Properly flashing a chimney requires a two-part overlapping metal system—step flashing and counter flashing —combined with a solid underlayment and correct water diversion.
Install a Water-Resistant Base and Underlayment
Ice and Water Shield: Lay a self-adhering polymer-modified bitumen membrane around the chimney penetration, running it up the brick or masonry face by at least 2 to 3 inches before layering shingles.
Build a Cricket (Saddle): For any chimney wider than 30 inches on the upslope side, build a small peaked cricket (saddle) covered with matching membrane or metal. This diverts standing water, melting snow, and debris away from the back of the chimney.
For a roofer, the key is to treat chimney flashing as a water-shedding system, not as a collection of pieces sealed together with caulk. The NRCA’s guidance emphasizes proper sequencing, drainage, and durable counterflashing/through-wall details.
A properly flashed chimney should be built as a layered water-shedding system, not as a collection of caulked seams. The most important techniques are:
1. Use both step flashing and counterflashing
For an asphalt-shingle roof, the basic system is:
Step flashing: Individual L-shaped pieces installed with each shingle course along both sides of the chimney. Each piece sheds water onto the shingle below.
Counterflashing: A second layer attached to or embedded in the masonry and covering the upper portion of the step flashing.
Front/apron flashing: A continuous piece across the downslope face of the chimney.
Properly flashing a chimney requires a two-part system—base/step flashing and counter-flashing—integrated with a continuous waterproof underlayment and a cricket for wider structures.
Proper chimney flashing relies on a sequenced, multi-layered approach that directs water safely over the roofing materials rather than allowing it to pool or find gaps in the masonry.
Core Installation Techniques
ChatGPT Search· Sep 19, 2026
A roofer who wants a chimney flashing job to last should treat it as a water-shedding system, not a caulking job. The goal is to create multiple overlapping layers so water is always directed back onto the roof surface. A properly flashed chimney typically uses base flashing, step flashing, and counterflashing working together.
1. Inspect and prepare the chimney first
Before installing metal:
Check the masonry for cracked mortar joints, loose bricks, or deteriorated areas that could let water enter behind the flashing.
Remove old tar patches, failed sealants, and improperly installed flashing.
Verify the chimney crown/cap is sound; flashing cannot compensate for water entering from the top.
Clean mortar joints where counterflashing will be installed.
2. Install a proper underlayment transition
Google AI Mode· Sep 19, 2026
To properly flash a chimney and prevent leaks, a roofer must utilize a two-part flashing system consisting of base/step flashing and counter flashing, integrated with a cricket for wide chimneys . Relying solely on roofing sealant or asphalt cement at joints is a temporary fix and will inevitably fail.
Essential Techniques for Chimney Flashing
Install a Cricket (Saddle) on Wide Chimneys : For any chimney 30 inches wide or more (measured up the slope), a roof cricket must be built behind it. The cricket diverts water and snow around the masonry stack, preventing standing water and ice dams from pooling against the back.
Apply Base and Step Flashing First : Lay down self-adhering ice and water shield around the base, then install individual step flashing pieces interlocked with every single course of shingles. The bottom flange of the base flashing should extend , while the vertical leg goes .
For a roofer, the key is to treat chimney flashing as a water-management system, not as a bead of caulk around the chimney. The most reliable approach is layered flashing that always directs water onto the roof covering.
Best-practice chimney flashing
Start with the chimney and roof intersection
Remove enough shingles and old flashing to expose sound substrate.
Inspect the masonry, mortar joints, roof deck, and framing for existing water damage.
Don't simply flash over failed or deteriorated masonry.
For a roofer, the key is to treat chimney flashing as a water-shedding system, not as a caulking job. A properly flashed masonry chimney generally uses base/apron flashing + step flashing + counterflashing, with a cricket or backer flashing where appropriate.
Best practices
Google AI Mode· Sep 11, 2026
The best technique for properly flashing a chimney is using a two-part system combining step flashing and counter flashing, integrated over a self-adhering waterproofing membrane and a rear cricket . Relying on caulk or roofing tar alone against masonry is a temporary fix that inevitably fails; a permanent, leak-free installation relies on overlapping metal and mechanical anchors.
Essential Chimney Flashing Techniques
Install Ice and Water Shield First : Run a self-adhering polymer-modified bitumen membrane around the base of the chimney, extending 2 to 3 inches up the vertical brickwork and onto the roof deck. This creates a vital secondary waterproof barrier beneath the metal.
