What are the best techniques for a roofer to properly flash a chimney to prevent leaks?

A properly flashed chimney is less about caulk and more about creating a layered water-shedding system that lets the roof and masonry move independently. The most reliable installations use base flashing, step flashing, counterflashing, and (when needed) a cricket.

1. Start with a sound chimney and roof deck

Before installing metal:

• Repair cracked mortar joints, deteriorated brick, or loose chimney caps.
• Replace rotted roof sheathing around the penetration.
• Remove old failed flashing rather than covering it with new metal.
• Use a compatible high-quality underlayment or self-adhered membrane around the chimney corners and uphill side.

2. Install the base flashing system correctly

A good chimney flashing assembly has several parts:

• Apron flashing (downslope side): A continuous piece of metal that runs across the bottom of the chimney and turns up the face.
• Step flashing (sides): Individual L-shaped pieces installed one per shingle course, woven into the shingles as the roof is installed. Each piece should shed water onto the shingle below.

Best practices:

• Use properly sized step flashing (many professionals prefer larger pieces for better coverage).
• Keep fasteners out of areas where water can enter. Fasten only where the next layer covers the penetration.
• Do not rely on exposed screws and caulk as the primary waterproofing method.

3. Install counterflashing into the chimney

This is one of the biggest differences between a temporary repair and a durable installation.

Proper counterflashing:

• Is cut into a mortar joint or reglet in the chimney.
• Is embedded mechanically into the masonry.
• Overlaps the step flashing below.
• Allows the chimney and roof to move independently.

Avoid simply fastening a metal strip to the chimney face and smearing the top edge with caulk; that commonly fails as the masonry and roof move at different rates.

4. Build a cricket on the uphill side when needed

A chimney sitting below the roof ridge can trap water, leaves, and snow on its uphill side. A properly framed and flashed cricket diverts water around the chimney instead of letting it pond against the masonry.

A good cricket:

• Has proper slope.
• Is covered with compatible underlayment.
• Has metal flashing integrated with the roof system.
• Directs runoff away from the chimney corners.

5. Pay attention to corners

The front corners and uphill corners are common leak points.

Good techniques:

• Fold or solder metal transitions rather than leaving open cuts.
• Overlap pieces in the direction water flows.
• Use compatible sealants only as secondary protection, not as the main waterproofing layer.

6. Choose appropriate flashing materials

Common choices include:

• Copper (long life, easy to form and solder)
• Lead-coated copper
• Galvanized steel
• Stainless steel in harsh environments

Avoid mixing incompatible metals that can cause corrosion, and be cautious with aluminum against masonry.

7. Final inspection points

Before leaving the job, verify:

• Water has a clear path around the chimney.
• Counterflashing is tight and mechanically secured.
• No exposed fasteners are in drainage paths.
• Step flashing is woven correctly with shingles.
• Corners are sealed and folded properly.
• A hose test (performed carefully from low to high areas) does not reveal leaks.

The key principle: the metal should shed water by design; sealants should only reinforce the assembly, not substitute for proper flashing geometry.

Proper chimney flashing is less about piling on sealant and more about creating a layered drainage system that sheds water even after sealants age or masonry moves.

The best-practice approach

1. Inspect the chimney and roof first

   • Replace damaged/rotted roof decking before flashing.
   • Check mortar joints, cracked bricks, chimney crown, and existing flashing.
   • Remove old flashing rather than simply covering it.
   • Make sure the chimney is structurally sound and the masonry joints are suitable for receiving counterflashing.

2. Use a multi-piece flashing system

   A durable chimney detail generally consists of:

   • Base/apron flashing at the downslope face.
   • Step flashing along both sides, interwoven with each shingle course.
   • Back-pan/cricket flashing at the upslope side.
   • Counterflashing over the base and step flashing.

