*I'm sorry about the lengthy description. I'm photographically challenged at the moment.

I went about installing a two side by side, double hung Marvin integrity windows. After doing that, my dad said, "wouldn't it be nice to have a large window sill on the inside?". I went to look at some recent construction. The carpenter had put in window sills on all the windows. At a glance I thought that he removed the entire bottom jam of the premade windows, replacing it with a larger piece of wood. I can't do that now.

These Marvin integrities have a strip of wood about 3/4 of an inch high which conceals the bottom of the window sash. My best guess right now is that I pull these off and put the window sill on that same plane, right up to where they butted to. I pulled one off and noticed that there was a vinyl barrier in front of it, and a foam strip inbetween them that I sort of ripped. So, maybe it would need a silicone bead there.

I want to clarify with you that this is the best way to do it. The other thing is whether I cut the vertical jam out to a bit to slip the sill into, which would look better than profiling the sill to fit it. I'd like the sill to be 1 inch thick, but that would make the bottom of the sash look shorter. I feel like its not quite proper, like maybe I missed the opportunity when I installed it. The alternative would be to stick it on the front, level with the jam, but then you would a seam.

These windows have the same strip of wood concealing the bottom of the sash. If I remove it, there is a nice flat surface.

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1 Answer 1


I happen to have a shop built with Marvin Integrity windows. As far as I know, the customary/expected way to provide an interior sill/trim is that it either butts up to or slips behind the wood frame provided with the window. Yes, there's a visible seam - it's not a big deal.

Your photographically-challegened-ness makes it difficult to determine if you already have the most minimal form of this installed, or not. Mine are currently sitting with no interior trim, waiting for my "tuit" to get "round."

  • Yea it's not a bit deal. I accidentally installed these side-by-side double hungs so they bow out in the middle a tad. It would need to be cut or planed to fit that. So are you poo-pooing my idea? That's how I sort of feel, too.
    – John Dee
    Commented Dec 13, 2017 at 19:38
  • I'm wondering how to connect butt it to the window, with corner screws? The finish trim underneath the sill would provide additional reinforcement, too.
    – John Dee
    Commented Dec 13, 2017 at 19:40
  • Huh? No, I'm merely stating the way I normally see it done. I'm not convinced there's any need to attach it to the window at all, if it's in the right location and supported to stay put there - pocket screws from the underside might be one way if you felt the need, but the frame wood is pretty thin, so you'd have to take care that those did not break out the surface. If looking for a wide sill, with no access to the rough framing for attachement, I'd use a beefy "trim" board underneath to provide support and attachment. Or just go whole hog and build a window-seat there.
    – Ecnerwal
    Commented Dec 13, 2017 at 19:48
  • Yea, I thought about screwing to the rough framing on the drive home.
    – John Dee
    Commented Dec 13, 2017 at 22:49

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