I am replacing my current windows with new construction windows. Because I am in a historic district I need to keep the same size windows (opening size) and exterior molding. My plan is to remove the exterior molding up to the j-channel (leaving the j-channel), installing the new window, and then installing new exterior molding to match the current size that is there now. After I remove the old windows, the rough opening will be too large... i.e. I think if I purchase a new window to fit the rough opening, I would not be able to have the same size molding which is a requirement of the historic district.

Am I correct that I will need to build up the rough opening to provide a place for the new construction window to nail into and also allow enough room for the exterior molding? And what is the correct size windows I should order? Is 30" x 63 1/2" correct?

I plan on using Andersen 400 Series windows like this although they will need to be custom sized.



interior detail


2 Answers 2


Yes, I think your plan is correct if you mean to shrink the rough openings. I'd be careful to only shrink as far as necessary, though. I'm very familiar with Andersen 400 series windows, and I think you could incorporate the outer vinyl-wrapped jamb rib into the width of your current casing. This means that you may only need to shrink your rough openings 3/4" or so.

At my previous home, circa 1956, I did work like this using 400s. I laid 3/4" thick vinyl stock over the mounting flanges and caulked it to the vinyl exterior jamb rib so that they appeared to be a continuous unit, with the rib protruding about 1/4". Actually, I caulked it to the window, the wall, and the J-channel and used no other fasteners. It looked nice and worked well for the decade I was there.


If you are located in a "Historic District", you'll need get the style, shape, color and finish approved. Where I live, that is through the State Historic Society. Make sure you have their approval in writing prior to ordering the windows.

I have been denied because I proposed a vinyl window frame (Andersen frames are vinyl) and because the window sill did not have a "stool and apron" like the historic window, (Andersen 400 Series does not have a stool and apron.) Rather their sill is the same profile as the head and jambs.

Also, Andersen windows can be ordered with or without a nailing flange. If you order with the flange, you'll need to allow sufficient space to nail to the framing.

I strongly recommend that you get the exact window style, shape, etc. approved prior to ordering. They have recommended manufacturers, model numbers, etc.

  • I already went before the historic commission and was approved. I am planning on getting the windows with a nailing flange and now I just need to figure out a place to nail it into and still be able to install the same size exterior molding. Apr 22, 2017 at 12:03
  • If you order without a nailing flange, they come with pre-drilled nail holes on the jambs. Without the nailing flange, will they fit tight to the exterior trim?
    – Lee Sam
    Apr 22, 2017 at 16:14
  • Wow I am suprised, we had to custom order in 2 different locations (states) for the exact same style that was existing.
    – Ed Beal
    Sep 17, 2018 at 20:15

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