Window Leaks and Water Damage: Where the Water Really Comes In (Usually Not the Window), the Stain Patterns That Point at Each Cause, What the Fix Costs, and When the Wall Has to Come Open

Windows & Doors

The Stain Under the Window: Nine Times Out of Ten, the Water Came From Somewhere Above It

A window leak is rarely the window. It is the joint between the window and the wall, the flashing above it that was never there, or water that got behind the siding ten feet higher and found the window on its way down. The stain tells you which, if you know how to read it, and the hose confirms it. Here is how to find the path, what each fix costs, and the point at which the wall has to come open.

Ontario repair prices, October 2026, before HST.

The numbers

$150 to $500most exterior fixes: resealing, a head flashing
$1,550full-frame replacement of one casement, including the flashing
$2,000 to $6,000when the sheathing and framing are wet and the wall opens
5 minwith a hose, per zone, to find the path

Reading the stain

Where the water showsLikely pathFirst fix
Top corners of the window, inside, after wind-driven rainNo head flashing; house wrap not lapped over the head; sealant failed at the top of the trimHead flashing or drip cap over the trim; reseal; $150 to $400
Bottom corners, sill stained, drywall soft belowWater down the jambs from above, collecting at a sill with no pan; or sealant across the sill trapping itHose test to find the entry above; if the sill is rotten, full-frame replacement
One side only, a streak down the jambJamb sealant or tape failed on that side; or a siding joint above that sideReseal that joint over backer rod; check the siding above
Stain appears in a thaw, not in rainCondensation or ice on the glass or frame running down, or frost in the wall cavity meltingCondensation: the house, not a leak
Stain well below the window, or beside it, not at the frameWater behind the cladding from higher up: a roof-to-wall joint, a deck ledger, a window above, a failed brick weepFind the source up the wall; the window is just where it came out
Wet at the sill of a window under a deck or a roof valleyConcentrated runoff hitting the wall above itDivert the water: a kickout flashing, a gutter, a deck drip edge

The hose test

One person inside with a flashlight and paper towels, one outside with a hose on a gentle spray, no nozzle pressure. Start low and work up, five minutes a zone, so you find the lowest entry first: the sill joint, then each jamb, then the head, then the wall above the window a foot at a time, then the next thing up (the window above, the roof edge, the deck). Stop when water shows inside; that is the zone. Wait for it to dry before testing the next zone up, or you cannot tell. Do not spray up under the siding; that is a leak you created. Half an hour and you know more than any contractor standing on the lawn.

What each fix costs

  • Reseal the exterior joints with backer rod and a good sealant, all four sides except where the sill is meant to drain: $150 to $300 a window.
  • Add a head flashing or drip cap where there is none, tucked behind the siding or the wrap: $200 to $500 a window, more in brick.
  • Divert water from above: a kickout flashing at a roof-to-wall joint, a gutter, a drip edge on a deck ledger: $150 to $600.
  • Pull the trim, foam, re-flash the jambs, re-trim: when the tape was never there and the sheathing is still dry: $400 to $800 a window.
  • Full-frame replacement, which is the only way to pan the sill and tape the jambs properly on a window that was never flashed: $1,550 a casement installed, explained here.
  • Open the wall: when the sheathing is soft, the insulation is wet or black, or the framing at the sill is rotten. Siding off, sheathing and framing replaced, new flashing, new window, siding back: $2,000 to $6,000 per opening depending on cladding. Brick is the expensive one.

When the wall has to come open

Push on the drywall below the window. Soft: the wet has been there long enough to rot the paper and the framing behind it is probably worse. Take the casing off and look at the jamb framing and the sill: black, soft, or a screwdriver goes in. Outside, if there is a gap at the bottom of the siding, look at the sheathing: black OSB is done. Any of those, and resealing is painting over it. The wall opens, the rot comes out, the opening gets flashed properly, a new window goes in, and the cladding goes back. It is the job nobody wants and it is cheaper this year than next, because rot does not stop.

Where homeowners go wrong

  1. Replacing the window for a leak above it. The new window gets the same water from the same roof joint.
  2. Caulking the inside. Water stays in the wall.
  3. Caulking the sill drain. The pan was meant to let water out; now it holds it.
  4. Retrofit insert into a leaking frame. The leak is in the frame or behind it; an insert seals it in.
  5. Waiting. A $300 reseal in year one is a $4,000 wall in year six.

Questions people ask

Why is water coming in around my window?

Usually from above: no head flashing, failed sealant, or water behind the siding from a roof or deck joint higher up. The hose test finds which in half an hour.

Does home insurance cover window leaks?

Sudden damage from a storm, often; slow leaks from failed sealant or missing flashing, usually not, because they are maintenance. Document it and ask, but budget as if it is yours.

Should I replace a leaking window?

If the frame or the sill is rotten, yes, full-frame, with proper flashing. If the frame is sound and the water is coming from a joint or from above, fix the joint and keep the window.

Not sure what you are looking at?

Photos of the stain, the outside of the window and the wall above it tell most of the story. The $99 review reads them with whatever quotes you have, and says whether this is a $300 reseal or a wall. The patio door page is the same story at floor level.

Review my quotes, $99How it should have been flashed

Sources and further reading