How to Install Counter Flashing Over Step and Apron Flashing
The second piece of metal that covers the top of the base flashing, so the wall and the roof can move without opening a leak.
- Advanced
- 2 to 6 hours
- Updated
The short answer
Counter flashing is a separate piece of metal that covers the top edge of base flashing (step, apron or valley metal) and is fastened to the wall, not the roof. It is installed in a cut mortar joint called a reglet, on the surface of the wall with sealant, or behind cladding in a two-piece receiver. It should overlap the base flashing by at least 4 inches and hang free.
Base flashing is lapped with the roof. Counter flashing is lapped with the wall. It is a second piece of metal that hangs over the top edge of the base flashing, so water running down the wall falls onto the outside of the metal and cannot get behind it. Because it is fastened to the wall and not the roof, the roof can settle and move seasonally without tearing the joint.
You find counter flashing on chimneys, brick and stone walls, stucco, parapets and any wall where a simple wall leg running up behind siding is not possible. It is the detail most often faked with a bead of caulking, which is the single most common reason masonry joints leak.
This guide covers the three common methods: reglet set, surface mounted and two-piece receiver. It pairs with step flashing, headwall flashing and, for chimneys, chimney counter flashing.
Tools
- Angle grinder with a diamond blade and dust shroud
- Hand seamer
- Tin snips (straight cut)
- Caulk gun
- Masonry drill and hammer drill bits
- Tape measure and level
- Hook blade utility knife
Materials
- Counter flashing or receiver, galvanized or prefinished steel, aluminum, copper or lead depending on the wall
- Base flashing already in place
- Masonry screws or anchors, stainless or coated
- Polyurethane or hybrid polymer sealant rated for masonry and metal
- Backer rod for wide joints
- Mortar or lead wedges for reglet fixings
Safety first
- Grinding masonry makes silica dust, which is a serious lung hazard. Use a shroud and vacuum, a respirator rated for silica, eye protection and wet methods where possible. See cutting a reglet.
- You are working at a wall face, often above the roof edge. Tie off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Sheet metal edges cut skin. Wear cut resistant gloves when fitting and bending.
Step by Step
Step 1: Confirm the base flashing is right first
Counter flashing cannot rescue bad base flashing. Look for short pieces, reverse laps and nails through the wall leg. If you are covering old base flashing, see whether it is rusted at the bend. On a reroof, replace it. See step flashing.
The wall leg of the base flashing should stand up at least 4 inches where possible. You need the counter flashing to lap over at least 4 inches of 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).
Step 2: Choose the method that suits the wall
Each method has a place. A reglet is a slot cut into the mortar joint. It is the most durable because the metal is protected from above and the sealant sits inside the slot. Surface mounted counter flashing is screwed to the wall face and sealed along the top. It is simpler, but the sealant is exposed to sun and needs maintenance. A two-piece receiver is a metal channel fixed to the wall with a separate piece that clips or slides in, so the roof can be replaced without disturbing the wall.
Match the method to the wall. Brick and block suit reglets. Poured concrete and stone with no suitable mortar joint often need surface mount. Stucco and cladding suit two-piece systems, covered in flashing a roof to a wall by cladding type.
Step 3: Cut the reglet or fix the receiver
Use a mortar joint, not the brick itself. Mark a level line so the counter flashing will lap the base flashing by at least 4 inches, and follow the stepped slope of the roof in steps with each course of masonry. Cut the slot with a grinder and diamond blade with a dust shroud, to a depth of at least 1 inch and a width that just takes the flashing lip. Clean dust out before setting metal.
For a receiver, screw the channel through the top edge into solid backing every 12 to 16 inches and seal the screw heads. Keep the cladding or stucco finish running over the top of the receiver. The step by step for cutting is in how to cut a reglet.
Watch out: Never cut into the brick face or across a wall that may be structural without checking. A cut that is too deep weakens the wall and opens a new water path.
Step 4: Form the counter flashing pieces
Pieces are commonly cut in lengths of about 8 to 10 feet on straight runs and in stepped lengths along sloped masonry. Each stepped piece follows the mortar courses and laps the next by at least 3 inches. Bend the top lip so it fits the slot. A drip bend at the bottom edge makes water break away from the wall face.
Match metal to the situation. Galvanized or prefinished steel, copper and lead-coated copper suit masonry. Do not use bare aluminum against wet mortar. See roof flashing types for material notes.