Sources AI cites
8% of citations to these sources link to brands' own websites.
Individual L-Shaped Pieces: Use individual, bent 90-degree metal step flashing pieces (aluminum, galvanized steel, or copper).
Shingle Integration: As each course of roofing shingles goes up the side of the chimney, interweave one piece of step flashing over the shingle and up the vertical brick face.
Fastening: Nail each step flashing piece strictly to the roof deck/sheathing through the top horizontal flange—never nail step flashing directly into the vertical masonry , as shifting and settling will cause leaks.
Install the Head and Sill Flashings
Front Apron Flashing: At the downslope base (front) of the chimney, install a wide apron piece that overlaps the top courses of shingles.
Back Pan / Cricket Flashing: At the upslope side, ensure the cricket or back pan channels water cleanly outward past the left and right corners of the chimney structure.
Cut and Insert Counter Flashing (Cap Flashing)
Grind Mortar Joints: Use an angle grinder to cut a horizontal reglet (a 1/4-inch to 3/8-inch deep groove) into the mortar joints of the brick, roughly 1 to 2 inches above the step flashing line.
Tuck and Secure: Insert a custom-bent lip of the counter flashing securely into the ground-out mortar joint rather than surface-mounting it with caulk alone.
Mechanical and Chemical Seal: Secure the metal inside the reglet using masonry anchors or lead wedges, then point up the joint with a high-grade, exterior structural sealant like polyurethane caulk (NP1 or Vulkem) to ensure water sheds completely over the step flashing below.
If you're working on a specific project, let me know:
What is the chimney material (brick, stone, or stucco)?
What is the roof slope and covering (asphalt shingle, tile, or metal)?
I can provide more tailored advice on material compatibility or flashing dimensions!
Repair deteriorated mortar joints, cracked masonry, loose bricks, and damaged chimney crown/cap before flashing.
Don't expect flashing to compensate for water entering through the masonry itself.
Use step flashing along the sides
Install individual step-flashing pieces with each course of shingles, rather than relying on one continuous L-shaped piece.
Each piece should overlap the next so water is continually directed onto the roof covering.
NRCA specifically recommends step flashing for asphalt shingles, slate, and wood roof systems because it directs water onto the roof surface rather than relying on the underlayment.
Install a proper front/apron flashing
At the downslope side of the chimney, use a properly sized apron/base flashing that extends onto the roof and turns up against the chimney.
Integrate it with the shingle courses rather than simply laying it over finished shingles.
Pay special attention to the upslope side
This is usually the most important part of the detail because water and debris can accumulate against the chimney.
A chimney cricket/saddle is particularly valuable on wider chimneys or where the roof geometry warrants one. It splits water around the chimney rather than allowing a large volume of water to hit the back wall.
The cricket should itself be properly flashed and integrated with the roofing underlayment.
Use counterflashing over the base/step flashing
The counterflashing should cover the upper portion of the base flashing and be securely integrated into the masonry.
Ideally, the counterflashing is installed as a separate component so the base flashing can be replaced during future reroofing without destroying the counterflashing. NRCA specifically describes two-piece counterflashing as preferable to a single-piece arrangement.
Don't rely on a shallow saw cut and caulk
This is a common failure point. A strip of metal inserted only slightly into a mortar joint may look like through-wall flashing but doesn't necessarily provide a functional drainage path.
Where the chimney/wall construction calls for it, true through-wall flashing should extend into the wall assembly and provide a path for water to exit. NRCA has specifically warned that superficial "saw-cut flashing" can be ineffective compared with properly installed through-wall flashing.
Make the corners watertight
The front corners, rear corners, and transitions between apron, step, and counterflashing deserve particular attention.
Flashing should be folded, soldered, welded, or otherwise joined according to the metal/system manufacturer's requirements—not merely patched with sealant.
Common choices include aluminum, galvanized steel, stainless steel, and copper, depending on the roof, chimney, environment, and expected service life.
Avoid incompatible metals that can create galvanic corrosion, and don't bury incompatible materials together without considering their compatibility.
Integrate the flashing with the underlayment
The roof's water-control layers should work together: roof covering → flashing → underlayment/membrane → drainage path.
Don't create a situation where water gets behind the flashing but has nowhere to go.
