Why does my window leak when it rains, and how do I fix it?

Milgard's own test for a window's drainage takes 15 minutes and a roll of tape. You tape over the weep holes, fill the track halfway with water, and wait. If the level holds, the weep system isn't defective, and your leak is coming from somewhere a pipe cleaner can't reach.

That's the whole problem with a window that leaks in the rain. Three or four different failures all put water on the same windowsill, and the first fix most people reach for, a bead of sealant, only addresses one of them.

Start with where the water appears

Before you buy anything, wait for the next storm and watch the window. Note three things: exactly where the water shows up, whether it happens in every rain or only in wind from one direction, and whether there's water when it hasn't rained at all.

Those cost ranges aren't from a primary source. They reflect what contractors commonly charge, and they move with your region, the story the window is on, siding type, and how much rotted wood turns up once trim comes off. Treat them as a sense of scale, not a quote.

If the water only appears on cold mornings, it isn't a leak; our condensation guide covers that. Fog trapped between the panes is a failed glass seal, a separate problem with its own warranty clause.

The free fix: weep holes

Vinyl and aluminum sliders and single-hungs are built to let some water in. Rain that gets past the sash collects in the sill track and drains out through small slots on the exterior face of the frame. When those slots clog with dirt, insects, or paint, the track fills like a gutter and overflows indoors.

Milgard's care guidance tells owners to inspect the weep openings on the exterior bottom rail periodically, clear them with a soft brush, and do it when heavy rain is in the forecast. Its sliding-door guide uses a pipe cleaner and then pours water into the track to confirm it drains. Never caulk or paint over a weep slot. Sealing it shut turns a drainage path into a reservoir.

If water in the track drains right out the weeps, the weep system is working. Move up the list.

Exterior sealant: a real fix, with limits

The bead of sealant between the window frame and the siding or trim is a weather seal, not the primary water barrier. When it cracks, pulls away from one side, or goes chalky, wind-driven rain gets behind the trim.

Replacing it is a reasonable first-floor job. The product matters more than the brand. A window-and-siding sealant like Henkel's OSI QUAD Max lists a recommended joint width of 1/4 to 5/8 inch and movement capability of plus or minus 50 percent. That's why you cut out old caulk instead of smearing new caulk over it: new sealant bonded to failing sealant fails with it.

Two cautions. First, EPA's mold guide says not to caulk or paint over moldy surfaces; clean and dry them first. Second, if your house was built before 1978, scraping painted trim can trigger EPA's Renovation, Repair and Painting rule. EPA treats disturbing 20 square feet or less of exterior paint as minor maintenance, but a paid contractor still needs to follow lead-safe practices, and window replacement is covered no matter how small the job.

Here's the rule: if you've recaulked the same window twice and it still leaks, stop buying sealant. The water is coming in behind the sealant, through the flashing, and only removing trim or the window will reach it.

Water at the head: look above the window

Water that drips from the top of the frame or runs down the inside of the side jambs usually entered above the window. Sometimes that's the window's own head flashing. IRC Section R703.4 requires corrosion-resistant flashing applied shingle-fashion at exterior window and door openings, and above projecting wood trim. A window with a wide painted head casing and no metal drip cap over it is a common miss.

Sometimes the water entered much higher: an overflowing gutter, a wall-to-roof intersection, or a roof leak that tracks down inside the wall and exits at the first opening it finds, which is often the window head. If the leak lines up with gutter overflow, start with the gutter checklist. If it lines up with heavy rain on the roof, a roof leak is the likelier source. A second-floor window head is not a ladder job for a homeowner. Have a contractor inspect it with fall protection.

Water at the lower corners: the sill pan

This is the expensive one, and it's the most commonly misdiagnosed. Water that appears at the bottom inside corners of the window, stains the drywall below the sill, or swells the interior stool points to a failure under the window, not around it.

IRC Section R703.4.1 says to flash window openings according to the manufacturer's installation and flashing instructions. Where no instructions are provided, pan flashing goes at the sill, and the pan has to be sealed or sloped to drain water to the exterior wall surface or the water-resistive barrier. A correctly installed pan catches whatever gets past the frame and sends it back outside. Without one, that water runs straight into the framing.

No sealant on the outside fixes a missing pan. The fix is pulling the window, installing a pan (formed membrane or a prefabricated pan with a back dam at the interior edge), and reinstalling or replacing the unit. If the sill framing is soft when the window comes out, that repair gets added on top. This is where a rot allowance on the bid matters; our full-frame vs. pocket replacement guide explains why a pocket insert can't fix this.

Before you call: the warranty question

Manufacturer warranties generally won't pay for a rain leak. Andersen's 400 Series warranty, for example, excludes water infiltration other than from a defect in manufacturing, materials, or workmanship, and separately excludes problems from improper installation. In practice, a flashing leak is the installer's problem, not the window maker's.

So if the window went in recently, call the installer first and ask what their labor warranty covers. Bring photos from the storm, the date it went in, and a note of the wind direction when it leaked. A contractor who tells you to "just caulk it" when the water is at the lower inside corners is guessing.

Whatever the cause, deal with the wet wall promptly. EPA's guidance is to dry water-damaged materials within 24 to 48 hours to prevent mold growth, and it notes that if mold appears you need to fix the water source, not just clean the surface. EPA says a homeowner can usually handle a mold patch of about 10 square feet or less; anything larger calls for more caution, and often a professional.

Research sources

• ICC: 2021 International Residential Code, Section R703.4 — Flashing
• ICC: 2021 International Residential Code, Section R703.4.1 — Flashing installation at exterior window and door openings
• Milgard: Window and door care tips
• Milgard: How to test the weep system (PDF)
• Milgard: How to clean weep holes on a vinyl sliding door
• Henkel: OSI QUAD Max Clear technical data sheet (PDF)
• Andersen: 400 Series Windows & Patio Doors Limited Warranty (PDF)
• EPA: A Brief Guide to Mold, Moisture and Your Home
• EPA: Renovation, Repair and Painting Program: Contractors
• EPA: RRP minor repair and maintenance thresholds (6 sq ft interior / 20 sq ft exterior)