Chimney Cricket Installation Stops Water From Damming Behind Your Chimney
A chimney cricket is a small peaked roof structure built on the uphill side of a chimney. Correct chimney cricket installation frames the peak, covers it with waterproof underlayment, and ties flashing into the roof and masonry so water flows around both chimney sides instead of pooling behind it.
For a chimney that is more than 30 inches wide on the uphill side, the IRC requires a cricket. Even for smaller chimneys, one can be a smart safeguard on St. Louis roofs exposed to heavy rain, snow, and freeze-thaw cycles. A properly sized cricket should slope at least 4:12 and extend beyond the chimney corners so its valleys drain clear of the flashing.
Without that runoff path, water, leaves, and ice can collect against the chimney’s back wall. Over time, that can shorten flashing life, rot roof decking, and damage porous brick and mortar.
I am John Martin of Martin & Sons, a family-owned roofing company founded in 1953, with more than 35 years of industry experience. In the sections ahead, I will explain the key decisions behind a durable chimney cricket installation, from sizing and framing through the flashing details that keep the system watertight.
Building Code, Sizing, and Materials for a Chimney Cricket Installation
When building on a roof, building codes set the baseline for longevity and safety. According to the International Residential Code (IRC Section R1003.20), an exterior chimney cricket is required on all chimneys where the uphill-facing width is 30 inches or wider. If your masonry chimney measures 36, 48, or 60 inches across the back face, leaving that section flat against the roof plane creates an unmitigated water dam.
Roofs that have a 4:12 pitch or steeper rarely leak in the open field of shingles. Instead, water finds its way inside where flow gets blocked by wide obstructions. In our St. Louis climate—where winters in Chesterfield, St. Charles, and O’Fallon bring heavy snow, freezing rain, and constant freeze-thaw cycles—standing water behind a brick chimney saturates porous mortar joints. When that trapped water freezes, it expands, causing cracked bricks, spalling mortar, and rotted roof decking.
To prevent this damage, you need the right construction materials:
- Framing Lumber: Dimensional 2x4s and 2x6s (SPF or Douglas Fir) for the ridge, sleepers, and jack rafters.
- Decking: 1/2-inch CDX exterior-grade plywood or structural OSB.
- Underlayment: Self-adhering modified bitumen ice and water shield rated for high temperatures (up to 240°F), which ensures stability behind hot masonry walls and metal flashings.
- Metal Flashing: Corrosion-resistant 24-gauge galvanized steel, aluminum, or 16-ounce copper.
- Fasteners & Sealants: Hot-dipped galvanized framing nails, ring-shank roofing nails, lead wedges for masonry reglets, and a commercial-grade polyurethane masonry sealant.
For a deeper dive into professional carpentry techniques and layout tricks, you can check out this Make a Watertight Chimney Cricket guide from Fine Homebuilding.
Geometry, Pitch, and Overhang Calculations
A cricket needs adequate pitch to shed water, snow, and falling debris effectively. The minimum slope for a cricket’s own ridge faces is 4:12 (a 1:2 rise-to-run ratio). Anything shallower allows water to sheet sluggishly rather than channel briskly into the valleys, leaving pine needles and leaves to pile up.
When designing a cricket, a great practice is to match the pitch of the main roof slope or build it slightly wider than the actual chimney footprint. Extending the base of the cricket 3 to 6 inches beyond each side corner of the chimney allows the valleys to discharge water past the chimney corners with ample clearance rather than dumping high-volume runoff right into the sensitive corner flashing seams.
| Chimney Width (Inches) | Recommended Cricket Base Width (Inches) | Minimum Ridge Projection Upslope (Inches) | Valley Length (Approximate Inches on 6:12 Pitch) |
|---|---|---|---|
| 30″ | 36″ | 18″ | 25″ |
| 36″ | 42″ | 21″ | 29″ |
| 42″ | 48″ | 24″ | 33″ |
| 48″ | 56″ | 28″ | 39″ |
| 60″ | 66″ | 33″ | 46″ |
To calculate your layout without complicated trigonometry, measure the chimney width along the uphill face, snap a center plumb line straight up the roof slope, and mark the apex. If you set the ridge length equal to half the total cricket base width, the rafters naturally form clean, manageable geometric angles that drain water smoothly.
