Reglet Flashing: How Counterflashing Locks Into a Wall

Reglet flashing is counterflashing whose top edge is tucked into a reglet, a narrow slot cut or cast into a masonry or concrete wall. The slot holds the metal and sheds water over it, so rain running down the wall can't get behind the flashing. You'll see it where a roof meets a chimney, a parapet or a brick wall, covering the base flashing that turns up from the roof.

Base flashing and counterflashing

Every roof-to-wall joint has two layers:

1. Base flashing turns up from the roof onto the wall, usually 8 inches or more. It's part of the roofing.
2. Counterflashing comes down from the wall and laps over the top of the base flashing. It's fastened to the wall, so the roof and wall can move separately without tearing anything.

The reglet is one way to anchor the counterflashing. On brick chimneys with shingle roofs, the same idea is done with step flashing and a cut-in counterflashing.

Types of reglets

Type	How it's made	Where it's used	Notes
Saw-cut	A slot cut into a mortar joint or concrete, about 3/4 to 1 inch deep	Existing masonry and concrete walls, reroofs	Most common retrofit; cut into mortar joints, not brick faces
Cast-in (embedded)	A preformed metal reglet set in the concrete pour or laid in a mortar joint	New concrete and masonry walls	Neatest; plan it before the pour
Surface-mounted	A Z-shaped bar fastened to the wall face with sealant along the top	Walls where cutting isn't possible	Depends entirely on the sealant; check it often
Two-piece spring-lock	A receiver set in the wall and a removable counterflashing that snaps in	Commercial roofs	Lets roofers remove the counterflashing at reroof time

Installing saw-cut reglet flashing

1. Cut the slot in a horizontal mortar joint above the top of the base flashing, typically 3/4 to 1 inch deep. Use a masonry blade and dust control.
2. Clean it out with a brush and blower; dust keeps sealant from bonding.
3. Form the counterflashing with a short leg bent to fit into the slot (often with a hem or a kink that grips) and a face long enough to lap at least 4 inches over the base flashing, with a hemmed drip edge at the bottom.
4. Insert and wedge the leg into the slot. Lead or plastic wedges, or spring clips, hold it every 12 to 16 inches.
5. Seal the slot with a sealant rated for masonry and metal, such as polyurethane, tooled so it sheds water.
6. Lap joints in the counterflashing by about 3 to 4 inches, sealed, and never stack a joint directly over a base flashing seam.

Metals

Stainless steel, copper and prepainted galvanized steel are common. Bare aluminum doesn't like fresh mortar; the alkalinity corrodes it, so coat it or choose another metal. Match the metal to the roof and gutters to avoid galvanic corrosion: no copper counterflashing above a Galvalume roof, for example.

Why reglets leak

• Sealant dried out or never bonded to a dusty slot
• Slot cut too shallow, so the flashing works loose
• Counterflashing too short to lap the base flashing
• Slot cut into soft, crumbling mortar; repoint first
• Joints in the counterflashing left unsealed

Resealing a reglet is cheap maintenance. Check it every few years, and any time you see staining inside the wall below a chimney or parapet. On membrane roofs, the base flashing is often held at the top with a termination bar before the reglet counterflashing goes on.