Minimize dependence on sealant
Sealant is useful as a secondary weather seal, but it shouldn't be the primary water-shedding mechanism.
Mechanical geometry, laps, bends, end dams, and drainage should do the bulk of the work.
Any sealant used should be compatible with both the masonry and flashing material and capable of accommodating movement.
A good mental model
Think of the installation as three overlapping defenses:
Water should always encounter a surface that laps over the surface below it and directs water back onto the roof, rather than having to depend on a bead of caulk to stop it.
For a professional roofer, I'd also follow the specific roofing manufacturer's flashing detail and the applicable local building code; the NRCA's current manuals contain detailed construction details for architectural metal flashing and steep-slope roof systems.
If you tell me whether this is an asphalt-shingle, tile, slate, or metal roof, I can give you the proper chimney flashing sequence layer-by-layer, including where each piece starts/ends and how the cricket, step flashing, apron, and counterflashing should be installed.
Backer or cricket flashing: At the upslope side, directing water around the chimney.
The 2024 IRC requires flashing at roof penetrations and says chimney flashing is to be installed according to the applicable shingle manufacturer's instructions.
2. Integrate the step flashing with every shingle course
Don't simply run one long piece of flashing up the side of the chimney and rely on sealant.
For each shingle course:
Install the shingle.
Place the step flashing over the shingle.
Install the next shingle so it covers the appropriate portion of the flashing.
Repeat all the way up the chimney.
The flashing should be large enough to provide adequate coverage on both the roof and chimney sides. NRCA describes step flashing as individual pieces installed at the end of each shingle course.
3. Make the counterflashing a mechanical detail—not a caulk detail
This is one of the biggest differences between a durable installation and a short-term repair.
Counterflashing should be mechanically integrated into the masonry, typically by installing it into mortar joints/reglets, then overlapping the step/base flashing. Sealant can provide secondary protection, but shouldn't be the primary thing keeping water out.
NRCA specifically warns about relying on surface-mounted or saw-cut flashing as though a shallow cut in masonry by itself constitutes a functional through-wall flashing system.
For an existing chimney, a roofer should avoid simply smearing new sealant over deteriorated old flashing and calling it repaired.
4. Pay particular attention to the upslope side
This is where a lot of chimney leaks originate because water, leaves, ice and debris accumulate behind the chimney.
A cricket/saddle diverts water around the chimney rather than allowing it to pond against the masonry. NRCA recommends crickets where significant water, snow, ice or debris is expected, and its architectural flashing guidance recommends them for chimneys around 24 inches wide or greater.
The IRC has a somewhat different threshold: for residential roofs, it requires a cricket or saddle on the ridge side of a chimney/penetration more than 30 inches wide, measured perpendicular to the roof slope.
So the roofer should check both the applicable code and the roofing manufacturer's requirements, rather than assuming one universal dimension.
5. Properly waterproof the roof deck around the chimney
Before installing the metal, the roofer should integrate the chimney flashing with the roof's underlayment/water-control layer.
A good assembly essentially works from the roof deck outward:
The Department of Energy's Building Science Education guidance describes the chimney transition as connecting the roof's water-control layer to the chimney's water-control layer, using base flashing, step flashing and counterflashing as the transition.
In cold climates, an ice-barrier membrane around the chimney can be particularly important.
6. Don't put fasteners where they create leak paths
Fastener placement matters. Ideally, flashing is secured where the fastener will subsequently be protected by the overlapping roofing or flashing.
Avoid the classic mistake of putting exposed screws through horizontal flashing surfaces and then assuming a blob of roofing cement makes the hole permanently waterproof.
Likewise, don't depend on roofing cement as a substitute for properly lapped metal.
7. Treat the corners as critical details
The four chimney corners are particularly vulnerable.
A good installer will form the apron, side flashing and rear flashing so that water is directed around the chimney, not into a corner where multiple pieces terminate.
Where appropriate, corners can be folded, hemmed, soldered/brazed, or otherwise mechanically detailed according to the metal and system being used. The goal is a continuous drainage path rather than a collection of butt joints covered with caulk. Fine Homebuilding's chimney-flashing details similarly emphasize mechanically connected counterflashing and carefully formed corners.
8. Use compatible metals
The flashing material needs to be corrosion-resistant and compatible with the chimney and other metals in the assembly. The 2024 IRC specifies corrosion-resistant metal for applicable flashing and gives minimum thickness requirements in several roof-flashing applications.