   Building Science describes this as the basic transition between the roof's water-control layer and the chimney, with step flashing integrated into the shingles and counterflashing overlapping the underlying flashing. bsesc.energy.govwww.nrca.net

3. Install step flashing one course at a time

   Don't use one long piece of flashing simply caulked to the chimney. Individual step-flashing pieces should be integrated with each shingle course so water always flows over the next layer rather than behind it. NRCA specifically identifies individual pieces at each shingle course as the standard step-flashing approach.

4. Make the upslope side the priority

   Water tends to accumulate and get driven against the chimney on the high side. A properly constructed back pan or cricket should collect that water and divert it around the chimney.

   In Florida, this is particularly important. The 2023 Florida Residential Code requires a cricket when the chimney is more than 30 inches wide parallel to the ridge and doesn't intersect the ridge.

5. Use proper counterflashing

   This is where many otherwise good chimney jobs fail. Counterflashing should be mechanically integrated with the masonry rather than relying on a bead of caulk at the chimney/metal interface.

   For masonry, a common high-quality detail is to cut a reglet into a mortar joint, insert the counterflashing, mechanically secure it, and seal the joint appropriately. IIBEC recommends a reglet roughly 1 inch minimum, with 1½ inches preferred, and describes bending the flashing to create a capillary break and a hemmed drip edge.

6. Don't rely on a shallow saw cut

   This is an important professional distinction. Simply cutting a shallow slot into the face of a brick and stuffing metal into it can look like proper counterflashing without actually connecting to the masonry's drainage plane. NRCA and Building Science both caution that conventional saw-cut counterflashing can be inadequate where water can penetrate behind the outer masonry.

   Where the chimney is being rebuilt, through-wall flashing integrated into the masonry is the more robust solution.

7. Keep dissimilar metals compatible

   Use flashing compatible with the roofing and masonry materials. Avoid creating galvanic corrosion by putting incompatible metals directly together. For a high-quality job, the roofer should follow the flashing manufacturer's compatibility requirements.

8. Use sealant as secondary protection—not the primary waterproofing

   Sealant should seal joints, reglets, laps, and appropriate transitions, but the geometry of the flashing should shed water without depending on caulk. Florida's roofing standard, for example, specifies properly detailed counterflashing joints and sealant rather than treating sealant as the entire flashing system.

A detail I'd consider a "red flag"

If a roofer says, "We'll put some flashing around it and seal the edges with roofing cement," I'd want considerably more detail before approving the work.

A good chimney-flashing job should have a clear answer for where water entering at the top of the chimney intersection goes, how it gets around the sides, and how the counterflashing remains effective after sealant eventually deteriorates.

For a reroof, I'd specifically ask the roofer to show you their planned side step-flashing, upslope cricket/back-pan, apron flashing, and masonry counterflashing/reglet details before the shingles go down.

To properly flash a chimney and guarantee a leak-free seal, a roofer must use a two-part flashing system layered from the bottom up . This method ensures that the roof and the chimney chimney can move independently without tearing the metal, while forcing water to shed like an umbrella over the shingles. Relying solely on caulk, roofing cement, or single-layered metal will eventually lead to failure.

The industry-best techniques and installation sequences are outlined below.

1. Preparation and Deck-Level Waterproofing

Never install metal flashing directly over bare wood or standard felt paper.

• Strip to the Deck: Remove all old flashing, shingles, and rotted wood to create a clean, nailable surface.
• Apply Ice & Water Shield: Install a self-adhering, rubberized membrane around the perimeter. Bring it at least 6 inches up the chimney walls and 12 inches out onto the roof deck. This acts as a secondary waterproof barrier if water ever gets past the metal.

2. The Bottom-Up Installation Sequence

Flashing must be integrated with the shingles row-by-row, starting from the lowest down-slope point.

