What Is Counterflashing?
Counterflashing is the upper component of a two-piece flashing system at wall-to-roof junctions. It is installed in a reglet (a groove in masonry, a surface-mounted metal extrusion, or a bend in the wall cladding) at a height above the base flashing. Its exposed leg hangs down and overlaps the top of the base flashing by at least 4", sealing the top edge of the base flashing without being mechanically attached to it.
The critical distinction is independence: counterflashing and base flashing move separately because the wall and roof structure move separately due to differential thermal expansion, settling, and live loads. A single-piece flashing system attached to both would tear itself apart over a few heating cycles. The two-piece system allows relative movement while maintaining the weather lap.
Where Counterflashing Is Used
- Chimney sides and back pan — counterflashing over sidewall flashing on chimney sides; back pan counterflashing at the high side of the chimney
- Parapet-to-roof junction — counterflashing in a reglet near the base of the parapet wall, lapping the base flashing that transitions from the roofing membrane up the wall
- Masonry wall to roof transitions — on buildings where the roof meets a masonry firewall, party wall, or adjacent building wall
- Skylight and HVAC curb surrounds — where the roofing membrane turns up the curb and counterflashing seals the membrane termination against the curb wall
Counterflashing Profile Geometry
The standard counterflashing profile is essentially an L-shape with two legs:
- Reglet leg (insertion leg) — the short horizontal leg that inserts into the reglet groove; typically ¾"–1½" long with a locked hem at the tip for mechanical engagement
- Face leg (exposed leg) — the vertical portion that hangs below the reglet and laps the base flashing; typically 4"–8" long depending on the required weather lap
Some counterflashing profiles add a drip hem at the bottom of the face leg to prevent water from wicking back up the face leg under capillary action. This is strongly recommended on chimney applications where the counterflashing face leg is exposed to direct weather.
Materials
| Material | Thickness | Application | Notes |
|---|---|---|---|
| Aluminum | .040" | Standard commercial and light commercial | Most common; good corrosion resistance, lightweight, Kynar available |
| Copper | 16 oz. | High-end light commercial, historical restoration | Develops a protective patina; long life; stains adjacent masonry green |
| Lead | 2.5 lb./sq ft | Specialty applications | Highly conformable, used around complex masonry; increasingly regulated |
| Galvanized steel | 26 ga. | Light commercial | Requires periodic paint or re-coating; shorter service life |
How Trimgy Handles Counterflashing
Counterflashing is one of the simpler trim profiles on Trimgy — typically a two- or three-leg profile with specific reglet insertion depth and exposed leg length. The critical dimension is the face leg length: it must be long enough to lap the base flashing by at least 4" while leaving the reglet insertion leg accessible for installation.
Roofing contractors who re-flash chimneys regularly benefit from saving a standard counterflashing profile in Trimgy. Reordering for a similar job takes seconds — just update the linear footage and reorder. For non-standard conditions (a parapet with an unusual reglet height, or a custom copper profile for an architectural chimney), the drawing grid makes it easy to specify the exact geometry.
See our complete roof flashing types guide →
Draw Your Counterflashing Profile on Trimgy
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Start Drawing Free →Frequently Asked Questions
What is the difference between base flashing and counterflashing?
Base flashing laps onto the roofing membrane and up the vertical wall — it moves with the roof. Counterflashing is installed above in a reglet or masonry joint and hangs down over the base flashing — it moves with the wall. The two-piece design allows independent movement while maintaining the weather lap.
How deep should counterflashing insert into a reglet?
At least 1" of insertion, with 1½" preferred. Surface-mounted reglets are caulked at the top after insertion. Saw-cut masonry reglets are 1" deep and sealed with backer rod and sealant.
What material is best for counterflashing?
Aluminum .040" for most commercial and light commercial. Copper for high-end or historical work. The material should be compatible with the base flashing and adjacent wall cladding — dissimilar metals in contact can accelerate corrosion.
How long should the counterflashing leg be?
The exposed leg should extend at least 4" below the top of the base flashing for adequate weather lap. On chimneys with slope exposure, 6" is preferred. Total height = reglet insertion depth + exposed leg length.
Can I order counterflashing cut to specific lengths?
Trimgy orders are priced per linear foot at your total footage. Standard pieces are 10-foot lengths. Cut to specific chimney or wall dimensions in the field.