How to Install Counter Flashing on a Masonry or Siding-Clad Chimney
The upper layer that caps the base flashing. Set into a cut reglet on brick, or surface mounted on a clad chase.
- Professional
- 3 to 6 hours
- Updated
The short answer
Chimney counter flashing is a separate piece of metal that laps over the wall legs of the apron, step flashing and back pan. On masonry, cut a reglet at least 1 inch deep into a mortar joint, set the bent top edge into it, wedge it every 16 to 24 inches and seal it. Lap the base flashing by at least 4 inches. On siding-clad chimneys, surface mount it under the cladding.
Counter flashing, sometimes called cap flashing, is the top layer of a chimney flashing. The base flashing (apron, step pieces and back pan) sits on the roof and turns up the masonry. The counter flashing covers the top of those turned-up legs from above, so water running down the chimney face drops onto the counter flashing and over the base flashing, never behind it.
It is a separate piece on purpose. The chimney and roof move independently, and a two-part system lets them move apart without tearing the flashing. On brick it is set into a cut joint called a reglet. On a clad chimney chase it is surface mounted and covered by the cladding. See chimney flashing parts explained for where it fits.
Tools
- Angle grinder with a diamond masonry blade and a dust shroud
- Shop vacuum
- Sheet metal brake or hand seamer
- Tin snips (straight cut and offset)
- Caulking gun
- Tape measure
- Respirator rated for fine dust
- Safety glasses and hearing protection
Materials
- Galvanized, prefinished steel or copper counter flashing, 26 gauge or heavier
- Lead or metal wedges
- Polyurethane or hybrid polymer masonry sealant
- Backer rod, where joints are wide
- Masonry anchors or screws for surface mounted pieces
Safety first
- Grinding mortar creates respirable silica dust. Use a grinder with a shroud and vacuum, wear a respirator suitable for fine silica dust and follow WorkSafeBC's silica requirements.
- Work tied off. You are close to the chimney at the ridge of the roof. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- A grinder kicks. Hold it with both hands, stand to the side of the cut and never use it one-handed on a ladder or steep roof.
Step by Step
Step 1: Install the base flashing first
Counter flashing cannot be fitted until the base flashing is on, since it is shaped to match it. Install and check the base flashing first: apron, step flashing and a back pan or cricket.
Step 2: Choose the joints and mark the steps
On each side of the chimney the counter flashing steps up in stages, following the roof slope. Choose the mortar joint at each step that is high enough to cover the wall leg by at least 4 inches, and mark it. The BC Building Code calls for counter flashing on masonry to be embedded at least 25 mm (1 inch) in the masonry, to run at least 150 mm (6 inches) down the face, and to lap the base flashing by at least 100 mm (4 inches), with stepped pieces lapped at least 75 mm (3 inches). On the front and back faces, the joint should be level. Cut in joints, not through brick.
Check the mortar. If the joints are crumbling, they need repointing by a mason before a reglet can be cut.
Step 3: Cut the reglet
Use an angle grinder with a masonry blade and a dust shroud, and run a vacuum while cutting. Cut on the lower side of the joint to keep the groove clean, and clear the dust. The groove is at least 1 inch deep, and deep enough to hold the bent edge of the metal securely. For the full method, see how to cut a reglet.
Blow or vacuum the groove clean so sealant will bond.
Watch out: Never cut reglets without dust control and a respirator. Silica dust is a serious hazard.
Step 4: Bend the counter flashing pieces
Each piece has a bent top edge that goes into the groove and a face that covers the wall leg of the base flashing by at least 4 inches. On the sides, cut stepped pieces that follow the roof, each overlapping the next by at least 3 inches. Copper, prefinished steel or galvanized steel are the usual choices. See how to use a sheet metal brake for the bends.
Cut the pieces so their lower edges follow the slope and hang within about 1/2 inch of the roof surface, but do not touch the shingles.
Step 5: Set and wedge the pieces
The wedges hold the metal in the groove while the sealant sets. Place one near each end and one every 16 to 24 inches. The metal should hang straight and lap over the base flashing without touching the wall leg tightly, so it can slide slightly with movement.
Overlap each piece over the next by at least 3 inches along the run, upper over lower, so water sheds down.