Copper, stainless steel, galvanized steel and other materials can all be appropriate depending on the roof, climate, masonry and design. Avoid creating galvanic-corrosion problems by mixing incompatible metals.
9. Inspect the masonry itself
A roofer can install beautiful flashing and still have a chimney leak if the masonry is absorbing water.
Check for:
Cracked or deteriorated mortar joints
Cracked bricks
Deteriorated chimney crown
Missing or damaged crown overhang/drip edge
Open mortar joints around counterflashing
Failed sealant
Missing or blocked weep/drainage provisions where applicable
If the chimney itself is admitting water, replacing roof flashing alone won't necessarily solve the leak.
The key principle
Think of the installation as "water always has somewhere to go." Each upper layer should overlap the lower layer so gravity takes water toward the roof surface, rather than forcing water to depend on caulk.
A particularly robust chimney-flashing assembly is therefore:
Cricket/backer → rear flashing → side step flashing → front apron → properly integrated underlayment → masonry counterflashing, with all laps and corners arranged to shed water continuously.
And for an existing leaking chimney, I'd recommend removing enough of the old flashing to see how the original system was constructed rather than simply applying new sealant over the old installation.
If you're evaluating a roofer's work, I can also give you a 10-point chimney-flashing inspection checklist showing exactly what to look for before the shingles go back on.
Install Ice and Water Shield First: Apply a self-adhering waterproof membrane around the base of the chimney, running it 2 to 3 inches up the vertical brick face and extending it generously onto the roof deck beneath the shingles.
Layer Step Flashing Interlocked with Shingles: Use individual 90-degree metal step-flashing pieces for every single course of shingles that meets the chimney. Each piece must overlap the one below it and extend at least 3 to 4 inches out onto the shingle and up the vertical wall.
Build a Cricket for Wide Chimneys: Construct a saddle or cricket on the upslope side of any chimney that is 30 inches wide or more . This diverts rain, melting snow, and debris safely around both sides of the masonry stack instead of letting standing water degrade the back flashing.
Cut and Tuck Counter-Flashing into Mortar Joints: Never surface-mount counter-flashing or rely solely on caulk for the upper seal. Grind a 3/8-inch deep groove into the mortar joints, insert the top edge of the custom metal counter-flashing, and secure it with masonry wedges or nails before sealing it with high-grade polyurethane or Vulkem sealant.
Overlap and Interlock Corners: Ensure proper sequencing where the front, back, and side flashings meet, crimping or soldering corners to maintain a continuous mechanical water barrier.
If you'd like to dive deeper, let me know:
Is the chimney made of brick, stucco, or metal?
What is the roof slope (pitch) where the chimney is located?
I can provide specific dimensions or troubleshooting advice for your exact setup.
A durable installation ties the roof’s water-control layer into the chimney flashing system:
Run self-adhered waterproof membrane (ice-and-water style membrane where appropriate) up the roof deck and onto the chimney sides.
Lap it correctly so water flows outward rather than behind the flashing.
Avoid relying on exposed caulk as the primary waterproofing layer.
3. Use step flashing correctly on the sides
The side walls of a chimney are where many leaks originate.
Best practices:
Use individual L-shaped step flashing pieces for each shingle course.
Install one piece per shingle course, weaving it with the shingles.
Each piece should overlap the piece below so water sheds downward.
Keep fasteners out of areas where water can travel; avoid unnecessary holes in the flashing.
Allow the step flashing to move independently from the chimney. Roofs and masonry expand and contract differently.
4. Install a proper front apron flashing
At the downslope side:
Use a continuous apron flashing that extends up the chimney face and out over the shingles.
Turn the corners so water cannot enter at the chimney edges.
Integrate it with the step flashing rather than simply butting pieces together.
5. Add a cricket on larger chimneys
For chimneys wide enough to collect significant runoff:
Install a properly sloped cricket (small roof behind the chimney).
The cricket diverts water around the chimney instead of allowing a pond of water and debris to build up behind it.
This is especially important in areas with heavy rain or snow.
6. Install counterflashing into the masonry
Counterflashing is what protects the upper edge of the base and step flashing.
A quality installation:
Cuts a groove (reglet) into a mortar joint.
Inserts the top edge of the counterflashing into that groove.
Secures it to the chimney, not the roof deck.
Allows the counterflashing to overlap the flashing below and shed water outward.
