How to Cut a Reglet in Masonry for Counter Flashing
A neat slot in a mortar joint, cut safely and sealed properly, is what lets masonry walls and chimneys stay dry above a roof.
- Advanced
- 2 to 5 hours
- Updated
The short answer
A reglet is a slot cut into a horizontal mortar joint to receive the top edge of counter flashing. Mark a level or stepped line, cut the joint with a dust-shrouded grinder to at least 1 inch deep, clean it, set the flashing lip into the slot, wedge it every 16 to 24 inches, and fill with a flexible sealant. Silica dust control and a respirator are mandatory.
Masonry cannot have flashing nailed into it, and surface-mounted counter flashing depends on a bead of sealant that ages. A reglet is a better solution: a slot cut into a horizontal mortar joint, into which the top edge of the counter flashing is set and sealed. The metal is protected from above and the sealant sits inside the slot rather than on the surface.
It is also a place where people get hurt and damage walls. Silica dust is a hazard to lungs, and a careless cut can chip brick faces or open a new leak path. This guide covers layout, cutting, cleaning, setting and sealing.
A reglet is used with step flashing on chimneys and brick walls, and is a common detail in chimney counter flashing.
Tools
- Angle grinder, 5 inch, with a dust shroud
- Diamond blade suited to cutting mortar
- HEPA vacuum connected to the shroud
- Respirator rated for silica dust, eye and hearing protection
- Level and pencil or chalk line
- Masonry chisel and small hammer
- Caulk gun and small brush
Materials
- Counter flashing with a return lip
- Lead or stainless steel wedges
- Backer rod, if the slot is wide
- Flexible sealant rated for masonry and metal
- Masonry primer if the sealant calls for it
- Tarp or drop cloths below the work area
Safety first
- Cutting mortar produces respirable crystalline silica, which causes serious lung disease over time. Use a shrouded grinder with a vacuum, wet methods where practical, and a respirator. Do not dry-cut without dust collection. WorkSafeBC publishes silica exposure requirements that apply to this kind of work.
- You are working at a wall face, often beside a roof edge. Tie off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- A grinder can kick back. Use both hands, guard in place, and keep the blade aligned with the joint. Wear eye and hearing protection.
Step by Step
Step 1: Inspect the masonry and choose the joint
Tap the masonry. Hollow or crumbling mortar, loose brick or a crack running through the joint is a sign the wall needs repair before any cutting. The reglet should be in sound mortar. Pick a joint that puts the bottom of the counter flashing at least 4 inches over the top of the base flashing wall leg, which is the BC Building Code minimum lap (100 mm).
Watch out: Do not cut into a wall you suspect is structurally unsound, or one with cracks through the bricks. Get a mason to look at it first.
Step 2: Set up dust control and protection
A shroud with a HEPA vacuum captures most of the dust at the cut. Keep a hose and a wet sponge for damping the surface if the shroud cannot reach. Clean up with the vacuum and never with a broom, which just puts dust back in the air.
Ask anyone nearby to stay clear and close windows on the downwind side. Silica dust drifts.
Step 3: Mark the cut line
On a chimney with a sloped roof, the reglet is cut in steps. Each step is one course of brick high, and follows the same rise as the step flashing. Mark each joint you will cut and make sure steps overlap by at least 3 inches where the counter flashing pieces will lap.
Level runs on horizontal stretches and plumb verticals at step ends help the metal sit correctly.
Step 4: Cut the joint to the right depth
Make the first pass shallow, about 1/4 inch, to set the line. Then deepen it in a second pass to at least 1 inch, because the BC Building Code calls for counter flashing to be embedded at least 25 mm in the masonry. Hold the grinder at a steady angle, in line with the wall, not tilted into the brick. Stay inside the joint. A slot that chews the brick corners is hard to seal.
At the ends of each step, stop short of the vertical joint and finish the corners with a hand chisel so you do not overcut. Mortar joints are commonly about 3/8 inch wide. A single blade pass may leave it narrow, so use a thin blade and two passes, or a blade about 1/4 inch thick plus a touch-up for the lip.
Step 5: Clean out the slot
Dust left in the slot stops sealant from bonding. Use the HEPA vacuum first and a stiff brush, then inspect. Let the masonry dry if it is wet. Masonry primer, if the sealant manufacturer calls for it, goes on now.