[ Chimney Brick ]
              || 
       +------|| <--- Counter-flashing (Cut into Reglet)

       |      ||
  +----|--+   || <--- Step Flashing (Weaved with Shingles)

  | Shingle | ||
================= <--- Roof Deck & Ice/Water Shield

• Install the Apron (Base) Flashing: Place a single wide piece of metal at the bottom base of the chimney. It sits on top of the shingles on the downslope side and extends up the chimney face. Secure it with clips or nails high enough up so you are not piercing the lower roof deck.
• Weave Step Flashing on the Sides: As shingles are laid up the sides, place an L-shaped piece of step flashing with every single course of shingles.
  • Each step piece must overlap the lower one by at least 2 to 3 inches.
  • Nail the flashing only to the roof deck, never to the masonry. If nailed to both, structural settling or thermal expansion will tear the metal apart.
• Install a Cricket (Saddle) on the High Side: If the chimney is wider than 30 inches, code requires building a small peaked roof structure (a cricket) on the upslope side. This prevents water and snow from pooling behind the chimney and diverts it around the sides.

3. Proper Counter-Flashing (The Reglet Cut)

Counter-flashing covers the top of the step flashing to prevent water from running down the brick face.

• The Right Way (Reglet/Mortar-Cut): Use a diamond-blade grinder to cut a 1 to 1.5-inch deep groove (a reglet) directly into the mortar joints or brick. Bend the top edge of the counter-flashing metal at a 90-degree angle and insert it firmly into this groove.
• The Wrong Way (Surface Mounting): Never simply surface-mount metal against the brick and seal the top edge with caulk. As the house expands and contracts, the caulk will pull away from the brick within a few seasons, creating an active leak path.
• Lock and Seal: Fasten the metal inside the reglet using lead wedges, then fill the remaining groove completely with high-grade, UV-resistant polyurethane or silicone masonry sealant.

4. Critical Water-Diverting Details

• Incorporate Kick-out Flashing: At the very bottom corners where the chimney meets the roof edge, bend the final piece of flashing outward to form a "kick-out". This aggressively forces streaming water away from the wall siding and onto the open roof or into the gutter.
• Hem the Edges: For premium performance, fold the bottom 1/2 inch of the counter-flashing under itself 180 degrees to form a hem. This strengthens the metal edge, keeps it straight, and stops wind from blowing water up under it.

If you are currently evaluating a specific chimney layout, tell me:

• What roofing material are you using (e.g., architectural shingles, metal, slate)?
• Is the chimney made of brick/masonry or is it wood-framed with siding?
• Is the roof steep or low-slope?

I can provide custom tail-end bending dimensions or specific material pairing recommendations (like copper vs. aluminum) based on your project.

Flashing a chimney correctly requires a multi-layered, water-shedding system of metal step flashing and counter flashing. Here are the essential techniques and sequential steps to ensure a watertight seal:

1. Clean and Prepare the Surface

• Remove old material : Tear away all old flashing, mortar, and roofing felt around the chimney base down to the bare roof deck and masonry.
• Inspect the masonry : Check for cracks or loose bricks and repoint the mortar joints if necessary before installing any metal.

1. Install the Ice and Water Shield (Underlayment)

• Wrap the base : Apply a self-adhering polymer modified bitumen membrane (ice and water shield) extending at least 6 inches up the chimney wall and 12 inches out onto the roof deck.
• Seamless corners : Ensure corners are folded and sealed without cutting shortcuts that could let water track underneath.

1. Layer the Step Flashing (Step-by-Step)

• Cut individual pieces : Use L-shaped individual step flashing pieces rather than one continuous long piece. Each piece should be roughly 5 inches by 7 inches with a 90-degree bend.
• Integrate with shingles : As each course of shingles is installed up the side of the chimney, place a piece of step flashing over the shingle and nail it to the roof deck (never nail it directly into the vertical brick wall).
• Overlap correctly : Each upper step flashing piece must overlap the one below it by at least 3 inches.

