The Leaking Patio Door Threshold: Why It Is Always the Patio Door, Where the Water Really Comes In, the Sill Pan Nobody Installed, and the Soft Floor That Tells You

Windows & Doors

The Patio Door Leaks Under the Sill, Not Through the Glass. Here Is Why, and the $12 Pan That Would Have Stopped It.

Every builder has stood in a family room with the flooring peeled back from the patio door and black plywood underneath. Nobody saw it leak. It never dripped. It ran under the sill, into the subfloor, for ten years, because the door was set on three shims and a bead of caulk on the outside, and water got past the caulk the second summer. Here is where the water actually comes in, and how the new door is supposed to be set so it does not.

Ontario repair prices, October 2026, before HST.

The numbers

$12for a sill pan or a roll of flashing tape
$450to cut out and patch a soft subfloor at the sill
$1,200+when the rim joist has gone with it
1/2 in.the threshold should sit above the deck or pad outside

Where the water comes in, in order of likelihood

  1. Under the sill. The sill is a hollow aluminum or vinyl extrusion sitting on the subfloor. If it was set on shims with no sealant bed and no pan, water that gets to the outside edge of the sill (rain off the glass, a leaky weep, snow melt against the door) runs under it and into the plywood. This is three-quarters of patio door leaks.
  2. The deck or pad is at or above the threshold. Water sits against the door with nowhere to go. A deck board flush with the sill is a dam. The code and the manufacturer both want the outside surface below the threshold, with slope away.
  3. Blocked weep holes. The outer track drains through small holes at the ends. Grit, leaves and ice block them, the track fills, and the water goes over the inside lip. Clean them every spring: $0.
  4. The sill-to-jamb corner. The joint where the side frame meets the sill, outside, should be sealed from the factory and again on site. It cracks open as the frame moves. $20 of sealant, if it is caught.
  5. Over the head. No head flashing, or a window above draining onto the door. Water runs behind the trim and down the jambs. The rarest, and the one that shows as a stain at the top corner inside.
  6. The glass. Almost never. Condensation on the inside in winter running down onto the sill looks like a leak and is your humidity. Condensation.

How to tell which you have

  • Bounce on the floor 12 in. inside the door. Soft or spongy: the subfloor is wet, and it is coming under the sill. Plan on replacing the door; a sill that has leaked that long will not seal with caulk.
  • Look at the deck or pad outside. Level with or above the threshold: that is the cause, or part of it.
  • Pour a cup of water in the outer track. It should drain out the weep holes in a minute. If it sits, the weeps are blocked.
  • Run a hose on the glass for five minutes with someone inside. Water appears at a corner: the corner joint. At the top: head flashing. Along the whole inside edge of the sill: under the sill.

What a proper sill looks like on the new door

The old door comes out and the subfloor at the opening gets looked at and repaired. Then either a formed sill pan (a plastic or metal tray with the ends turned up, sitting on the subfloor and lapped over the sheathing outside) or a self-adhered flashing membrane shaped the same way with the corners patched. The pan slopes out or is dead level and drains out, never in. The door sill sits in the pan on a bed of sealant, not on shims; shims go under the sill only where the pan allows, and the sealant fills around them. Outside, backer rod and sealant between the sill nosing and the deck or pad, which is at least 1/2 in. below the threshold. Then the jamb flashing tape laps over the pan’s upturned ends, and the head flashing laps over the tape. Water that gets anywhere ends up outside.

Ask your installer one question: “What goes under the sill?” The right answers are “a sill pan” or “flashing membrane and a sealant bed”. The wrong answer is “it sits on the floor and we caulk the outside.” That is the door you are replacing now.

What the repairs cost

FixCostWhen
Clean weeps, reseal the corner joints and the exterior sill joint$150 to $300Floor is sound; leak is small and recent
Cut the deck back or lower the pad; add a drain strip$200 to $800The outside is above the threshold
Pull the door, pan the sill, reset the same door$600 to $1,000Door is good, sill was never panned, floor is sound
New door with pan, subfloor patch$3,650 and upFloor is soft within a foot of the sill
Plus rim joist and sill plate repair+$1,200 to $3,000Floor is soft further in, or the outside wall sheathing is black

Where homeowners go wrong

  1. Caulking the inside. It traps the water in the floor.
  2. Replacing the door the same way it was set. Same leak, new door, five years.
  3. Building the deck up to the threshold so there is no step. A dam with a nice view.
  4. Blaming the glass. Sealed units do not leak liquid water. Fog between the panes is a different problem.

Questions people ask

Why does my sliding door leak when it rains?

Usually under the sill, because it was set without a pan or sealant bed, or because the deck or pad outside is at threshold height. Blocked weep holes are the cheap one to check first.

Can a leaking patio door be repaired?

If the floor is sound, yes: weeps, corner seals, and sometimes pulling and re-setting the door on a pan. If the floor is soft, the door comes out and the floor gets fixed, and at that point a new door is the sensible move.

Does a patio door need a sill pan?

Every manufacturer’s instructions say so, and the house would agree. The code requires the opening to be flashed so water drains out; a pan is how that is done at a sill.

Price the door with the floor

The door calculator carries the subfloor and rim joist allowances as their own menu, and the scope it writes asks for the sill pan by name.

Price a new patio doorWindow leaks

Sources and further reading