Step 6: Form and test-fit the counter flashing
The lip should fit freely without forcing. The flashing should hang over the base flashing by at least 4 inches and sit about 1/8 inch off it, not tight. See counter flashing for the lap details and material choices.
Step 7: Set the flashing and wedge it
Wedges hold the metal firmly while the sealant cures and as a mechanical backup. Tap them in gently. Do not use nails or screws in place of wedges in a slot. Lap the next piece over the last by at least 3 inches.
Step 8: Seal the reglet
Use backer rod if the gap is wider than about 1/4 inch so the sealant has the right depth. Tool the bead slightly concave or sloped and smooth it into the masonry. The bead is the weather seal, but the lapped metal is the flashing. Plan to check it every few years. See roofing sealants.
Step 9: Clean up and dispose of the dust correctly
Silica dust on clothing travels. Change out of dusty clothes before entering a vehicle or home. Dispose of vacuum contents and waste according to local rules.
Reglet Depth and Size at a Glance
- Depth: at least 1 inch into the mortar joint (25 mm embedment is the BC Building Code minimum).
- Height of slot: just wider than the flashing lip, commonly about 1/4 to 3/8 inch.
- Counter flashing lap over base flashing: at least 4 inches (100 mm), and at least 6 inches of metal down the face of the masonry.
- Wedge spacing: about 16 to 24 inches and at each end.
- Piece-to-piece lap: at least 3 inches (75 mm) in the direction of water flow.
Reglet Alternatives
Where the mortar is poor, where a joint would have to be cut across brick, or where a wall is painted or sealed, surface-mounted counter flashing or a two-piece receiver may be better. They rely more on sealant, so check them more often. See flashing a roof to a wall by cladding type.
Fraser Valley Notes
Freeze-thaw cycles in winter open hairline cracks in old mortar, and wind-driven rain pushes water into them. If a wall has been wet for some time, allow it to dry before sealing. Wet masonry stops sealant from bonding, and trapped moisture freezes behind it.
Common Mistakes
- Cutting without dust control. Silica dust causes permanent lung damage and spreads across the property.
- Cutting into the brick face instead of the joint, which damages the wall and leaves a slot that will not seal.
- Making the slot too shallow, so the lip barely enters it and the metal pops out in wind.
- Sealing over dust, so the bead never bonds and peels away within a season.
- Relying on sealant with no wedges, leaving the metal unsupported while the bead cures.
- Cutting into failing masonry and loosening brick that then needs a mason.
When to Call a Professional
Most homeowners should leave reglet cutting to a roofer or mason. It combines silica dust, a grinder, heights and masonry that may not be sound. Any chimney over a single storey is worth professional help.
If the mortar is crumbling, bricks are loose, or the chimney leans, the wall needs masonry repair before flashing. A roofer can advise on sequence with a mason.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Cut a Reglet for Counter Flashing: Questions
What is a reglet in roofing?
A reglet is a narrow slot cut into a masonry wall, usually along a mortar joint, that receives the top edge of counter flashing. The flashing lip sits inside the slot, is held by wedges and is sealed with flexible sealant. It protects the top of the flashing better than a bead of sealant on the wall surface, and it is common on chimneys and brick walls.
How deep should a reglet be?
At least 1 inch deep, cut into a horizontal mortar joint, because the BC Building Code calls for counter flashing to be embedded at least 25 mm in the masonry. Follow the flashing manufacturer's instructions and any local requirements. The slot must be deep enough to hold the lip firmly and wide enough to take it without forcing, then clean and dry before the flashing and sealant go in.
Do you cut a reglet in the brick or the mortar?
In the mortar joint. Cutting into brick damages the wall and leaves a slot that is difficult to seal. Choose a horizontal joint at the right height, cut inside it with a thin diamond blade, and clean out the slot. If the mortar is poor, a mason should repair it first or another flashing method should be used.
Is cutting masonry with a grinder dangerous?
It can be. Cutting mortar and brick releases respirable crystalline silica dust, which causes serious lung disease with repeated exposure. Use a grinder with a dust shroud connected to a HEPA vacuum, wear a respirator rated for silica and eye protection, and never dry sweep the dust. Grinder kickback is also a hazard, so use both hands.
Can you use caulk instead of a reglet?
You can surface mount counter flashing and seal along its top edge, but that relies entirely on sealant that ages in sun and weather. A reglet holds the metal mechanically and keeps the bead in a protected slot, so it lasts longer. Either way the flashing needs to lap over the base flashing, with sealant as backup.
Related Guides
All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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