1. Install the Front Apron Flashing

• At the low side (downslope) : Install a continuous apron piece at the bottom (downslope side) of the chimney over the underlayment and underlying shingles, extending up the face of the chimney and overlapping the bottom row of shingles.

1. Install the Cricket (for wide chimneys)

• Divert water : If the chimney is wider than 30 inches, build a framed and metal-clad cricket (saddle) on the upslope side to actively divert water and snow around the obstruction rather than letting it pool behind the flat brick wall.

1. Cut Reglet and Install Counter Flashing

• Grind a reglet : Use a masonry grinder to cut a horizontal groove (reglet) about 1 4 one-fourth 1 4 to 1 2 i n c h deep into the mortar joints, roughly 1 to 2 inches above the top edge of the step flashing.
• Insert counter flashing : Insert the top edge of the metal counter flashing into the regletted groove and bend it down to overlap the step flashing by at least 3 inches.
• Seal the joint : Fill the reglet groove with high-grade polyurethane or silicone masonry sealant to lock out driving rain.

If you'd like, I can also share:

• Common mistakes roofers make that cause premature leaks around chimneys
• The best choice of metals (copper vs. aluminum vs. galvanized steel) for longevity

Let me know how you want to proceed.

The most reliable chimney flashing is a layered drainage system, not a bead of caulk. For a roofer working on a typical asphalt-shingle roof, the key techniques are:

1. Start with the underlayment

   • Remove enough roofing to inspect the deck and existing flashing.
   • Run the roof underlayment correctly into the chimney intersection.
   • In cold climates, use an appropriate self-adhered ice/water membrane around the chimney, integrated with the roof's water-control layer.

2. Use individual step flashing on the sides

   • Install a separate L-shaped piece with each shingle course rather than trying to make one long piece of flashing.
   • Each piece should be layered so water always flows over the flashing and onto the shingle below.
   • The step flashing should generally be substantial enough to provide adequate coverage above the roof and up the chimney; dimensions need to be appropriate to the roofing system and local requirements.

3. Install proper base/apron flashing at the bottom

   • The downslope side needs an apron/base flashing that carries water away from the chimney and onto the shingles.
   • Don't depend on sealant to make the bottom transition watertight; the metal should be shaped to drain water mechanically.

4. Build a proper head flashing at the uphill side

   • The upslope portion is particularly important because water accumulates against the chimney.
   • Ideally, install a cricket/saddle behind the chimney to split and divert water around it. NRCA recommends a cricket for chimneys 24 inches or wider.
   • The cricket needs its own properly integrated flashing and counterflashing.

5. Counterflash the masonry

   • This is one of the biggest differences between a good chimney flashing job and a temporary repair.
   • The counterflashing should cover the upper edge of the base/step flashing and be mechanically integrated into the chimney, typically by cutting into a mortar joint/reglet rather than simply fastening it to the face of the brick.
   • Seal the reglet appropriately, but don't make caulk the primary waterproofing mechanism.

6. Pay special attention to the four corners

   • Chimney corners are difficult because several planes of sheet metal intersect there.
   • Carefully formed corners, folded/hemmed edges, and designs that direct water away from corner joints can substantially reduce dependence on sealant. A newer "winged" flashing approach specifically addresses these vulnerable three-plane corners.

7. Don't rely on exposed caulk

   • Sealant is useful as secondary protection, particularly at joints and reglets.
   • It shouldn't be the thing keeping the chimney from leaking. Proper overlaps, bends, mechanical attachment, and drainage should do that job.

8. Respect metal compatibility

   • Choose flashing material appropriate for the roof, chimney masonry, fasteners, and environment. Common professional materials include galvanized steel, stainless steel, copper, and lead.
   • Avoid creating galvanic-corrosion problems by combining incompatible metals.

The basic water-shedding sequence

Think of the assembly as:

roof deck → membrane/underlayment → base flashing → step flashing integrated with shingles → counterflashing integrated with masonry

Every upper layer should shed water onto, rather than behind, the layer below. The U.S. Department of Energy's Building America guidance describes the same basic three-part system—base, step, and counterflashing—with the elements layered to drain incident rainwater.