Step-by-Step Framing and Flashing Guide
Building a cricket on the roof while balancing on a steep slope can be awkward and dangerous. Whenever possible, seasoned roofers build the cricket frame on the ground, transport it up the ladder, and set it onto the stripped decking. This reduces open-roof exposure to sudden rain showers and ensures accurate, square cuts.
Before installing framing, familiarize yourself with proper chimney and flashing repair techniques so you understand how the underlying metal and water barriers interconnect with the chimney’s vertical surfaces.
Step 1 to 5: Framing and Decking the Chimney Cricket Installation
Follow these steps to frame, anchor, and deck the structure:
- Draw a 1:1 Layout Template: On a sheet of scrap plywood on the ground, draw a full-scale template matching the width of the chimney plus 6 inches of total overhang (3 inches on each side). Mark the centerline where the ridge board will sit.
- Cut the Gable and Sleepers: Cut a 2×6 gable backboard that will sit flush against the masonry wall. Bevel the bottom edge of your base sleeper plates to match the roof pitch so they lie flat on the roof deck without rocking.
- Set the Ridge Board and Jack Rafters: Cut a 2×6 ridge board running from the top center of the gable backboard down to the uphill apex on the roof deck. Cut 2×4 jack rafters spaced 16 inches on center with compound birdsmouth bevels to connect the ridge to the sleeper plates. Fasten everything with 3-inch structural wood screws.
- Fasten the Frame to the Deck: Hoist the open frame onto the roof deck behind the chimney. Align it squarely with your chalk lines and fasten the sleeper plates through the existing roof sheathing directly into the underlying structural rafters using 3-1/2-inch framing nails or structural screws.
- Sheath and Membrane the Cricket: Fasten 1/2-inch CDX plywood sheathing to the rafters using 8d ring-shank nails. Cover the entire cricket with a continuous layer of self-adhering modified bitumen underlayment (ice and water shield). Extend the membrane at least 6 inches up the vertical brick masonry, 6 inches past the cricket base out onto the roof deck, and 12 inches past the uphill apex.
Step 6 to 10: Flashing, Shingling, and Waterproofing the Chimney Cricket Installation
Flashing is the true waterproof barrier of the cricket system. Shingles shed water, but metal flashings divert it away from seams.
- Install the Lower Apron and Corner Flashing: Fasten the front apron flashing and wrap base corners around the lower chimney before working upward. For detailed regional building requirements, consult these detailed chimney cricket code and build specifications.
- Cut Reglet Slots in the Brick Mortar: Using an angle grinder equipped with a diamond tuckpointing blade, cut a clean horizontal groove (reglet) 1 inch deep directly into the chimney mortar joints roughly 4 to 6 inches above the cricket deck. Blow out all dust with a blower or hand brush.
- Step-Flash the Cricket Valleys and Masonry: As you lay courses of asphalt shingles up the roof deck, weave individual 4×6-inch or 8×8-inch metal step flashing pieces into every single shingle course along the cricket valleys. Each piece of step flashing must overlap the one below it by at least 2 inches.
- Shingle and Cap the Cricket Peak: Shingle the two sloping faces of the cricket, cutting the valley shingles precisely along a snapped chalk line. Fold a cap flashing piece across the uphill apex where the two valley slopes converge to protect the high-point intersection.
- Install Counterflashing with Lead Wedges: Fabricate custom counterflashing with a 1/2-inch top return hem. Insert the hem 1 inch deep into the mortar reglet joints, overlapping the base step flashing by at least 3 inches. Mechanically secure the metal inside the mortar joint by driving tightly folded lead wedges every 12 inches using a hammer and plugging chisel. Finish by packing the joint with a flexible, UV-resistant polyurethane masonry sealant.
Retrofit vs. Reroof Costs, Common Mistakes, and Maintenance
Installing a cricket as a standalone retrofit on an existing roof is more labor-intensive than installing one during a complete roof tear-off.
