wedges every 8 to 12 inches and seals the top edge with a polyether sealant ⁵.

Some quick repairs skip the reglet and just glue a strip of metal to the face of the brick with caulk. That surface-mount strip depends entirely on the sealant. When the caulk lets go, water runs straight behind it.

What a roof cricket is, and when the code requires one

A cricket, also called a saddle, is a small two-sided roof built on the uphill side of a chimney.

Without one, water and snow pile up against the back of a wide chimney, sit there, and work under the flashing.

The cricket's peak splits the flow and sends it around both sides, where the step flashing carries it away.

The 2021 International Residential Code covers this twice. In the chimney chapter, R1003.20 says: "Chimneys shall be provided with crickets where the dimension parallel to the ridgeline is greater than 30 inches (762 mm) and does not intersect the ridgeline" ¹.

In the roof chapter, R903.2.2 says: "A cricket or saddle shall be installed on the ridge side of any chimney or penetration more than 30 inches (762 mm) wide as measured perpendicular to the slope" ².

The roof chapter adds that the cricket may be covered with sheet metal or the same material as the roof ².

The code also says how the cricket meets the chimney: "The intersection of the cricket and the chimney shall be flashed and counterflashed in the same manner as normal roof-chimney intersections" ¹. In other words, a cricket is not a shortcut around flashing. It gets its own base and counterflashing.

Thirty inches is the code minimum, not a ceiling. One roofer in Fine Homebuilding builds a cricket on chimneys wider than 24 inches and uses a one-piece pan on smaller ones ⁵. If your chimney is close to 30 inches and you get heavy snow, ask your roofer whether a cricket makes sense anyway.

Sources: [1] [2]

Cricket height by roof pitch (IRC Table R1003.20)

The code sets the cricket's height (H) as a share of its width (W), which is the width of the chimney. Steeper roofs need taller crickets. As written in Table R1003.20 of the 2021 IRC ¹:

Not sure of your slope? Our roof pitch guide explains how it is measured.

Sources: [1]

Worked example: sizing a cricket

A brick chimney sits in the middle of a roof slope, well below the ridge. It measures 36 inches across, parallel to the ridge. The roof pitch is 6:12.

Illustrative chimney. If the same chimney were 28 inches wide, the code would not require a cricket, and a back pan would usually do the job.

Where chimney flashing is found

• Brick and stone chimneys for fireplaces and wood stoves, the most common case
• Chimneys clad in siding or stucco over a wood frame, where the wall wrap and siding act as the counterflashing
• Old chimneys no longer in use, which still need full flashing as long as they pass through the roof
• Chimneys at the ridge, which straddle the peak and do not need a cricket under R1003.20 ¹
• Other wide roof openings, since R903.2.2 applies the 30-inch cricket rule to "any chimney or penetration" ²

Benefits of flashing done right

• Keeps rain out of the most leak-prone joint on most roofs
• Lets the chimney and the roof move separately without tearing open
• A cricket stops water and snow from piling up behind the chimney
• Mechanical laps keep working after sealant has aged
• Protects the roof deck, rafters and ceiling around the chimney from slow rot

What chimney flashing cannot do

• It cannot stop water that enters through the top of the chimney. That is the crown and the cap.
• It cannot keep water out of porous brick or cracked mortar above the roof line.
• It cannot hold in soft or crumbling mortar. A reglet cut into bad mortar fails, so the joints may need repointing first.
• It cannot fix rot already in the deck around the chimney. That wood is replaced before new flashing goes on.

Sealant vs flashing

Caulk and roof cement show up on almost every chimney. They have a place, but they are not the same thing as flashing.

The DOE guide does use mastic: it calls for base flashing "sealed with a full bed of mastic" to the top of the roof membrane underlayment ³. That is sealant working with flashing, not instead of it.

Sources: [3] [5]

Flashing leak or masonry leak?

Chimney leaks have many sources. The Chimney Safety Institute of America (CSIA) points to caps, crowns, masonry and flashing, and notes that "water causes more damage to masonry chimneys than fire" ⁶. Where you see the water gives a clue:

A good contractor checks the whole chimney before blaming one part. A CSIA-certified sweep inspects the cap, crown, masonry and flashing together.

Sources: [6]

The crown and cap: the top of the chimney

The crown is the sloped slab on top of the brick. The cap is the metal cover over the flue opening. Both keep rain from entering from above, which no flashing can stop.

The 2021 IRC says: "Masonry chimneys shall have a concrete, metal or stone cap, a drip edge and a caulked bond break around any flue liners in accordance with ASTM C1283. The concrete, metal or stone cap shall be sloped to shed water" ¹.

The CSIA advice says many crowns are made of common mortar that does not hold up, and recommends a Portland cement-based crown with an overhang that projects "beyond all sides of the chimney by at least 2 inches" ⁶.

It calls a chimney cap "the most inexpensive way to prevent water damage to a chimney" ⁶.

For brick that soaks up water, the CSIA advice is a "vapor-permeable waterproofing agent to let moisture out, but prevent it from coming in," and to avoid paint or sealers that trap moisture ⁶.