Biggest red flags: one-piece flashing simply caulked to the chimney, step flashing missing between shingle courses, counterflashing screwed to the brick face without a proper masonry joint/reglet, no cricket behind a wide chimney, exposed fasteners in drainage paths, and relying on roofing cement as the primary waterproofing.

If you're evaluating a roofer's work, the counterflashing and the uphill cricket/head-flashing detail are probably the two areas I'd inspect most carefully.

For a roofer, the key is to treat chimney flashing as a layered water-management system, not as a bead of caulk around the chimney. The National Roofing Contractors Association (NRCA) emphasizes that roof-to-vertical-surface intersections are among the most vulnerable areas of a roof.

Best-practice chimney flashing

1. Use a two-part base-and-counter-flashing system

   • Base flashing is integrated with the roof covering and directs water away from the chimney.
   • Counterflashing is installed into/against the masonry and covers the upper edge of the base flashing.
   • The two layers should remain independent enough to accommodate movement between the masonry and roof.

2. Install step flashing along the sides For an asphalt-shingle roof, use individual step-flashing pieces at each shingle course rather than relying on one long L-shaped piece. Each piece should be properly integrated with the shingles and underlayment so water is directed onto the roof surface. NRCA specifically recommends step flashing for this type of intersection.

3. Build the front/apron flashing correctly At the downslope face of the chimney, the apron flashing should extend sufficiently onto the roof and be integrated with the shingle courses. Avoid simply laying metal over the finished shingles and caulking the edges.

4. Pay special attention to the back of the chimney This is frequently the most troublesome area because water accumulates where the chimney meets the upslope roof. Ideally, install a proper backer flashing and a cricket/saddle where required by the roof/chimney configuration. GAF's current field guide, for example, recommends crickets for certain larger, steep-slope, and northern-climate chimney applications.

5. Use counterflashing that actually engages the masonry The upper flashing shouldn't merely be surface-mounted and caulked to the brick. Where masonry construction permits, the counterflashing should be properly integrated into the masonry/weather-resistive system. NRCA cautions that superficial "saw-cut" flashing can be largely ineffective compared with functional through-wall flashing.

6. Maintain positive laps Every layer should shed water onto the layer below it. As one manufacturer example, GAF specifies at least a 2-inch overlap of counterflashing over step flashing in its chimney-flashing detail. Follow the specific roofing manufacturer's requirements rather than applying a universal dimension to every roof system.

7. Don't make sealant the primary waterproofing Sealant is useful as a supplemental weather seal, but it shouldn't be what's keeping the chimney dry. Proper metal geometry, overlaps, drainage paths, and masonry integration should do the primary work. NRCA notes that sealant joints can be particularly vulnerable when used as the fundamental solution at masonry flashing interfaces.

8. Use compatible, durable metal Select flashing material appropriate for the roof covering and masonry, and avoid incompatible metals that can cause corrosion. The flashing's expected service life should be compatible with the roof system.

A good installation sequence

For a typical asphalt-shingle chimney, the sequence is roughly:

roof deck → underlayment → apron/base flashing → step flashing integrated with each shingle course → backer/cricket flashing at the uphill side → shingles → counterflashing integrated with chimney masonry

The important principle is that water should always have a continuous downhill path to the roof surface, with each upper layer overlapping the layer below.

Common mistakes that cause chimney leaks

• Caulking the top edge of flashing instead of properly counterflashing it
• Using one continuous piece of side flashing instead of properly installed step flashing
• Neglecting the uphill/back side of the chimney
• Cutting a shallow groove in mortar and relying entirely on sealant
• Driving fasteners through exposed portions of flashing where they can become leak paths
• Failing to integrate flashing with the underlayment
• Reusing deteriorated or corroded flashing during a reroof
• Installing a cricket/backer detail incorrectly or omitting one where the roof geometry calls for it

For a professional roofer, I'd also have the installer follow the particular shingle manufacturer's flashing detail and the applicable local code rather than relying on a generic chimney-flashing recipe. The NRCA's current architectural-metal-flashing manual was updated in 2026 and contains detailed construction guidance.

