Roof Counter Flashing: Reglet Cuts, Surface Mount, Sealant

Where a roof meets a brick wall or chimney, the shingles alone can never keep water out. The joint needs two layers of metal working together, and the upper one is the part most homeowners never notice until it starts to leak. Roof counter flashing, also called cap flashing, is the metal that comes down out of the masonry and laps over the base flashing on the roof, shedding water away from the gap between the wall and the roof deck.

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The base flashing (step flashing or a continuous apron) is tied into the roofing and turns up the wall. The counter flashing is tied into the wall and turns down over that upturned leg. Because the two pieces are attached to different surfaces, the house and the roof can move independently with temperature and settling without tearing the seal. If the counter flashing is loose, cracked, or held on only by caulk, water runs behind the base flashing and shows up as ceiling stains, damp attic sheathing, or rotted framing around the chimney.

There are two main ways to attach it: cutting a reglet (a slot) into the mortar or masonry and inserting the metal, or fastening it to the surface of the wall with a sealed termination bar. Each has a place, and both can be replaced without tearing off the shingles if you work carefully.

How Roof Counter Flashing Works With Base Flashing

Think of the system as two overlapping shields. The base flashing turns up the wall at least 4 inches, often more, and is never fastened to the masonry, so it can move with the roof. The roof counter flashing is anchored into the wall above that and drops down to cover the top of the base flashing by at least 2 to 4 inches. Water running down the wall hits the counter flashing and gets kicked out onto the face of the base flashing, then onto the shingles.

On a sloped side wall or the side of a chimney, the counter flashing is usually cut in a stepped pattern that follows the mortar courses, with each step lapping the one below. Along the bottom (front) of a chimney, it is one continuous piece over the apron. At the top, the counter flashing laps the cricket or back pan flashing.

Brick counter flashing has an advantage over flashing on siding: the mortar joints give you a natural line to cut into, so the metal can be tucked into the wall rather than relying on surface adhesives alone.

Reglet Cut vs Surface Mount

Reglet-cut counter flashing

A reglet is a slot about 3/4 to 1 1/4 inches deep cut into a mortar joint with a masonry grinder. The top edge of the counter flashing is bent back into a small hem, pushed into the slot, wedged with lead or plastic shims or metal clips, and then sealed with a flexible masonry sealant. Because water running down the wall face passes over the slot rather than into it, a reglet detail is the longest-lasting option on brick, block and stone.

The downside is labor and dust. Cutting reglets means grinding mortar, which creates crystalline silica dust and requires care not to chip the brick faces.

Surface-mounted counter flashing

Surface mounting uses a metal strip or termination bar fastened directly to the face of the wall with masonry screws or anchors, with a bead of sealant along the top edge. It is faster and avoids grinding, which makes it common on stucco, smooth concrete, or block where clean mortar lines are not available. Some surface-mount profiles have a built-in kick at the top that holds a sealant bead.

The weakness is that the sealant is the only thing stopping water at the top edge. Even good sealant needs inspection every few years and periodic replacement. On brick with sound mortar, most roofers prefer a reglet.

Quick comparison

Choosing Metal and Sealant

Aluminum counter flashing is the most common choice for DIY repairs. It is light, rust-free, easy to bend and available prepainted to blend with brick or trim. Avoid putting bare aluminum in direct contact with fresh mortar or wet masonry for long periods, as alkaline mortar can corrode it; a coated or painted product holds up better. Galvanized steel is stiffer and cheaper per foot but can rust at cut edges. Copper and lead-coated copper are traditional on chimneys and last for generations, though they cost considerably more and require compatible fasteners.

For sealant, pick a flexible polyurethane or hybrid polymer masonry sealant rated for exterior use and for bonding to both metal and masonry. Avoid plain acrylic caulk and generic silicone, which either harden and crack or will not accept paint and do not bond well to dusty masonry. Roofing cement (asphalt mastic) is fine for patching shingles but makes a poor primary seal at a reglet because it dries out and cracks in the sun.