Sources: [1] [6]

Who does the work: roofer or mason

Chimney flashing sits on the line between two trades. The apron, step flashing and cricket belong to the roof, so a roofer installs them along with the shingles.

The counterflashing goes into the brick, so it may be done by the roofer or by a mason, depending on who has the tools and skill to cut clean reglets.

Crowns, caps, repointing mortar and replacing brick are masonry work. A chimney sweep certified by CSIA can inspect the whole chimney and tell you which parts need attention.

On a reroof, the best time to redo chimney flashing is when the shingles around the chimney are off anyway. If the mortar is soft, ask for the masonry repair to happen first, so the new counterflashing has solid joints to grip.

How a crew flashes a chimney on a reroof

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

1. Old shingles, flashing and tar come off around the chimney, and the deck is checked for rot.
2. Ice and water membrane goes on the deck around the chimney and up the brick a few inches.
3. The apron goes across the downhill face, on top of the shingles below.
4. Step flashing goes up both sides, one piece with each course of shingles.
5. On a wide chimney, the cricket is framed and covered; otherwise a back pan goes on behind.
6. Reglets are cut into the mortar, and counterflashing is set in, lapped over all the base pieces and sealed at the top edge.
7. The crew checks every corner, where the front, side and back pieces meet.

What drives the cost of chimney flashing

• Chimney size. A chimney over 30 inches wide needs a cricket, which adds framing, metal and flashing ¹.
• Metal. Galvanized steel, aluminum, painted steel and copper cost different amounts. Fine Homebuilding's roofers prefer copper for how it bends, lasts and can be soldered ⁵.
• Reglets vs surface mount. Cutting real reglets into mortar takes more time than gluing on a strip.
• Mortar condition. Soft joints may need repointing by a mason before counterflashing can go in.
• Roof pitch and height. Steep or high roofs need more safety setup.
• Timing. Flashing done during a full reroof costs less than a separate repair later.

We do not publish prices here. To plan a budget for the whole roof, try the roof cost calculator.

What to check as the chimney ages

• Counterflashing still tight in its slot, not pulled out or bent away from the brick
• Sealant at the reglets not cracked, shrunk or missing
• No gaps at the front and back corners
• No piles of leaves or debris behind the chimney or on the cricket
• Crown free of cracks, and the cap in place
• No loose mortar or crumbling brick near the roof line
• From the attic, no stains or soft wood around the chimney after rain

Check from the ground with binoculars or from inside the attic. Leave the roof itself to a pro.

Common misconceptions

Myth A thick coat of roof tar fixes a leaking chimney.

Reality Tar can stop a drip for a while, but it cracks as it ages and often hides the real problem. Lapped metal is the fix.

Myth Every chimney needs a cricket.

Reality The code requires one only when the chimney is more than 30 inches wide and does not meet the ridge. Narrow chimneys usually get a back pan.

Myth If the chimney leaks, the flashing has failed.

Reality Crowns, caps and porous brick cause many chimney leaks too. Look at where the water shows up.

Myth One piece of metal is stronger than two.

Reality A single piece fixed to both roof and chimney tears when they move. Separate base and counterflashing slide past each other.

Myth Old counterflashing can always be reused.

Reality It often can if it is sound, but bent, rusted or loose pieces should be replaced while the shingles are off.

Local code and chimney flashing

The rules on this page come from the 2021 International Residential Code. States and cities adopt the code on their own schedule and can change it, so your building department has the final say on which edition applies.

Climate matters too. In snowy areas, snow piles behind chimneys and melts slowly, so a cricket and ice barrier around the chimney are worth more.

Where wind-driven rain is common, the DOE guide stresses that the chimney-roof joint is vital for "controlling rainwater entry during high wind events such as hurricanes" ³.

Ask your roofer which code edition your area uses and whether the inspector checks chimney flashing on a reroof.

When chimney flashing matters, and what to ask

• If you are reroofing, ask: will you replace all the chimney flashing, or reuse some?
• Is my chimney wider than 30 inches, and will you build a cricket?
• Will the counterflashing be set into reglets, or surface-mounted with caulk?
• What metal will you use, and will it work with the other flashing on my roof?
• Does the mortar need repair first, and who will do it?
• If the chimney leaks, have you checked the crown, cap and brick as well as the flashing?

Keep going

• Roof repairFixing leaks around chimneys
• Roof replacementNew chimney flashing goes on with a reroof
• Ice and water shieldThe membrane under flashing at the chimney
• Roof underlaymentThe layer the flashing laps over

Related terms

Roof Flashing

Roof flashing is thin, water-shedding material, usually bent metal, installed at the joints of a roof where shingles alone cannot keep water out: along walls, around chimneys and pipes, in valleys and at the roof edges.

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.

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 Pitch

Roof pitch is the steepness of a roof, written as the inches it rises for every 12 inches it runs horizontally, so a 6/12 roof climbs 6 inches per foot.

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.

Reroofing

Reroofing is the work of putting a new roof covering on a house that already has one, either by tearing the old covering off down to the deck (a replacement) or by laying the new covering over the old one (a recover, often called an overlay).

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: Roof Flashing, Step Flashing, Kickout Flashing, Pipe Flashing