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I am replacing windows in my 1920s house. In modern homes, the sill plate is part of the framing and with rubber flashing, a sill pan is created.

However, my 1920s house has a sloped sill plate made of of redwood that protrudes through the stucco and also acts as part of the exterior window trim:

enter image description here

From the picture you can see there is currently a gap between the bottom of the window and the tilted sill plate.

How does the bottom part of this window need to be properly waterproofed?

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  • You have flashing tape on the jambs. Was this also applied to the sill? It looks like you had molding removed on the jambs. to accommodate the nailing flange. How are you planning to finish the window butting up to the stucco?
    – DelphicOracle
    Commented Sep 18 at 10:26

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I have done many of these sloped sill retrofits; you should:

  • install wood blocks/shims underneath to support the bottom frame
  • fill the space with low expansion foam
  • install quarter round, strip, or other aesthetically pleasing molding to cover the gap
  • seal window and molding perimeter with high quality caulk
  • paint as appropriate

Notes-

  • there are retrofit windows specifically designed for sloped sill application that have an outer flange that projects down from the bottom frame to cover more of that gap. Perhaps consider these as a future option.
  • be careful not to block weep holes. Interestingly, I don't see weep holes on your window, which is not normal for windows that open (perhaps that is a picture window?)
  • you don't need a "sill pan" per se, because your redwood sill already has flashing under it and probably also has weep channels/ports underneath. However, I do usually install flashing tape under the new window (prior to also flashing the sides and top), out to the exterior edge of the new frame, even while I believe this is not strictly necessary.
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  • Thanks! I think the lack of weep holes is because of wood window (Marvin Ultimate). Can that be? In this case, anything specific to take care of?
    – divB
    Commented Sep 18 at 18:50
  • Wrt to your last point (sill pan), how can I make sure it’s properly done? imgur.com/a/Vf0EX1M Shows a more detailed picture how it currently looks like
    – divB
    Commented Sep 18 at 18:53
  • I don't think you can assess the original window construction without removing the new window and then disassembling the entire original sill, which defeats your strategy of using a "insert" style replacement window. I feel that the old original wood looks sound (no rot or signs of water damage), which would give me confidence that it was constructed and flashed appropriately. I will say that you have a hybrid installation going here; usually when stucco work will be involved, we remove all original window structure (including sill) and treat as new "nail on" installation.
    – Jimmy Fix-it
    Commented Sep 19 at 15:39
  • Thank you, I think this is a good answer. I have one more question on the order of flashing and paper ... but I'll create another question on it.
    – divB
    Commented Sep 19 at 17:08

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