Most window leaks after rain trace back to failed flashing, blocked weep holes, or worn out caulking, not a cracked pane of glass. Your first move isn’t to grab a caulking gun. Document where the water shows up, dry out the interior, and run a controlled hose test to find the actual entry point. If you spot soft framing, mould, or water that keeps coming back no matter what you try, stop troubleshooting and get a professional inspection before the damage spreads.
TL;DR:
- Most window leaks result from failed flashing, blocked weep holes, or worn caulking, not a cracked glass pane, requiring inspection before sealing.
- A hose test performed zone by zone helps identify whether water enters at the sill, jambs, or head, indicating blocked drainage or failed flashing.
- Recurring leaks after resealing often suggest underlying issues with flashing or sill pan, not just caulk failure, and demand professional inspection.
- Proper DIY intervention involves clearing weep holes and replacing failed caulk with weather-rated sealant, avoiding sealing blocked drainage paths.
- Regular inspections of caulk lines, gutters, and trim, plus seasonal maintenance, prevent leaks and reduce the need for costly repairs later.
Table of Contents
- Why is my window leaking, and how do I find the source?
- What actually causes water to seep through windows
- How do you fix a window leak yourself, at least temporarily?
- When should you call a professional instead of DIY-ing it?
- How do you prevent window leaks before the next storm?
- What a caulking pro actually checks before touching a joint
- A contractor’s honest take on quick fixes
- Get a proper inspection instead of another temporary patch
- Sources
- FAQ
Why is my window leaking, and how do I find the source?
You can’t fix what you can’t find. Before anyone touches a tube of sealant, the entry point needs to be pinned down, and the most reliable way to do that is a controlled garden-hose test done zone by zone.
Here’s how it works on the job:
- Get a helper inside the house with a flashlight and a phone camera. You’ll want them watching the frame, not just standing there.
- Set your hose to a fan spray, not a jet. A jet forces water where it normally wouldn’t go and gives you a false reading.
- Start low, at the sill, and hold the spray there for a full five minutes before moving up.
- Move to the jambs (the sides) for another five minutes each, then finish at the head (the top).
- If wind was blowing rain sideways during the actual leak, angle the hose to mimic that direction once you’ve done the straight test.
What the results tell you matters as much as the test itself. Water showing up at the bottom usually points to blocked weep holes or a failed sill pan. Leaks at the jambs often mean the caulk joint between the frame and the cladding has let go, or the housewrap behind it tore. And if water shows up at the head, even though you sprayed the sill first, that’s a strong sign of missing or improperly lapped head flashing, because water is travelling down inside the wall before it ever reaches your interior trim.
Pay attention indoors, too. A stain that appears within minutes of rain starting suggests a direct path. One that shows up hours later, or only during a multi-day soak, often points to water travelling through the wall assembly first. Soft drywall, bubbling paint, or a musty smell near the frame are all clues worth photographing for your own window water test record, or for a contractor later.
One limit worth knowing: a hose test recreates gravity-driven water, but it can’t fully reproduce wind-driven rain hitting a wall at an angle during a real storm. Skip the ladder work in bad weather, and never test above the first storey without proper fall protection.
Pro Tip: Run each test zone for the full five minutes, even if you think you’ve already found the leak. Water often needs that long to travel through a gap and show up where you’re watching.
What actually causes water to seep through windows
Where the water shows up on the inside is your best clue to what’s failing on the outside. After 25 years of chasing these leaks around the GTA, the pattern almost always sorts into one of four categories.
- Sill problems: Clogged weep holes, a missing or damaged sill pan, or sill caulk that’s cracked and pulled away from the frame.
- Jamb problems: Caulk between the frame and the brick or siding has failed, or the housewrap behind the trim is torn, letting water track sideways into the wall.
- Head problems: No head flashing, or flashing that’s lapped backward so water runs behind it instead of over it. Sometimes it’s not the window at all. It’s a gutter overflowing right above the frame.
- Systemic problems: Saturated cladding or plugged gutters dumping water faster than the wall can shed it, or condensation being mistaken for intrusion.
That last point trips up a lot of homeowners. Condensation forms on cold glass or a cold frame and pools on the sill, but it doesn’t leave a trail up the wall or stain the drywall below the window the way true intrusion does. If you’re not sure which you’re dealing with, the hose test above will settle it, since condensation won’t respond to water hitting the exterior.
It’s worth understanding why some fixes hold and others don’t. Building science guidance consistently favours drained installations over barrier-only ones, meaning the assembly is built to let a little water in and then guide it back out through pan flashing and shingle-lapped layers, rather than trying to seal everything perfectly on the surface. A barrier approach that relies entirely on caulk to keep water out has no backup plan when that caulk eventually cracks.
How do you fix a window leak yourself, at least temporarily?
Not every leak needs a contractor on day one. Some genuinely resolve with careful DIY work, and even the leaks that need a pro benefit from you limiting damage in the meantime.
Start inside the moment you notice water:
- Photograph the stain, the timing, and the weather conditions. This record becomes useful for insurance and for any contractor you eventually call.
- Dry the area with towels and run a fan or dehumidifier to stop moisture soaking further into drywall or trim.
- If wet insulation or trim is loose and easy to pull without damaging anything structural, remove it so it can dry rather than mould.
On the exterior, weep holes are the easiest win. A thin wire, a toothpick, or a purpose-built weep hole cleaning tool will clear the debris that’s blocking drainage at the sill. Do this before you touch a caulking gun. Sealing over blocked weeps traps water inside the wall instead of letting it drain, which makes the problem worse, not better.
If the caulk itself has failed, cut it out completely rather than caulking over the old bead. Wait for a dry day, clean the joint, and apply an exterior-grade polyurethane or hybrid sealant built for movement and Canadian temperature swings.