A common failure is simply attaching a metal strip to the chimney face and covering the top edge with caulk. That may work temporarily, but sealants eventually fail from UV exposure and movement.
7. Use compatible, durable materials
Common flashing materials include:
Copper (long life, easy to form and solder)
Lead-coated copper
Galvanized steel
Galvalume
Material choice should consider the roof material, climate, and compatibility with masonry. Some metals can have compatibility problems with certain environments.
8. Pay special attention to corners
The four chimney corners are frequent leak points because several planes meet.
Good techniques include:
Properly folded and layered corner pieces
Soldered or carefully sealed seams where appropriate
Avoiding exposed cuts or gaps that depend only on caulk
Using formed corner details rather than shortcuts
Common mistakes that cause chimney leaks
❌ Installing only a metal collar around the chimney
❌ Using roofing cement as the main waterproofing method
❌ Reusing old flashing during a reroof
❌ Nailing through horizontal flashing surfaces unnecessarily
❌ Skipping counterflashing
❌ Relying on caulk instead of mechanical overlaps
❌ Failing to install a cricket where one is needed
A properly flashed chimney should have visible layered metal details, with water moving from the chimney → counterflashing → step/apron flashing → shingles, without needing sealant to do all the work.
Grind Reglets for Counter Flashing : Rather than surface-mounting counter flashing with exposed caulk, use an angle grinder with a masonry blade to cut a horizontal groove (reglet) at least 7/8 inch deep into the mortar joints.
Insert and Secure Counter Flashing : Bend the top edge of the metal counter flashing at a 90-degree angle (about 3/4 inch) and insert it firmly into the ground mortar reglet . Secure it with lead wedges or masonry anchors, then seal the joint cleanly with high-grade polyurethane or silicone exterior sealant.
Use Kickout Flashing at the Bottom Corners : Place a kickout (diverter) flashing precisely at the bottom transition where the step flashing meets the eave/gutter. This crucial piece kicks water away from the vertical sidewall and directly into the gutter, preventing runoff from soaking the siding or corner flashing.
If you'd like to dive deeper, let me know:
Is the chimney on a shingle roof or a metal roof?
What is the approximate width of the chimney?
I can provide specific dimensions and custom-bend instructions tailored to your project.
Properly flashing a chimney requires a two-part defense system: step flashing to shed water away from the shingle line, and counter flashing embedded into the masonry to cap the system . Relying on caulk alone is a recipe for future failure; true waterproofing relies on overlapping metal and mechanical bonds.
Best Techniques for Chimney Flashing
Install a Waterproof Underlayment Base
Before any metal touches the deck, wrap the base of the chimney with a self-adhering high-temperature ice and water shield.
Run the membrane at least 3~4 inches up the vertical walls of the chimney and out onto the roof deck by 6~8 inches, ensuring a seamless secondary barrier.
Layer Individual Step Flashing Pieces
As you lay each course of shingles up the side of the chimney, interweave individual, factory-bent 90° Amerimax Step Flashing pieces.
Each L-shaped step flashing piece should overlap the shingle below it, extend about 4 inches up the chimney, and extend 4 inches out onto the roof deck. Secure each piece to the roof deck with a single nail at the top edge (never nail directly into the chimney brick).[2]
Build a Custom Cricket (for wide chimneys)
If the chimney is wider than 30 inches and sits on a sloped roof, construct a roof cricket (a small peaked saddle) on the upslope side.
This diverts standing water, snow, and debris around the masonry rather than letting a massive dead-valley pool against the back wall.
Cut and Anchor Counter Flashing into Mortar Joints
Never surface-mount counter flashing with just sealant. Use an angle grinder to cut a horizontal reglet (a 3~4 inch deep groove) into the mortar joints of the brick, roughly 1~2 inches above the step flashing line.
Insert the top edge of the custom-bent counter flashing into the mortar reglet and secure it with masonry wedge anchors or lead plugs.
Seal the Reglet Correctly
After mechanically locking the counter flashing into the brick joint, point up or seal the cut reglet groove using a high-grade polyurethane or specialized exterior masonry sealant (like NP1 or Vulcum) to prevent wind-driven rain from sneaking behind the metal.
Ensure the lower skirt of the counter flashing overlaps the vertical rise of your step flashing by at least 3 inches, completely shielding it from direct exposure.
