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  • Window Jamb Extensions


    We have a little cabin up north. We are slowly renovating the entire thing. We are replacing evey window. Our contractor is doing a wonderful job but the labor cost is something that I want to avoid by doing the rest of the project myself. I have never done anything to a window except to open and close one and occasionally wash one. I am comfortable with DIY projects and have no trouble attemtping things.

    I need to know how to make and install a jamb extension on a new window. Its's a vinyl framed window already installed and in place. The interior sashes are pine. The cabin is old and very little is square and plumb. The finish material on the walls wil be 3/4 #2 tongue and groove pine so the extensions will need to come out far enough to meet that. I have never done this before ever. Should i build the extension in place or build the box first and set it in place and shim? I really need a very simple plain explanation hw to do this.

    Could someone give me a good explaination how to do this or at least maybe point me in a direction where I can find the information? I have searched all over the internet and haven't really come up with anything that helps.

    Thank you in advance

  • #2
    Re: Window Jamb Extensions

    Tom5151 - I see you've posted your question in several forums, so I'll respond to this second post you made in "DIY Chatroom":

    "There are no jambs currently. Just the vinyl frame. I would be butting the outside edge of the box right up against the vinyl window frame. Given that, I cannot nail into the vinyl so should i nail through the extension into the rough 2X4 framing? Do I just nail right through the shims? Do I shim the bottom of the extension or does that just get nailed directly to the sill plate?"

    Yes you'll be nailing into the 2x4s.
    Yes you'll be nailing through the shims.
    Yes you'll need to shim the bottom extension just like the other sides in order to get a consistent reveal of the window frame.
    When you rip the extensions on your table saw, you'll cut them to the width of the largest measurement that you find between the window frame and your finished wall on any of the four sides of the window. This will leave some areas proud (sticking out farther than the T&G pine). You'll then use a hand plane to trim down to match the finished wall depth. This shouldn't be difficult since you're using a soft wood. Pre-drill, counter sink and screw the extension jamb "box" together.
    Last edited by Dairylander; 09-23-2009, 04:28 AM.


    • #3
      Re: Window Jamb Extensions

      Dont know if they are Andersen windows or not, but this is a good article on Andersen extensions, could be a good reference regardless of brand:

      I did something similar to new replacement windows were a lot slimmer than my original windows, so when they went in they were inset from the surface of the wall by 2" or so. I built each frame in place, top and bottom first (windows were taller than they were wide) with a rabbet at each end so I could just slide the side pieces in. Shimmed the whole thing relatively parallel to the lines of the window and nailed into the rough opening.

      Its not hard, just time consuming (especially considering at the time I didnt have a table saw to rip the 1x4 stock down, so I was planing everything). I used pre-primed 1x4 pine, but you would use raw wood.


      • #4
        Re: Window Jamb Extensions

        Thank you so much....this is incredibly helpful. I know the walls are not plumb so I anticipate a lot of planing once everything is in place. In order to get a consistent, small reveal (I want to cover as much vinyl as possible) I think i am going to need pretty thick shims and prety long finish nails.

        Thanks again.


        • #5
          Re: Window Jamb Extensions

          I did most of the planing on the bench, but marked a line about 1/8" past the drywall to allow for some finish planing after the jambs were installed. Be prepared it gets really messy

          You might find it doesnt need to be perfectly flush with the wall, the trim will have a little give and its OK within reason if the jamb or the walls are a little proud because you wont really see the trim on an angle anyway.

          I actually left more vinyl visible than I thought I looked kinda funny if I hid too much, I found 1" or 1.5" visible looked nice (also cool was having all of the 45 degree angles between the window frame, extension jamb and casing lining up perfectly....makes the job look more "quality").


          • #6
            Re: Window Jamb Extensions

            got it.......that makes very good sense. I'll have to experimanet and see what looks good. Did you put the extension jambs in after your finish wall was up or before? It's going to be a while before we actually put the finish walls (tongue and grove pine) up. I was hoping to gain a little time and install the extensions now so when the time comes I will have that done. I have a lot of scraps from the tongue and groove pine we already installed in the bathrrom and bedrooms. I could use that as a template now. Your thoughts?


            • #7
              Re: Window Jamb Extensions

              My finished walls were already in place.

              I suppose you could install the jambs now just making sure there is plenty of "depth" to them, taking into account where your finished walls would be. Dont want to end up with not enough jamb to come out flush with the wall, then you'll just be adding even more work.

              On the other hand, if you make them now and make sure you have extra depth just in case you are going to do a lot of planing once the walls are in.

              I think I would leave them be for now, and add the extensions once your walls are in place so you know your exact dimensions.


              • #8
                Re: Window Jamb Extensions

                got it....that makes perfect sense.........can i ask you a similar question of extension jambs for interior doors?


                • #9
                  Re: Window Jamb Extensions

                  Are the doors already ordered? You might be able to get custom depth jambs to be close to your finished wall depth.

                  Otherwise, I think I would glue/brad nail similar material right to the stock door jamb. Might not come out as nice as the window version but there is only so much you can do for a door frame.

                  You could try to route a groove into them to accept tongued wood, but that might be hard if the doors are already installed (and not necessarily easy if they arent already installed)


                  • #10
                    Re: Window Jamb Extensions

                    actually the doors are already installed. They are pine prehung doors. Menard's had a great deal on them and they have a standard 4-9/16in. jamb on them. They were installed on two bedrooms and a bathroom. The insides of those rooms are completely finsihed and the doors were installed so that the jambs were flush with the finish walls in those rooms. But outside of those rooms is where I will need the extensions. The finishi walls on the outside of the rooms will be 3/4 inch tongue and groove pine so I basically need extenions that are 3/8 inch think around the whole door. That seems really thin for an extension. Is that fairly normal? Also for the door extensions there should be no reveal, correct? Those should be totally flush with the existing door jamb? Do you just plane any inconsistencies where the extension meets the existing jamb?