Step 5: Set the counter flashing over the base flashing
Do not press the counter flashing tight to the base flashing. A small gap, about 1/8 inch, lets the two move independently. If the two are fixed together, the roof pulls the joint open the first time the house shifts.
In a reglet, secure each piece with lead wedges or stainless steel wedges at about 16 to 24 inches and at each end. Wedges hold the metal while the sealant cures. Nails or screws into the mortar above the slot are not a substitute for wedges.
Step 6: Lap each piece over the next and seal the ends
Lap the upper piece over the lower at every joint. At exposed ends and at each corner, a small dab of sealant is acceptable, but the lap itself does the work. See roofing sealants for what to use.
Step 7: Seal the top of the reglet or the wall line
Use backer rod if the gap is wider than about 1/4 inch, so the sealant has a correct depth of roughly half its width. Tool it smooth and slightly sloped so it does not hold water. This sealant is the weather seal on top of a detail that is already lapped. It is maintenance, and should be checked every few years.
Step 8: Tie the wall barrier in over the top
On stucco or siding walls, the building paper must lap over the receiver so water running behind the cladding lands on the outside of the metal. Where the barrier cannot be reached, a membrane strip over the top edge carries the lap.
Reglet vs Surface Mount vs Two-Piece
- Reglet set: the most durable and cleanest finish. Needs a good mortar joint and a grinder.
- Surface mounted: quick and suitable for retrofits. Needs regular resealing along the top edge.
- Two-piece receiver: best when the roof will be replaced before the wall. The counter piece slides out for reroofing.
- Behind cladding: the wall leg of the base flashing is covered by the siding and no counter flashing is used. This only works on wood walls where cladding can be lifted. See step flashing.
Fraser Valley Notes
Wind-driven rain hits chimneys and masonry walls hard, and freeze-thaw cycles open hairline cracks in mortar joints over winter. A surface mounted counter flashing with a cracked bead of sealant will leak in the first real storm of autumn. Stainless or coated fasteners matter in a wet climate, because rust weeps down the masonry and stains it.
Common Mistakes
- Relying on sealant alone at the top of the base flashing with no counter flashing. The bead fails in a few seasons and the wall leg is wide open.
- Fastening counter flashing to the base flashing. The roof and the wall move separately, so a fixed joint tears.
- Cutting the reglet too shallow, so the lip barely enters the slot and the metal falls out or opens in wind.
- Using bare aluminum against mortar. Wet lime and cement corrode it within a few seasons.
- Not lapping counter flashing pieces in the direction of water flow, leaving open joints along the wall.
- Covering rusted base flashing with new counter flashing rather than replacing it.
When to Call a Professional
Reglet cutting near chimneys, or on walls of uncertain condition, calls for a roofer or mason. Silica dust, unstable masonry and heights combine. If mortar is crumbling, cutting into it can loosen bricks.
If the wall is stucco, stone or a multi-storey masonry face, or if you cannot reach the wall safely from the roof, leave counter flashing to a professional. A leak here often does not show inside for months.
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 Counter Flashing: Questions
What is counter flashing on a roof?
Counter flashing is a second piece of metal that covers the top edge of base flashing where a roof meets a wall or chimney. It is fixed to the wall, not the roof, and hangs over the base flashing so water running down the wall falls on the outside of the metal. It lets the roof and wall move independently without opening the joint.
What is the difference between step flashing and counter flashing?
Step flashing is base flashing that is woven into the shingle courses along a wall, one piece per course. Counter flashing sits over the top of step flashing and is attached to the wall, usually in a reglet, on the surface or in a receiver. Step flashing deals with water on the roof, and counter flashing keeps water off the top of the step flashing.
How deep should a reglet be for counter flashing?
At least 1 inch deep, cut into a mortar joint and not the brick itself, and wide enough to take the flashing lip. Check the flashing manufacturer's instructions and any local requirements. The slot is cleaned of dust, the flashing wedged in with lead or stainless wedges, then the opening is sealed with a flexible sealant rated for masonry.
Can you use caulk instead of counter flashing?
Caulk alone is not a substitute. It fails as it ages and moves, and there is no lap to fall back on once it cracks. Sealant has a place at the top of a reglet or along a surface mounted counter flashing, but only on top of a detail where the metal already laps over the base flashing.
How far should counter flashing overlap base flashing?
At least 4 inches on masonry, which is the BC Building Code minimum lap (100 mm), and the same is good practice on other walls. Leave a small gap of about 1/8 inch rather than pressing it tight, so the two pieces can move independently. Confirm the lap against the flashing manufacturer's detail for the specific product.
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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