I just finished a window in the basement about a month ago and these are the steps I took:
- I determined the inside dimension of the window frame and planed on a 1/2" reveal around the entire inside frame.
- Then made my 2x4 frame around the window to allow for my pre-made extension jambs (around 9" deep) which equaled 1-1/2" of material.
- Once the frame was made, I pre-trimmed the boxed extension jambs so when I was done I had what looked like an open Birch plywood box with trim on one face.
- Then I simply slid the whole thing inside framed opening and put it up against the window frame and lo' and behold I had a 1/2" reveal around the entire window.
So....let's say the dimension of your window is 15" x 32" (standard American Craftsmen window from Home Depot
) you should have a measurement of 9-1/2"h x 26-1/2"d
for an inside
dimension(this is the key dimension to work with and is the inside of the frame).
Considering 1-1/2" for your boxed extension(2pcs 3/4" thick) and about a 1/4" of space for your framed rough opening (to allow room for the boxed extension). The final dimension for your 2x4 framed rough opening should be 12-1/4" H x 29-1/2" w
.(added 3" for sides and 2-3/4" for top to bottom from "inside dimension
Now for the extension jambs/box:
I used 3/4" birch plywood to create the box, and from the face of the 2x4 framing to the face of the window it was 9 inches deep. So in this case, my side pieces measured 9"d x 10-1/2"h (1" bigger then the inside diemension 9-1/2"
)and my top and bottom pieces measured 29"w
, (2-1/2" bigger then the 26-1/2"
wide inside dimension). Reason being, to allow 1/2" on both sides for the reveal and it has to sit on the side pieces 3/4's of an inch, plus allow 1/4" spacing to fit opening.
Now to build the box extension:
Once you have your sides, top & bottom pieces cut(MDF or Birch), nail them all together (stand the sides up(10-1/2" up&down) and nail the top over them, flip over and nail the bottom piece....there's your box. Now apply whatever trim you had in mind and keep a standard 1/4" reveal on the edge.
Now you have a box all trimmed out and ready to just slip into the framed opening and up against the window frame. 1st nail trim on and then do the sides, top and bottom of the extension box. Keep in mind that this box will not have to be set up a 1/4" from the frame sill, just set it on the 2x4 frame going across and the reveals on the window will be the same.
I hope this is not greek to you
. But just read a couple times and it should all makes sense. It's easy for me to write all this, but I'm sure a slight challange "at first" for you to comprehend. If there's any questions just ask.