Why do new windows leak at the sill and how is it prevented

Spacious and bright room with large glass windows overlooking a serene outdoor setting.

Water that reaches the sill has to be sent back outside, and the sill pan is the only part that does it. The running record window installers riverside museum house on bloor holds shows that leaks after a window replacement in Riverside, CA start at the sill far more often than at the glass or the sash.

What a sill pan is

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A sill pan is a shallow tray under the window made of flashing tape, a formed metal piece or a molded plastic liner. It has a raised back edge, called a back dam, and two end dams at the jambs. The floor of the pan slopes toward the outside, so any water that gets past the frame runs out and over the wall face instead of into the wall.

The order of the tape

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The sill tape goes on first and extends a few inches up each jamb. Then the jamb tape goes on, lapped over the sill tape so that water running down the jamb falls onto the sill layer. The head tape goes last and laps over the jamb tape. Every layer overlaps the one below it the way shingles do. A layer put in backward becomes a channel that carries water inward.

How winter storms find the weak point

Riverside gets few rain days, but when a winter storm comes through, wind drives rain sideways against stucco and into any seam. A hairline gap at a sill corner passes little water in a drizzle and a lot of it in a hard storm, and dry months in between mean nobody notices the damage building behind the wall until the stain shows on the drywall.

Signs the sill has failed

Bubbled paint under the window, a soft or darkened wood sill inside, a musty odor and a line of stain along the wall below are the usual signs. On a vinyl window the frame itself is rarely at fault, since the sealant joint at the corners and the missing pan below are. Fogging between the panes of double pane insulated glass is a different problem, a seal failure in the unit.

What gets checked before leaving

The sill is checked with a level for slope, the pan is checked for open corners, and the weep holes in the frame are cleared on every slider and casement. A good sill pan job is invisible when it is finished, which is why a photo of the tape stage before the unit goes in is worth taking.