Tools and Materials

• Counter flashing coil or preformed pieces, plus a hand seamer or bending brake
• Angle grinder with a diamond masonry blade and a dust shroud connected to a HEPA vacuum
• Spray bottle or water feed for wet cutting
• Masonry sealant and caulk gun; lead or plastic wedges
• Masonry screws or anchors for surface mounting
• Aviation snips, tape measure, chalk line, flat pry bar
• Safety glasses, hearing protection, gloves and an N95 or P100 respirator

Step-by-Step: Replacing Counter Flashing Without Disturbing Shingles

The good part about counter flashing is that it lives on the wall, not the roof. In most cases you can replace it while leaving the shingles and base flashing alone.

1. Inspect the base flashing first. Gently lift the old counter flashing and look at the step or apron flashing behind it. If the base flashing is rusted through or missing, new cap flashing will not solve the leak, and that part needs to be replaced too.
2. Remove the old cap flashing. Rake out old sealant, pull wedges, and slide the metal out of the joint. Work the pieces free with a flat bar rather than prying against the brick.
3. Clean or recut the reglet. If the existing slot is crumbly or shallow, grind a fresh one about 1 inch deep in the mortar joint, roughly 6 to 8 inches above the roof surface. Use wet cutting or a shrouded grinder with a HEPA vacuum. Brush and vacuum out the slot.
4. Form the new pieces. Bend a 1/2 to 3/4 inch hem on the top edge that inserts into the reglet. On side walls, cut stepped pieces that follow each mortar course and overlap each other by at least 2 inches, with the upper piece lapping over the lower one.
5. Set the flashing. Insert the hem into the slot, wedge it every 12 to 16 inches, and confirm the lower edge covers the base flashing by at least 2 inches without being fastened to it.
6. Seal the reglet. Run a continuous bead of masonry sealant into the joint over the metal and tool it smooth so water sheds off it.
7. For surface mount: mark a level line, predrill, fasten the strip or termination bar every 8 to 12 inches with masonry screws, and apply sealant along the top edge and over each fastener head.

Troubleshooting Leaks at Walls and Chimneys

• Leak only during wind-driven rain: often a gap at the top of the counter flashing or open joints in the stepped pieces.
• Leak after long, steady rain: may be water soaking through porous brick above the flashing. Consider a breathable masonry water repellent or through-wall flashing, which is a masonry job.
• Counter flashing nailed through base flashing: this ties the two layers together and causes tearing as the roof moves. Refasten so they stay independent.
• Caulk-only repairs: a thick bead of caulk smeared over a loose flashing edge will fail within a season or two. Reset the metal properly.
• Cracked mortar at the reglet: repoint the surrounding joints before setting new flashing, or the seal will fail with the mortar.

Cost Ranges

Materials for a typical chimney or short wall run are inexpensive: aluminum coil and sealant usually come to well under a hundred dollars, while copper can push materials into the low hundreds. Professional replacement of counter flashing on a single chimney generally falls in the few hundred dollar range, and rises when the job includes new step flashing, a cricket, masonry repointing or a steep, high roof. Long wall abutments are priced by the linear foot.

Silica Dust and Ladder Safety

Cutting a reglet into mortar or brick releases crystalline silica, which can cause permanent lung damage. Cut wet whenever possible, or use a grinder with a dust shroud connected to a HEPA vacuum, and wear an N95 at minimum, with a P100 respirator for extended cutting. Clean up with a HEPA vacuum or wet wiping, never by dry sweeping or blowing dust off the roof. Keep children and pets away from the work area.

Chimney and wall flashing often sits high on the roof. Use a properly secured extension ladder that extends 3 feet above the eave, roof brackets or a harness on steeper pitches, and never run a grinder while balanced on a ladder. On two-story homes or steep roofs, hire a roofer or mason.

When to Call a Pro

Call a roofer when base flashing needs replacement, when water damage has reached decking or framing, or when the chimney needs a cricket. Call a mason if the brick is spalling, the mortar is badly deteriorated, or the leak seems to come through the masonry itself rather than the flashing joint.