The Master Craftsman’s Guide to Chimney Flashing: Mastering the Reglet Cut
- Apr 6
- 4 min read
At Winchester Roofing, our mission is simple: providing Lasting Protection for Generations. One of the most common calls we receive involves a leaky chimney. More often than not, the issue isn't failing bricks or bad shingles—it is failing chimney flashing.
As a family-owned Class A Residential Building Contractor (#2705178523), we believe in doing things the right way, the first time. We are not storm chasers looking for a quick fix; we are local experts dedicated to Craftsmanship and Integrity. That means adhering strictly to manufacturer guidelines, specifically the CertainTeed Shingle Applicator’s Manual (SAM), which sets the gold standard for roofing installations.
Today, we are pulling back the curtain to educate homeowners on the proper way to flash a masonry chimney using the "Reglet Cut" method.
The Problem: The "Caulk and Run" Epidemic
When a roof meets a vertical masonry wall, water finds its way into the smallest vulnerabilities. Many roofers attempt to seal this transition by taking a piece of sheet metal, bending it against the brick, and slathering a thick bead of roofing tar or caulk across the top.
We call this the "caulk and run" method. Why? Because the moment the house settles or the metal expands and contracts with Virginia's changing seasons, that surface caulk breaks its bond. Water sneaks behind the metal, runs down the masonry, and rots your roof decking and interior drywall.
At Winchester Roofing, we know that Value means delivering a solution that actually lasts. Surface caulking is not flashing; a mechanical seal is.
The Solution: The Reglet Cut Method
To ensure absolute water tightness, the metal flashing must actually become part of the chimney. This is achieved through a two-part flashing system (Step Flashing and Counter Flashing) integrated directly into the masonry using a Reglet Cut.
A "reglet" (or raggle) is a precise groove cut horizontally into the mortar joints or brick of the chimney. Instead of just resting against the brick, the top piece of metal flashing (the counter flashing) is bent, tucked deeply inside this groove, and permanently sealed.
Step-by-Step: The CertainTeed Approved Process
When our Master Craftsman™ certified crews flash a chimney, we meticulously follow CertainTeed’s installation standards to ensure your warranty remains valid and your home remains dry. Here is how we do it:
1. Deck Preparation & Waterproofing
Before any metal touches the roof, the foundation must be secure.
Solid Decking: We verify the roof deck is structurally sound (minimum 3/8" plywood or 7/16" non-veneer).
Ice and Water Shield: We apply CertainTeed WinterGuard™ (a high-temperature, self-adhering waterproofing membrane) directly to the wood deck, wrapping it securely around the base of the chimney.
2. Installing the Base and Step Flashing
Flashing works exactly like shingles: it overlaps to shed water with the force of gravity.
Base Flashing: Applied to the bottom face of the chimney, installed over the shingles and set in asphalt plastic cement.
Step Flashing: We weave individual pieces of L-shaped metal flashing under each successive course of shingles along the sides of the chimney. Step flashing must be routed up the wall and overlap appropriately so water cannot travel sideways into the house.
3. Executing the Reglet Cut
This is where true craftsmanship shines.
We mount a level guide or ledger on the side of the chimney.
Using a dry masonry diamond wheel on a grinder, we cut a straight groove parallel to the roof slope directly into the masonry.
Manufacturer Standard: According to CertainTeed guidelines, this reglet cut must be a minimum of 1-inch deep into the masonry, though 1¼ to 1½ inches is preferred for optimal security.
4. Forming and Inserting the Counter Flashing (Cap Flashing)
The counter flashing is the metal layer that drops down over the step flashing, protecting it completely.
The Spring-Loaded Hem: We don't just shove flat metal into the hole. We pre-bend the top edge of the counter flashing with a 3/8” to 1/2” reverse fold (hem).
When this hemmed edge is inserted into the 1.5-inch reglet cut, it acts like a compressed spring. Once inside, the tension locks the flashing firmly in place so it cannot be easily dislodged by high winds or structural settling.
5. The Final Seal
Once the metal is locked into the reglet under spring tension, the joint is refilled. We use high-grade polyurethane sealant or mortar to permanently seal the groove. Because the water is now shedding over the top of the metal rather than relying on a surface bond, leaks are virtually eliminated.
6. The "Cricket" Rule
Does your chimney face the upper slope of your roof? If a chimney is wider than 24 inches, CertainTeed standards dictate that a "cricket" (a small, peaked roof structure) must be built behind the chimney. This diverts water, snow, and debris away from the flat back wall of the masonry, preventing standing water and ensuring proper drainage.
Why Trust Winchester Roofing?
Roofing is a highly technical trade, and what you can't see matters just as much as the beautiful new CertainTeed asphalt or metal shingles you can see.
If you suspect your chimney flashing is failing, or if you are considering a full roof replacement, don't wait for a leak to show up on your ceiling.
Contact the professionals at Winchester Roofing today. We are your neighbors, and we are here to provide lasting protection for your family's home.
📍 Visit Us: 6 S Braddock Street, Winchester, VA 22601 📞 Call Us: 540-315-1125 💻
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