If you're tackling a specific project, let me know:
Is the chimney brick, stucco, or stone?
Does it sit on the ridge or down the slope of the roof?
I can give you more tailored advice on whether you need a cricket or custom fabrication tips.
NRCA emphasizes that flashing failures are commonly caused by poor design or installation details rather than the flashing material itself.
Install the underlayment correctly
Run compatible self-adhered membrane or other specified underlayment around the chimney in accordance with the roofing-system manufacturer's detail.
Turn it up the chimney and integrate it with the surrounding roof underlayment.
Pay particular attention to the upslope corners, where water can get trapped.
Use step flashing along the sides
On a shingle roof, use individual pieces of step flashing with each shingle course rather than relying on one continuous L-shaped piece.
Each piece should be integrated with the shingle course so water flows from one layer onto the next and ultimately onto the roof surface.
NRCA specifically recommends step flashing for asphalt-shingle, slate and wood roof systems because it keeps water on top of the roofing rather than directing it underneath the underlayment.
Build a proper front/apron flashing
At the downslope face of the chimney, use an apron/base flashing arrangement that catches water coming down the roof.
The apron should extend far enough onto the roof to discharge water safely over the shingles.
Avoid relying on sealant as the primary waterproofing mechanism.
Give the back of the chimney special attention
The upslope side is often the most critical area because water accumulates against the chimney.
For a sufficiently wide chimney, install a properly designed cricket/saddle that splits and redirects water around the chimney.
A cricket is particularly important on larger chimneys or where the roof geometry creates a substantial drainage area above the chimney.
Install counterflashing over the base/step flashing
The masonry-side counterflashing should overlap and protect the exposed top of the base flashing.
Ideally, counterflashing is mechanically integrated into the masonry rather than simply caulked to its face.
NRCA distinguishes between base flashing and counterflashing and recommends a two-piece approach that permits the counterflashing to be installed after the base flashing and maintained without damaging the roof system.
Avoid the “saw-cut and caulk” shortcut
A common failure is cutting a shallow groove into the mortar and stuffing sheet metal into it, then relying on sealant.
NRCA has specifically warned that this type of saw-cut flashing can be essentially decorative rather than functional when the underlying wall-flashing/drainage detail is missing.
Where the chimney construction permits it, properly integrated through-wall flashing provides a much more robust drainage path. NRCA describes through-wall flashing as the appropriate long-term correction for certain chimney/wall flashing failures.
Pay attention to corners and end dams
The front corners and especially the rear corners of the chimney are frequent leak points.
Form the metal so water cannot simply run around the end of the flashing.
Properly close and seal end dams and transitions rather than depending on exposed caulk joints.
Use compatible, durable metal
Copper, stainless steel, aluminum, galvanized steel, or another appropriate flashing metal can work, but compatibility with the chimney materials, roofing, sealants and expected service life matters.
Don't mix incompatible metals in a way that promotes galvanic corrosion.
Flashing thickness and material should be appropriate for the expected life of the roof system. NRCA, for example, recommends appropriately sized metal step flashing rather than treating flashing as a disposable accessory.
Make the water test systematic
After installation, inspect the entire flashing assembly rather than merely flooding the chimney.
If troubleshooting an existing leak, test one section at a time, starting downslope and progressively moving upslope.
Remember that water can travel surprisingly far underneath roofing and masonry before appearing inside.
The basic assembly
For a typical asphalt-shingle chimney, think of the layers roughly as:
roof drainage → cricket/saddle where appropriate → flashing → around the chimney → downslope discharge
The fundamental rule is: every upper layer should shed onto the layer below it, and no exposed joint should require caulk to remain watertight.
For professional work, I'd also follow the current NRCA Roofing Manual and the specific shingle manufacturer's flashing detail rather than relying on a generic dimension. NRCA's current manual series includes dedicated architectural-metal and steep-slope guidance, and NRCA notes that its details need to be adapted to the actual roof/deck/wall construction.
If you tell me whether you're dealing with asphalt shingles, metal roofing, tile, or slate, I can give you a detailed chimney flashing sequence with a simple labeled diagram and recommended overlaps/dimensions.
Repair loose or deteriorated mortar before flashing.
Replace rotten roof decking around the chimney.
Don't try to compensate for structural/masonry problems with sealant.
Install an underlayment/leak barrier first
Run the roof underlayment properly into the chimney intersection.
