Water enters most BC houses through the windows, not the roof. Building envelope consultants report it, and it is what our crews find every time we open a wall around a leaking sill. The assembly can be a perfect rainscreen with three inches of exterior insulation, and if the head flashing is missing or the sill pan slopes the wrong way, water finds it and rots the framing inside two winters.
Window flashing in BC is the small, cheap layer of the wall that decides whether a $180,000 exterior renovation lasts twenty years or ten.
Why do windows leak in BC houses even when the wall looks fine?
Almost every window leak we open in BC comes from one of three places: no head flashing above the frame, a sill pan that slopes back into the wall instead of out to the drainage plane, or membrane tapes lapped in the wrong order so that water running down the WRB is directed behind the flange instead of over it. Water always wins against caulk. It only loses against gravity, sequencing, and drainage.
The four flashing details that actually keep water out
A window or door in a BC wall lives inside four layers of water management, all of which have to work together. Miss any one and the assembly leaks. The other three cannot compensate.
Sill pan flashing. A rigid or flexible pan under the window that catches any water past the frame and drains it back out to the WRB. Sloped away from the interior at 5-10 degrees, with a back-dam at least 1/4 inch tall and end-dams turned up at both jambs.
Jamb membrane. A self-adhered flashing tape (typically 4-6 inches wide) sealing the vertical sides of the rough opening to the WRB. Applied AFTER the sill pan and BEFORE the head flashing, so the sequencing shingle-laps.
Head flashing. A metal or membrane cap above the window that directs water outward and away from the frame. In cladding with cavities (rainscreen, siding, stucco with a drainage plane), this is a separate metal cap. In face-sealed assemblies (the older stucco most BC houses wear), it is either a formed metal drip or a membrane tape terminated onto the WRB above.
Weather-resistive barrier tie-ins. The WRB (Typar, Tyvek, membrane wrap) has to lap OVER the head flashing and UNDER the sill pan, in that order. Water draining down the outside face of the WRB has to run over the top of the head flashing and clear of the opening.
Everything else on a window install (the fasteners, the shims, the exterior sealant bead) is secondary. Those four details are the whole job.
Where BC installers cut corners
We have opened enough failing walls in Metro Vancouver to know the pattern. Roughly two-thirds of the leaking windows we retrofit share the same three shortcuts.
The first is skipping the sill pan entirely and relying on a bead of caulk under the flange. Caulk will not last ten years against BC's freeze-thaw and UV, and when it cracks the water goes straight into the framing. On coastal exposures (Delta, Tsawwassen, Bowen, Powell River) the failure runs faster because wind-driven rain hits sideways, not down.
The second is reversing the membrane lap order. The tape goes on in a hurry and someone runs the head tape first, then jambs over it, then sill over both. The physics of that stack sends any drainage water directly behind the window flange on the jambs. It is a subtle mistake that inspectors miss, because from the outside the tape looks continuous. It only shows up eighteen months later when you see paint bubbling on the interior drywall below the sill.
The third is using butyl or acrylic flashing tape not rated for local temperature swing. BC's coast runs from -8°C to 32°C in a single year on the wall face. A tape rated for interior work will lose adhesion in the first cold snap.
Head flashing: the small metal piece that fails last but costs most
If you can only fix one detail on an existing wall, fix the head flashing. It keeps roof-driven water off the top of the window, and it is the one most often missing on 1980s-2000s BC houses.
A proper head flashing is a formed 24-gauge galvanized or prefinished metal piece: one continuous length across the top of the window, hemmed at the front edge, kicked out at 6-10 degrees, with end-dams turned up at least 1/2 inch. It sits on top of the head casing (or the WRB in a flanged install) and the cladding above laps down onto it, leaving a 1/4 inch clear gap so drainage water can escape.
On a rainscreen assembly the head flashing is straightforward, because the drainage cavity behind the cladding gives the flashing somewhere to shed to. On a face-sealed stucco wall, which is what most BC houses built between 1985 and 2004 have, the head flashing has to be integrated into the stucco system with a two-piece drip and a proper reveal, or the water runs behind the stucco and rots the sheathing.