If you tell me whether this is asphalt shingles, metal, slate, or tile, I can give you a step-by-step chimney flashing detail—including recommended dimensions, overlap sequence, and where each fastener/sealant goes.

Properly flashing a chimney requires a two-part system —base/step flashing and counter flashing—working in tandem with a reliable underlayment and diversion strategy. Relying on surface caulk alone is a recipe for a future leak.

Essential Chimney Flashing Techniques

1. Install Ice and Water Shield First
   • Before any metal touches the deck, wrap the base of the chimney with a self-adhering rubberized ice and water membrane. Extend it a minimum of 4 to 6 inches up the vertical masonry walls and out onto the roof deck to create a fully sealed waterproof pan.
2. Weave Step Flashing Up the Sides
   • Use individual 90-degree bent L-shaped metal step flashing pieces. Interweave a single piece of step flashing with every single course of shingles, making sure each piece overlaps the one below it by at least 2 to 3 inches. Fasten only to the roof deck—never nail step flashing directly into the vertical brick face , as the roof and masonry will expand, contract, and settle at different rates.
3. Install a Front Apron and Rear Cricket (Saddle)
   • Front (Downslope): Install a continuous custom apron pan at the low side of the chimney that overlaps the top course of the transitioning shingles.
   • Back (Upslope): If the chimney is wide (typically wider than 30 inches), build a cricket or saddle behind it. A cricket diverts rainwater and snowmelt around the obstruction so it doesn't pool and compromise the upslope joint. Cover the cricket with its own layer of ice and water shield and step flashing.
4. Grind and Tuck Counter Flashing (Cap Flashing)
   • Never surface-mount counter flashing with just caulk. Use an angle grinder with a masonry blade to cut a horizontal groove (reglet) about 1 inch deep into the mortar joints of the chimney, roughly 2 to 3 inches above the step flashing line.
   • Insert the top edge of the bent L-shaped counter flashing into the mortar reglet, secure it with lead or masonry plugs/wedge keys, and step-overlap the pieces vertically down the rake of the roof.
5. Seal the Reglet Joint Properly
   • Fill the masonry groove above the counter flashing using a high-grade exterior urethane sealant (such as Vulkem/NP1) or repoint it cleanly with mortar to lock the counter flashing permanently into a mechanical and chemical seal. The counter flashing will hang down over the step flashing, allowing the roof to move independently while keeping water completely out of the joint.

If you're tackling a specific project, let me know:

• Is the chimney brick, stucco, or stone?
• What is the roof pitch and position of the chimney (upslope vs. middle of the roof)?

I can provide tailored dimensions or advice on material choices (like copper vs. aluminum).

Proper chimney flashing is a layered water-management system, not just metal and caulk. The goal is to direct water back onto the roofing while allowing the roof and chimney to move independently without tearing the flashing.

Here are the key techniques professional roofers use to prevent leaks:

1. Install an ice-and-water membrane first

• Extend a self-adhering waterproof membrane several inches up the chimney and onto the roof deck before any metal flashing.
• This acts as the secondary defense if wind-driven rain or ice dams force water beneath the shingles.

2. Use individual step flashing pieces

• Install one L-shaped step flashing piece for every course of shingles.
• Each piece overlaps the one below so water continually sheds downhill.
• Never rely on one long strip of flashing along the side of the chimney, as it doesn't move well with the roof and is more prone to leaks.

3. Separate base flashing from counterflashing This is one of the most important principles.

• Base/step flashing is attached only to the roof.
• Counterflashing is attached only to the chimney.

