In Canadian construction, a building's durability is won or lost at its seams. Improper flashing around windows and doors is the leading cause of costly callbacks, interior damage, and structural decay. This visual guide provides a clear, code-informed methodology for achieving a watertight, airtight, and durable installation, tailored to the demands of rain, snow, and freeze-thaw cycles.
The Core Principle: Drainage Over Dam
Modern flashing follows the "drainage plane" philosophy. We cannot guarantee a perfect seal; instead, we design a system that manages any incidental water that penetrates the primary weather barrier, guiding it safely back outside. The key is layering materials shingle-style so each layer laps over the one below, shedding water downward and outward.
Essential Materials & Tools
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Flashing Tape: High-performance, vapor-permeable but liquid-water-resistant tape (e.g., acrylic or butyl-based). Must be compatible with your weather-resistant barrier (WRB).
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Flexible Flashing Membrane: Self-adhered bituminous or synthetic membranes (e.g., Vycor, Blueskin) for complex details.
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Rigid Head Flashing: Metal (aluminum, coated steel) or pre-formed PVC. Mandatory for doors and large windows.
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Sill Pan/Pre-formed Sill Flashing: Critical for creating a "tub" at the bottom of the opening. Can be custom-fabricated metal or pre-formed composite.
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Sealants: High-quality, UV-resistant, paintable exterior sealant compatible with all substrates.
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Utility Knife, Shears, Squeegee: For cutting and bonding materials.
Part 1: Wood Frame Construction with Absorbent Cladding (Vinyl, Wood, Fiber Cement)
Step 1: Prepare the Rough Opening
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Ensure the opening is sized correctly (per manufacturer's specs, typically 1/2" larger than unit dimensions).
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The rough opening sill must be sloped a minimum of 3° (1/4" per foot) to drain. Use a tapered sill seal or shim to create slope.
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Apply a bead of sealant along the exterior face of the sheathing at the sill, just inside the rough opening line. This seals the WRB to the sheathing.
Step 2: Integrate with the Weather-Resistant Barrier (WRB)
This is the most critical step for integration.
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Install Sill Flashing: Apply a wide strip of flexible flashing membrane over the sill, extending up the side jambs at least 6". Bond it to the sloped sill and sheathing. This is your primary sill pan.
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Integrate Side Flashing: Lap the WRB (housewrap) from the wall over the side legs of the sill flashing. Tape this seam with flashing tape.
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Create the Head Condition: Before installing the unit, install a piece of rigid head flashing (metal) or a wide strip of flexible membrane above the opening, lapping over the WRB. This directs water over the WRB plane.
Visual: A cross-section showing the WRB lapping over the side flashing, which laps over the sill pan, with head flashing lapping over everything.
Step 3: Install the Unit & Final Flashing
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Set the window/door into the opening, shimming it level, plumb, and square from the interior.
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From the exterior, apply flashing tape in this order:
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Sill: Tape the unit's nailing flange to the sill flashing below.
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Sides: Tape the side flanges to the integrated WRB/side flashing, lapping over the sill tape.
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Head: Finally, tape the head flange, ensuring the tape laps overthe side tape and seals to the pre-installed head flashing/WRB above.
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Result: Every joint is shingled. Water hitting the head is directed over the sides; water at the sides is directed over the sill.
Part 2: Masonry Veneer (Brick) over Wood Frame
The cavity wall introduces a secondary drainage plane, but the primary seal at the rough opening is even more vital.
Critical Difference: The Through-Wall Flashing
At the sill of the rough opening, a continuous through-wall flashing (e.g., flexible membrane with end dams) must be installed. It passes behindthe weather-resistant barrier on the sheathing, extends out over the brick shelf angle or foundation, and is turned up at the ends to form "end dams" that prevent water from running sideways inside the cavity.
Flashing Sequence for Brick Veneer:
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Through-Wall Sill Flashing: Installed first, integrated with the WRB on the sheathing.
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Unit Installation: Window/door is installed as in wood frame.
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Sealing to WRB: Tape the unit's flanges to the WRB as described in Part 1.
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Lapping with Masonry: The brick shelf angle and mortar must lap over the exterior leg of the through-wall flashing. A 1-inch minimum clearance must be maintained between the back of the brick and the window frame to allow for drainage and tooling of sealant.
Visual: A detail showing the through-wall flashing directing water from the sheathing plane out over the brick ledge, with a clear air cavity behind the brick.
Part 3: Insulated Concrete Forms (ICF)
ICF presents a monolithic, solid wall. The challenge is creating a durable seal on the foam surface and managing water at openings.
Key Adaptations:
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Surface Preparation: The expanded polystyrene (EPS) foam must be smooth and cut cleanly. Irregularities can compromise tape adhesion.
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Reinforced Sill Pan: Because you cannot slope the concrete sill, the sill pan flashing is your entire drainage plane. Use a robust, pre-formed composite or metal sill pan sealed meticulously to the ICF foam with compatible sealant and tape.
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Flashing Integration: ICF-specific WRBs (liquid-applied or sheet membranes) are often used. The flashing sequence is similar to wood frame, but all tapes and membranes must be 100% compatible with the EPS foam and any adhesives used to prevent foam degradation.
Best Practice for ICF: Consider a fully sealed, liquid-applied flashing system for the entire rough opening, creating a continuous, monolithic drainage plane bonded to the ICF, before installing the taped window unit.
The Canadian Climate Imperatives
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Freeze-Thaw & Sealant Joints: All exterior sealant joints must be properly sized (depth-to-width ratio) and tooled. Undersized joints will fail under the expansion/contraction of Canadian temperature swings.
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Air Barrier Continuity: Your flashing tape is part of the air barrier. In regions with stringent energy codes (e.g., BC Step Code, Ontario SB-12), the continuity of this plane must be documented. Use tapes with approved air barrier assembly ratings.
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Managing "Ice Damming" at Heads: In snowy regions, ensure head flashings extend well beyond the plane of the exterior finish and are securely integrated under the siding or masonry above to prevent meltwater from being directed behind the unit.
The Final Checklist
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[ ] Rough opening sill is sloped to drain.
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[ ] Sill pan/flashing is installed first and is continuous.
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[ ] WRB is integrated (lapped or taped) over side flashings.
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[ ] Head flashing is installed before the unit and directs water overthe WRB.
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[ ] Window/door is installed level, plumb, and square.
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[ ] Flashing tape is applied in the correct shingle sequence: Sill -> Sides -> Head.
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[ ] All sealant joints are properly sized, tooled, and compatible.
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[ ] A minimum 1-inch clearance is maintained between the unit and any masonry cladding.
By treating window and door openings as integrated system details rather than punch-throughs to be sealed, you build resilience directly into the envelope. This meticulous approach, documented here for Canada's specific challenges, is the hallmark of professional, durable, and high-performance construction.