We regularly find single-family homes in Vancouver, Burnaby and Coquitlam where every window was installed with a bead of caulk at the head and nothing else. Twenty years later the framing above every opening is soft. The fix, at that point, is a full window replacement with a rebuilt head detail, not a caulk touch-up. A proper stucco and EIFS contractor will price that reveal as part of the scope; a low bid will paint over the problem.
Sill pans: what a proper one looks like
The sill pan is the most important flashing detail on any BC window installation and the one most likely to be done badly. A proper sill pan has three things: slope, a back-dam, and end-dams.
Slope. The pan surface tilts outward at 5-10 degrees so any water inside the opening drains toward the exterior. A flat pan collects water and holds it against the framing.
Back-dam. A vertical lip on the interior edge of the pan, at least 1/4 inch tall, that stops water from being blown or pushed back into the wall cavity by wind pressure. On coastal BC exposures we spec 3/8 inch minimum.
End-dams. The pan turns up 1-2 inches at each jamb, forming a shallow tray. Water landing in the pan cannot escape sideways into the framing. It can only exit out the front.
The pan itself is either a formed metal piece (folded from coil stock, or a proprietary product like SureSill), a flexible self-adhered membrane bent into shape, or a hybrid with membrane over a metal L. For BC exposures we recommend the metal or hybrid options for exposed elevations, membrane for sheltered ones. Pure membrane sill pans work but require the installer to form the corners properly, which under time pressure often does not happen.
Membrane sequencing: shingle-lap, not caulk
The word installers should have tattooed on their forearms is "shingle-lap." Water running down the WRB is directed by gravity. If every horizontal joint laps OVER the piece below, water runs off. If it laps under, water is directed into the wall.
The correct order at a window is:
- Cut the WRB in an inverted-Y or modified-I pattern.
- Fold the flaps in and staple.
- Install the sill pan first, extending 6 inches past the jambs on each side.
- Install jamb membrane second, running from 4 inches below the sill up to 4 inches above the head. The jamb membrane laps OVER the sill pan flanges.
- Set the window.
- Install head membrane third, laid across the top of the head flange, extending 4 inches past the jambs on each side. It laps OVER the jamb membrane.
- Fold the WRB flap above the window DOWN over the head membrane and tape.
- Install the metal head flashing, integrated with the cladding.
Skip any lap or reverse any order and you have created a channel that directs water into the wall. Every hour of care on this sequence saves five figures of remediation later.
Retrofit vs new construction flashing
The details above assume new construction, where the WRB is exposed and the installer can work with a clean rough opening. Retrofit is harder.
On a retrofit, say a strata council doing window replacement on a 1998 mid-rise, or a homeowner replacing failed vinyl windows on a Vancouver Special, the original flashing is buried behind cladding that is not being removed. The installer has three options.
Option one is remove enough cladding around each opening (typically 12-18 inches of stucco or siding around the perimeter) to expose the WRB, install proper flashings, then patch the cladding. This is the correct approach and the one we default to on projects where the envelope has any history of leaks. Cost per opening runs $2,800-$4,500 depending on cladding type and access.
Option two is "flanged install with exterior detailing." Set the new window into the existing opening, seal the flange to the exterior of the cladding with membrane and a metal head flashing, and rely on that exterior layer alone. Fine on sheltered exposures, marginal on exposed ones. Cost runs $1,400-$2,200 per opening. This is what most low-bid installers do.
Option three is "no flashing at all, just caulk." Do not accept this on any project. It is a five-year fix that creates a fifteen-year problem.
For strata properties working through a depreciation report, the difference between option one and option two is often the difference between a real repair and a repeat inspection three years later. Any building envelope contractor worth hiring will price option one by default and tell you why.
Real 2026 BC cost to do the flashing properly
Rough per-window cost, delivered by a licensed contractor in Metro Vancouver, current pricing:
- New construction, standard vinyl or aluminum window, proper flashings, no cladding rework: $180-320 per opening for flashing labour and materials (window and install separate).
- Retrofit with cladding cut-back, stucco patch, full flashing rebuild: $2,800-$4,500 per opening. This is the number that surprises homeowners.