A self-adhered leak barrier at the chimney is good secondary protection, but it doesn't replace metal flashing. GAF's chimney detail, for example, calls for leak barrier extending at least 5 inches up the wall.
Use individual step flashing along the sides
Install one piece of step flashing with each shingle course.
Each piece should turn up the chimney and extend sufficiently onto the roof.
Nail high enough that the next piece of flashing/shingle covers the fastener. Avoid exposed fasteners wherever possible.
Build a proper front/apron flashing
At the downslope side of the chimney, use appropriately sized continuous base/apron flashing to direct water around the chimney and back onto the shingles.
Don't simply bend a piece of thin L-metal around the chimney and rely on caulk. GAF specifically warns against using simple "L" metal in these transition areas.
Install true counterflashing
The counterflashing goes over the top of the step/apron flashing and is secured to the chimney.
Ideally, cut/prepare the masonry joint so the counterflashing is mechanically integrated into the chimney rather than merely surface-caulked.
Keep the counterflashing tight to the chimney and overlap the base/step flashing by at least 2 inches according to GAF's current field guide.
Pay special attention to the upslope/back side
This is often the most important part of the detail because water accumulates against the chimney.
For larger chimneys or appropriate roof configurations, install a cricket that diverts water around the chimney rather than allowing a large volume of water to run directly into it.
GAF currently recommends crickets for chimneys wider than 24 inches, roofs steeper than 6:12, and all chimneys in northern climates.
Use a backer flashing when a cricket isn't required
A properly formed backer flashing should extend up the roof and behind the chimney, with its dimensions tailored to the chimney.
GAF's detail calls for the backer flashing to be at least 6 inches wider than the chimney and extend at least 18 inches up the roof plane.
Don't make caulk the waterproofing system
Sealants have a role at appropriate joints, but they shouldn't be the primary defense against water.
Flashing should physically direct water away; caulk is supplementary. GAF specifically identifies "just use caulk" approaches as a common chimney-flashing failure.
Use compatible, corrosion-resistant metal
Select flashing material appropriate to the roofing system and compatible with adjacent metals/masonry.
Avoid mixing metals that can cause galvanic corrosion.
Step flashing should be non-corroding and appropriately sized for the shingle exposure.
Integrate flashing with the shingle courses
The sequence matters: underlayment → flashing → shingle → next step flashing → next shingle, working progressively up the roof.
Don't install all the step flashing on top of the completed shingles as an afterthought.
A good mental model
Think of the assembly like this:
Roof water → shingles → step/apron flashing → counterflashing → downhill and away from chimney
The base flashing catches water, while the counterflashing protects the top edge of the base flashing. Neither component should depend primarily on exposed caulk to remain watertight.
For a roofer evaluating another contractor's work, the biggest red flags are exposed fasteners, missing step flashing, counterflashing merely caulked to the brick, a flat/poorly detailed back side of the chimney, and large amounts of sealant being used instead of properly interlocked metal.
If you're interested, I can also give you a proper chimney-flashing installation sequence for an asphalt-shingle roof, including typical dimensions, nail placement, and the exact order each piece goes in.
Build a Chimney Cricket for Wide Chimneys : Construct a small, pitched saddle (cricket) behind any chimney that is 30 inches wide or more . The cricket diverts rain, melting snow, and debris around the sides of the chimney rather than letting it pool and stress the uphill joint.
Interlock L-Shaped Step Flashing : Install individual pieces of step flashing (90-degree bent metal) interwoven with each course of shingles. Each piece must extend at least 4 inches out onto the roof deck and turn up the chimney wall, overlapping the piece below it by at least 3 inches.
Cut Reglets for Counter Flashing : Never surface-mount counter flashing with just adhesive. Grind a 3/8-inch deep horizontal groove (reglet) into the mortar joint of the chimney, insert the top lip of the custom counter flashing into the joint, and secure it with masonry wedges or a bead of high-grade polyurethane sealant.
Use Kick-Out Flashing at the Bottom : Place kick-out (diverter) flashings at the bottom lower corners where the roof edge meets the chimney. This forces run-off water away from the vertical sidewall and directly into the gutter instead of behind the siding or wall face.
If you're working on a specific project, let me know:
What is the chimney's width and roof pitch?
Is the chimney made of brick, stucco, or siding?
I can provide tailored sizing guidelines or troubleshoot an active leak issue for you.