The two pieces overlap but are not fastened together. This allows the roof framing and masonry chimney to expand and settle independently without breaking the seal.

4. Cut counterflashing into the masonry For brick or masonry chimneys:

• Grind or cut a reglet (groove) into a mortar joint.
• Insert the counterflashing into the groove.
• Secure and seal the groove with an appropriate masonry-compatible sealant.

Surface-mounted flashing sealed only with caulk generally has a shorter service life than reglet-installed counterflashing.

5. Install an apron flashing on the downslope side At the front (downslope) of the chimney:

• Use one continuous apron flashing.
• Extend it far enough onto the shingles to shed water effectively.
• The side step flashing should overlap the apron at the corners.

6. Build a cricket behind wider chimneys A chimney on the uphill side of the roof can trap water and debris.

Best practice is to install a cricket (also called a saddle) behind chimneys approximately 30 inches (760 mm) or wider, and many building codes require one at that size. The cricket splits water and snow around the chimney instead of allowing it to pool against the back wall.

7. Pay close attention to corners Most flashing failures occur at inside and outside corners. Professionals:

• Fold metal instead of making unnecessary cuts.
• Use properly formed corner pieces.
• Minimize exposed seams.
• Avoid depending solely on sealant at critical joints.

8. Don't depend on caulk as the waterproofing Sealant is a supplement—not the primary defense. A properly flashed chimney should remain watertight because:

• every layer overlaps the one below,
• gravity directs water back onto the roof,
• and exposed sealant is minimized.

Common mistakes that lead to leaks

• Continuous side flashing instead of step flashing
• Counterflashing merely caulked to the brick surface
• No cricket behind a wide chimney
• Flashing nailed to both the chimney and roof
• Reusing old flashing during a roof replacement when it's deteriorated
• Heavy reliance on roofing cement or caulk as the primary waterproofing

A properly flashed chimney should function as a layered drainage system: underlayment beneath, step flashing integrated with each shingle course, a correctly lapped apron and back pan (or cricket), and counterflashing embedded into the chimney. When those layers are installed in the correct order, water is directed harmlessly off the roof rather than into it.

Properly flashing a chimney requires a two-part system (base/step flashing and counter flashing) designed to let the roof and masonry move independently during thermal expansion or home settling without breaking the water seal.

For a visual walkthrough of how these metal components integrate with the roof deck and masonry, watch this step-by-step tutorial:

2m How to Install Brick Chimney Flashing and Counter ...340K views  · 5 years ago YouTube · RoofCo Roofing Experts

Core Techniques for Flashing a Chimney

• Layer the Underside with Ice and Water Shield: Before any metal touches the deck, install a high-temperature self-adhered rubberized underlayment. Wrap it completely around the base of the chimney, running it at least 4 to 5 inches up the vertical masonry walls and out onto the roof deck.
• Install a Front Apron Flashing: At the downslope (front) base of the chimney, install a continuous piece of apron/pan flashing. The bottom portion rests flat over the top of the finished shingles, while the vertical leg goes against the chimney.
• Weave Step Flashing Up the Sides: Use individual 90° pre-bent metal pieces (aluminum, copper, or galvanized steel) for each shingle course along the sides. Each individual step flashing piece must overlap the one below it by at least 3 inches, resting on top of a shingle and extending up the chimney wall. Fasten it to the roof deck, never to the masonry wall, to allow for natural movement.
• Build a Cricket (Saddle) on Wide Chimneys: For chimneys wider than 24 to 30 inches (depending on local building codes) located on the middle or lower slope of a roof, build a framed, metal-covered cricket upslope. This triangular "tent-like" structure diverts water and debris around the back of the chimney instead of letting it pool and compromise the rear seal.
• Cut Reglets for Counter Flashing: Never rely strictly on surface caulk against a brick wall. Use a diamond tuck-point blade on an angle grinder to cut a 1/2-inch deep horizontal groove (a reglet) into the mortar joints of the chimney. Tuck the top edge of the L-shaped counter flashing directly into this mortar slot.
• Secure and Seal the Joints: Secure the counter flashing into the reglet using masonry anchors or lead wedges, then seal the entire cut groove with a high-grade exterior polyurethane or silicone sealant designed for masonry/metal transitions. The counter flashing must overlap the top edge of your step flashing by at least 2 to 3 inches.