During a full reroof, adding a cricket for a standard 30- to 42-inch chimney typically costs between $800 and $1,200. The roof deck is already exposed, allowing the framing and self-adhering membranes to tie seamlessly into the rest of the underlayment.
In contrast, a standalone retrofit on an existing roof costs between $1,200 and $1,800. A technician must carefully remove existing shingles in a 24-inch perimeter around the chimney, cut back underlayment without damaging the surrounding roof deck, build the cricket, tie in new membranes, install all-new custom-bent step and counterflashing, and weave matching shingles back into the existing field.
Over a 20-year span, investing $1,200 into a properly built cricket prevents an estimated $4,000 to $8,000 in cumulative roof deck rot repairs, ceiling drywall restoration, structural rafter sistering, and chimney repointing. On an 8:12 roof without a cricket, the back-pan flashing sealant typically fails within 5 to 8 years, leading to active leaks behind walls within 8 to 12 years.
Three Mistakes That Cause Cricket Leaks
- Building Too Shallow of a Slope: Framing a cricket with less than a 4:12 pitch causes slow drainage and invites standing water.
- Surface-Caulking Instead of Cutting a Reglet: Fastening counterflashing with masonry anchors and smearing caulk along the top edge is a shortcut that fails within 2 to 3 years due to thermal expansion. Flashing must always be let into a 1-inch saw-cut reglet joint and locked with lead wedges.
- Neglecting Ice and Water Shield: Relying purely on standard 15 lb or 30 lb felt paper beneath the cricket metal rather than a seamless, self-adhering high-temp modified bitumen membrane is a recipe for moisture penetration.
Even a flawlessly built cricket requires upkeep. Autumn leaves, pine needles, and storm twigs love to settle in the valley convergence behind a chimney. Homeowners should perform an annual inspection and read through preventing debris buildup in cricket valleys to keep roof valleys free of damp blockages.
Frequently Asked Questions About Chimney Crickets
Does every residential chimney require a cricket?
No. The International Residential Code (IRC) mandates a cricket only when the chimney’s uphill-facing dimension is 30 inches or wider perpendicular to the roof slope. Standard single-flue chimneys measuring 16×16 inches or chimneys located directly at the roof ridge peak do not legally require one. However, installing a cricket behind any upslope chimney penetration on steep roofs or in snowy regions remains a smart best practice.
Can a chimney cricket be added without replacing the entire roof?
Yes. A cricket can be added as a standalone retrofit. The roofer strips away shingles in a 24-inch perimeter around the uphill and side faces of the chimney, builds and anchors the frame directly to the exposed decking, installs high-temperature underlayment, cuts new mortar reglets, installs flashing, and weaves new shingles into the existing roof covering.
How much does a chimney cricket installation cost in 2026?
In 2026, building a chimney cricket during a complete roof replacement costs between $800 and $1,200 for a standard 30- to 42-inch chimney. If completed as a standalone retrofit on an existing roof, expect costs between $1,200 and $1,800 due to the additional labor required to strip and tie in shingles and underlayment. High-end architectural custom copper installations range from $2,200 to $4,000.
Conclusion
A chimney cricket is one of the highest-value structural investments you can make for your roof. By splitting the uphill water flow and channeling heavy runoff safely around the masonry corners, a well-built cricket extends the lifespan of your chimney flashing by 10 to 15 years and protects your attic, decking, and living space from expensive water damage.
At Martin & Sons, we believe that roof work should be done right the first time with zero cutting of corners. Whether you are in St. Louis, Florissant, Creve Coeur, Maryland Heights, St. Peters, or Chesterfield, our crew brings decades of craftsmanship to every job. We offer best prices upfront, lifetime labor warranties, and a strict no upfront deposit policy—you only pay when your project is 100% complete and you are fully satisfied. While we provide complete exterior water management solutions during our reroofs, Martin & Sons only provides gutter services when performing a roofing project; we are not a standalone gutter company.
If you want to ensure your chimney and roof penetrations are completely protected from water intrusion, review our guide on best practices for installing a roof cricket or contact our team today to schedule an inspection.