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Old 05-16-2009, 01:23 PM   #1
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Sill Flashing for Replacement Windows

I am replacing an aluminum kitchen window with a vinyl window in an 27 year old house in Oklahoma. I have the old window out and the siding cut to accommodate the nailing fins on the new window. There didn't seem to be any sort of sill flashing or any kind of flashing for that matter. There is just the siding, the styrofoam type sheathing, insulation and the framing.

How do I install the flashing and what type? I have the sticky tape type but would like to be sure. I can send pix if necessary.

Thank you


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Old 05-18-2009, 08:12 PM   #2
Craig O
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Sill flashing

Nail fin windows aren't usually installed with any type of sill flashing like a door, sliding door or block style window. Since the "nail-fin" covers the face of the sill, there is no need for sill flashing since typically water cannot reach back to the sill because it will be stopped at the face of the window.

Using an AAMA approved flashing method A or B will be sufficient for waterproofing the window however, if you are concerned that water will penetrate to the sill, you can create you own sill flashing by using a bitumen based peel and stick flashing. Start 8 inches up on the jambs and extend the flashing down and across the sill to the other jamb and up again eight inches. If you are installing on a 2x4 and your flashing is 9" (recommended) you will have to cut the face of the flashing and wrap it down and across to finish it off.

This will create a weak corner at the bottom but wil be covered by your standard flashing and the nail fin. You can caulk the weak corner and fan it out in the shape of a ping pong paddle. Make sure you allow two gaps of approximately 1" to allow for weeping. If water gets onto the sill but you caulk the exterior tightly, water will have nowhere to go but to the inside of your home.

For more information see


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Old 05-19-2009, 01:10 PM   #3
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Thank you. That's pretty much what I did.

But you mentioned the following:

>Make sure you allow two gaps of approximately 1" to allow for weeping.

Where are these gaps supposed to be?

Thanks, again.
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