Roofer Experiences

“I found using sealant instead of mortar for the reglets makes sense since sealant flexes with lead and brick, while mortar can crack over time.”

“The step flashing needs to overlap and interweave to divert water away from the chimney and roof.”

Are you working with a brick, stone, or stucco chimney, and does it require a cricket/saddle based on its width and roof position? Let me know and I can give you more targeted advice!

A properly flashed chimney is less about applying sealant and more about creating a layered water-shedding system that lets the roof and masonry move independently while directing water back onto the roof surface. The most reliable chimney flashing details use base flashing (including step flashing), counterflashing, and proper underlayment integration rather than relying on caulk.

Key techniques for a roofer:

1. Start with a proper water-control layer

• Extend a self-adhered ice-and-water membrane or compatible roof underlayment up the chimney sides and behind the flashing.
• Make sure the roof-to-chimney transition is sealed before installing metal.
• Avoid depending on exposed sealant as the primary defense.

2. Install step flashing correctly on the sides

• Use individual L-shaped step flashing pieces with each shingle course.
• Each piece should overlap the one below so water naturally sheds downward.
• The horizontal leg goes under the roofing material; the vertical leg runs up the chimney.
• Do not simply run a single continuous piece of metal down the side of a chimney on a shingle roof; it can fail as materials expand and contract differently.

3. Use a proper front apron flashing

At the downslope side:

• Install an apron/base flashing that extends onto the roof enough to shed runoff.
• Integrate the corners carefully so water cannot bypass the side step flashing.
• Avoid cuts or seams that create upward-facing pockets where debris can trap water.

4. Build a back-pan or cricket on wide chimneys

For larger chimneys, especially those wider than about 30 inches (common industry practice), install a cricket/saddle on the uphill side:

• It diverts water around the chimney instead of allowing a large volume of runoff to hit the flashing.
• The cricket should be integrated with the roof membrane and flashing system.

5. Install true counterflashing

This is where many leak-prone jobs fail:

• Cut a reglet (groove) into the mortar joint or masonry.
• Embed the counterflashing into that groove.
• Let it overlap the step/base flashing.
• Keep it mechanically secured so sealant is not doing all the work.

6. Handle corners carefully

Chimney corners are common leak points:

• Folded corners, soldered seams, or properly sealed transitions are preferred.
• Avoid excessive caulk buildup as a substitute for correct metalwork.
• Make sure no fasteners penetrate exposed horizontal flashing where water can enter.

7. Use compatible metals

Good choices depend on the roof environment:

• Copper: long-lasting and easy to form/solder.
• Stainless steel: durable in harsh environments.
• Aluminum: workable but must be protected from incompatible materials.
• Galvanized steel: common but coating quality and longevity vary.

Avoid mixing metals that can create galvanic corrosion.

8. Inspect the masonry too

A perfect flashing job can still leak if:

• Mortar joints are cracked.
• The chimney crown is failing.
• Bricks are absorbing excessive water.
• The chimney cap is damaged.

Quality-control checklist before leaving the job

✅ Step flashing exists under shingles on both sides
✅ Counterflashing is cut into masonry, not just surface-caulked
✅ Uphill side has proper diversion/cricket when needed
✅ No exposed nail heads in vulnerable areas
✅ Metal laps direct water downward
✅ Sealants are secondary protection, not the primary waterproofing method

A good chimney flashing installation should still work if the sealant eventually deteriorates; the metal layering should be doing the waterproofing.