- Retrofit with membrane wrap and metal head flashing, no cladding removal: $1,400-$2,200 per opening.
- Strata whole-building window replacement, per unit: $4,500-$9,000 per opening depending on floor, access, cladding type, and the state of the substrate once opened.
A typical Vancouver single-family with 14 windows and 2 exterior doors runs $45,000-$70,000 for a proper full retrofit with flashings and stucco patch. The number that shocks people is that this is 80-85% labour and 15% materials. Good flashings cost $40-90 per window in tape and metal. The rest is time, and time is what a low bid strips out first.
What good specification actually looks like
If you are writing an RFP for exterior work or reviewing a contractor's scope of work, ask the contractor to specify these details in writing:
- Sill pan material and slope
- Jamb membrane product and lap direction
- Head flashing gauge, profile, kick-out angle, end-dam height
- WRB-to-flashing lap sequence
- Sealant type at head and jambs (or absence of sealant: a proper head detail does not need it)
A contractor who cannot write those five lines has not thought about the detail. A contractor who can may still cut corners on site, so ask for on-site inspection points at rough opening, post-flashing, and post-cladding. Every strata renovation we do carries those hold points as a standard clause. On a homeowner project, adding a third-party envelope consultant for $2,500-$5,000 pays for itself if it catches even one bad installation.
Ten years of exterior building envelope work across Greater Vancouver has shown us the same lesson every time: the flashing detail is the cheapest line item on the job and the one that decides whether the wall lasts. MV Construction is fully licensed and WCB-insured, with warranty-backed workmanship on every envelope project. That warranty is only underwritable because we do these details properly the first time.
The direct recommendation
For any BC window or door install: specify a formed metal or hybrid sill pan with 5-10 degree slope, a 1/4-3/8 inch back-dam, and turned-up end-dams. Use a temperature-rated self-adhered membrane on jambs, applied over the sill pan. Install a formed metal head flashing with 6-10 degree kick-out, hemmed front edge, end-dams up 1/2 inch, integrated properly with the cladding above. Sequence the WRB shingle-lap: WRB over head flashing, jamb tape over sill, head tape over jamb. Do not rely on caulk to do the water management job.
For a retrofit on an older BC home, budget for cladding cut-back around each opening. It is more expensive up front and dramatically cheaper over twenty years than the alternative.
MV Construction handles exterior renovation, stucco, and building envelope work across Greater Vancouver, Burnaby, Coquitlam, Surrey, and the Fraser Valley. If you are planning a project where windows or doors are being replaced and want the flashing detail done right, request a fixed-scope estimate or call (778) 375-5672.
FAQ
Do I really need a sill pan on a small bathroom window? Yes. Water finds any opening, and a bathroom window is often on a coastal exposure where wind-driven rain hits sideways. The sill pan takes 15 minutes to install and prevents 10 years of hidden rot.
Can flashing tape replace metal head flashing? On sheltered exposures and residential retrofits, membrane head flashing can work if the tape is temperature-rated and the WRB integration is correct. On exposed elevations, coastal locations, and any commercial or strata project, use formed metal. Membrane alone does not have the rigidity to hold its shape over 20 years.
Why does new caulk fail so fast in BC? UV exposure, temperature swing (-8°C to 32°C on the wall face), and wind-driven rain all attack caulk joints. Even top-tier polyurethane sealants have a real-world lifespan of 7-12 years on exterior joints in BC. Water management should never depend on caulk as the primary line of defence.
Is head flashing required by BC Building Code? The BC Building Code requires wall assemblies to manage water intrusion; it does not spec every flashing detail. That said, most municipalities' inspectors expect visible head flashing on new construction, and its absence is a common inspection flag. On existing homes there is no code trigger to add one, but if you are opening the wall for any reason, adding a proper head flashing is the least expensive envelope upgrade you will ever make.
How do I check if my existing windows have proper flashings? Without opening the wall, you cannot. What you can check: is there a visible metal head flashing above each exterior window? If not, that is a red flag. Does the exterior caulk at heads and jambs look intact? If it is cracked, split, or missing, water has been getting in. Any staining or paint bubbling on the interior wall below a window is a sign of water damage that a licensed contractor should investigate.