Step 6: Seal the reglet
Apply the sealant into the joint so it bonds to both the masonry and the metal. Use backer rod if the joint is wider than about 1/4 inch. Tool it so water runs off rather than into the joint. Sealant is the weather seal on the top edge, not the flashing itself. See how to choose and apply roofing sealants.
Plan to check the sealant yearly. It is the part that ages.
Step 7: Siding-clad chimneys: surface mount and cover
A framed chase is treated like a sidewall. The counter flashing is fastened high on the chase, under the siding, with the building paper lapped over its top edge and the siding finished with a clearance above the roof. No reglet is needed. See how to flash a roof to wall by cladding type.
Where counter flashing is surface mounted on brick without a reglet, fasten it with masonry anchors and seal the top edge, but understand it is a weaker detail than a reglet.
Step 8: Check the lap, the drip and the finish
Look up each side and across the front and back. Every piece should lap over the one below, with no bare masonry visible above a wall leg and no place where the counter flashing rests against the roof. Corner joints should be folded and sealed.
Water test with a hose, starting high on the chimney and working down, with a helper watching the attic.
Reglet Depth and Height
- Depth: at least 1 inch into the mortar, enough to give the 25 mm embedment the BC Building Code calls for without cutting into the brick.
- Height: the joint should sit so the counter flashing covers the base wall leg by at least 4 inches.
- Wedges: every 16 to 24 inches, plus one near each end.
- Sealant: polyurethane or hybrid, with backer rod in wide joints.
- Lap: at least 3 inches between sections of counter flashing, upper over lower.
Freeze-Thaw on Masonry
Mortar joints absorb water, and in the Fraser Valley's freeze-thaw cycles, water in a crumbling joint expands and breaks the mortar around the reglet. Repoint any loose joints before cutting, and use a sealant that stays flexible in the cold. Always confirm the sealant's temperature range and the manufacturer's instructions.
Common Mistakes
- Skipping counter flashing. A single piece of metal glued to the brick is not a flashing system.
- Reglet too shallow. A groove under 1/2 inch cannot hold the bend, and the metal pulls out.
- Counter flashing that does not lap the base. If it only just touches the wall leg, wind-driven rain gets behind it.
- Relying on sealant alone. The lap does the work. Sealant is the backup at the top edge.
- Cutting a reglet in bad mortar. The groove crumbles. Repoint first.
- Grinding without dust control. Silica dust is a serious health risk.
- Cutting into the brick instead of the joint. It weakens the masonry and is very difficult to repair.
When to Call a Professional
Counter flashing on masonry means grinding mortar joints on a chimney at the ridge of the roof, with silica dust, sharp metal and tall masonry. If the mortar is loose, the chimney is high or the roof is steep, use a roofer, and a mason for any repointing.
If the chimney leaks despite good flashing, the crown, cap or brick may be the problem. See how to diagnose and fix a leaking chimney.
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 Install Chimney Counter Flashing: Questions
What is counter flashing on a chimney?
Counter flashing, or cap flashing, is a separate piece of metal set into the chimney masonry that laps over the top of the base flashing, which is the apron, step flashing and back pan. It is usually set into a cut mortar joint called a reglet. It lets the chimney and roof move slightly without tearing the flashing.
How deep should a reglet be for counter flashing?
A reglet is cut at least 1 inch deep into a mortar joint, since the BC Building Code calls for 25 mm of embedment, deep enough to hold the bent edge of the metal. The joint is chosen so the counter flashing laps over the wall leg of the base flashing by at least 4 inches. Follow the flashing manufacturer's or local specification for the exact depth.
Is counter flashing necessary on a chimney?
Yes. Without counter flashing, the top of the base flashing is fixed to the masonry with sealant or screws, which cracks as the roof and chimney move. Counter flashing laps over the base flashing so water runs over the joint, and it is the standard detail for masonry chimneys on shingle roofs.
Can counter flashing be surface mounted instead of cut into the mortar?
It can be, using masonry anchors and a sealed top edge, and this is the normal method on a siding-clad chimney chase. On brick it is a weaker detail than a reglet because the seal depends entirely on sealant. A cut reglet with wedges and sealant holds better and lasts longer.
Related Guides
All Chimneys 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.
Want someone to look at your roof?
Kory & Johnny have roofed homes across the Fraser Valley since 2016. Free inspections, honest estimates, no pressure.
604-728-0146Text UsEmailServing Hope, Agassiz, Chilliwack, Rosedale, Abbotsford & the entire Fraser Valley