Pro Tip: Never caulk a weep hole shut, even by accident. It’s the one place on a window where you want water to escape, not get sealed in.
These fixes buy time and sometimes solve the problem outright. But if the leak keeps returning after a proper re-caulk, the issue usually isn’t the sealant. It’s the flashing or sill pan underneath it, and that’s a different repair entirely.
When should you call a professional instead of DIY-ing it?
Some signs mean it’s time to stop testing and start booking an inspection. Recurring leaks after you’ve already re-caulked, soft or rotting framing, visible mould, leaks you can’t reach safely on a second storey, or water showing up in multiple spots around the same window all point to something beyond a surface fix.
A competent contractor doesn’t guess. Expect them to run their own hose test, open up trim or siding where necessary to inspect the weather-resistive barrier and flashing, and address the sill pan or head flashing directly rather than just laying new caulk over old problems.
Cost depends heavily on scope:
- Clearing weep holes and re-caulking a joint: usually a modest job, often in the range other trades quote around $50 to $400 for materials and labour on a single window.
- Sill pan or flashing repair: a bigger job, since it involves removing trim or siding to correct the drainage path.
- Full reflash or window removal: the most involved scope, reserved for windows with structural rot or repeated systemic failure.
Ask any contractor about the sealant class they’re using, whether it’s rated for freeze-thaw movement, and whether they’ll document the repair with before-and-after photos. That documentation protects you if the same spot ever acts up again.
How do you prevent window leaks before the next storm?
Prevention is cheaper than repair, every single time. A short seasonal routine catches most problems before they become drywall stains.
- Inspect caulk lines around every window each spring and fall, looking for cracking, gaps, or sections that have pulled away from the frame.
- Clear weep holes as part of the same walk-around. It takes two minutes per window.
- Clean gutters and downspouts before the fall rains start, since overflow above a window is a common and avoidable cause of head leaks.
- Check trim and siding condition near windows for soft spots or discoloration that suggest water is already tracking through.
When it’s time to reseal, quality matters more than price. Sealants rated under ASTM C920 for movement capability hold up far better through Ontario’s freeze-thaw cycles than a bargain tube that skips the movement rating entirely. And know the difference between re-caulking and re-flashing: caulk handles surface gaps, but if water is getting behind the cladding, no amount of fresh caulk fixes that. Keep a simple leak log with dates, weather, and photos. Patterns in that log make contractor diagnosis faster and cheaper.
What a caulking pro actually checks before touching a joint
Before removing anything, a trained eye tests adhesion on the existing bead. Sealant that’s still bonded and flexible sometimes gets left alone; sealant that’s cracked, chalky, or lifting away from the substrate gets cut out completely; there’s no in-between with failed caulk.
Joint prep is where most amateur re-caulk jobs fall apart. You need clean, dry substrate, the right primer for the material you’re bonding to, and a sealant class rated for the joint’s expected movement, not just whatever’s on the shelf. In a climate that swings 60 degrees between January and July, that movement rating is the whole game.
Fixing drainage and flashing solves the leak. Fresh caulk over a bad flashing detail just hides it for another season.
A contractor’s honest take on quick fixes
Quick cosmetic caulk jobs fail constantly in Ontario because freeze-thaw cycling stresses joints that weren’t prepped or rated for movement. I’ve opened up plenty of “recently caulked” windows and found the real problem sitting untouched behind the trim. Document every leak with a date and a photo. It saves you money later, and proper flashing work costs more upfront but it’s the repair you don’t revisit every spring.
— Felix
Get a proper inspection instead of another temporary patch
This company offers professional caulking services with climate-rated materials and transparent pricing for window leak repairs, aiming for durable results suitable for freeze-thaw conditions rather than short-term patches.
A service call starts with the same diagnostics covered above: a full inspection, a hose test where needed, and a written scope and quote before any work begins. If the fix is a straightforward reseal, we handle it through our Residential Window Caulking service. If the hose test points to a compromised flashing detail or drainage plane, that falls under our Building Envelope Sealing work, which addresses the sill pan and flashing directly instead of layering caulk over a bad detail. Book an inspection and get a clear, itemized quote before anything is touched.
Sources
- Window Leaks: Testing With a Hose to Find the Source
- Water management for residential construction (Building America)
- Windows and doors are fully flashed (BASC/PNNL resource guide)
- Storm-driven rain penetration of windows, skylights and doors (FGIA white paper)
FAQ
Will insurance cover a window leak?
It depends on the cause and how long the leak went unaddressed. Sudden, accidental water damage is often covered, but damage from a maintenance issue you knew about and ignored, like clogged weep holes for two seasons, usually isn’t. Keeping a leak log with dates and photos gives you the documentation an adjuster will ask for.
Is a window leak an emergency?
Not always, but treat active water intrusion during a storm as urgent enough to contain immediately with towels and a fan. Extraordinary wind-driven rain can occasionally exceed even a compliant window’s tested rating, so a one-time leak during a severe storm isn’t necessarily a sign of failure. A leak that recurs in normal rain is the one that needs prompt attention.
Can you reseal a leaking window?
Yes, resealing fixes leaks caused by cracked or failed caulk, provided the underlying flashing and drainage are intact. If the leak keeps returning after a proper reseal, the flashing or sill pan is the actual problem, and that requires opening the trim rather than another bead of caulk.
Why would a window leak from the bottom?
Bottom leaks almost always point to blocked weep holes, a missing sill pan, or sill caulk that’s pulled away from the frame. Clearing the weep holes is the first thing to check, since a fully drained sill design is meant to shed water automatically when those channels are open.
