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Thread: Help with jamb extensions

  1. #1
    Join Date
    May 2008

    Help with jamb extensions

    I had new windows put in about a month ago and the installers put in the jamb extensions also.

    Well now it's time for me to stain, varnish and put up the casing.

    Ha, what a cruel trick, alot of the jamb extensions aren't flush with the drywall. Some by maybe 1/8 of an inch.

    Tried with a plane and my ROS to make them flush and I'm not having alot of luck.

    Any ideas on how to do this? Maybe a trim router with a flush trim bit?

    I really don't want to rip the extensions out and do them over either.

    Should've read my tag line and hired it out.
    Confidence: The feeling you experience before you fully understand the situation

  2. #2
    Join Date
    Feb 2014
    Lake Gaston, Henrico, NC
    Probably easier to take them out, and do it right at install, rather than trying to correct them in place.

  3. #3
    Join Date
    Feb 2011
    Central WI
    Builders like the extensions attached by the factory. I made sure they were flush or a little under and had the builder shim the outside, under the flange and then rabbit the cedar trim. On occasion, I've added a strip to the back outside 1/2" of the trim to compensate but that is a PITA too. Dave

  4. #4
    #2 for Tom's advise

  5. #5
    Tom's right. sometimes you just have to back up and re-do things.

  6. #6
    I've had to deal with this problem many times. You could rabbet the casing on the inside edge as was mentioned above or you could pull the extensions and rip them down narrower than the jamb width then install flush to the drywall ignoring the odd gaps back to the frame which you can cover with a small piece of whatever, cove , some rounded over something. It adds another step in the jamb ext. which I like . As far as it goes the first fix would be the easiest with a tablesaw or router table . Even in new construction you have issues like this and it is usually better for the jamb to be proud of the wall because when you look at where the casing meets the jamb it is all tight then deal with any issues going back to the wall. When you case it , if mitered dry fit then glue it up using nails or biscuits or both let it dry a bit then nail it on as a three or four legged beast depending on whether you have a stool or not. Getting it perfect on a flat surface prior to install is key. Hope my dithering helps in some way, Good Luck

  7. #7
    Join Date
    Mar 2009
    Sterling, Virginia
    If I read your post correctly the extension jambs are proud of the wall? If they are straight with the window (not cut on tapers) I would just go ahead and trim the windows. Nail the trim to the extension jambs and shim under the other edge to keep the trim flat. Stain and finish you woodwork and then last caulk to the wall. No one but you will ever notice. Good luck.

  8. #8
    Even though the 1/8" proud of the drywall seems like a lot, I almost always just plane it off with a plane, I use a Stanley low angle block plane, kept sharp.With a little practice, you'll spend maybe 5-10 mins per jamb, and also be able to bevel the jamb away from the opening so the casings fit tight to the wall. A power plane with a fence also works, but can be tricky and messy. An ROS is an exercise in futility in this case IMO
    If the windows came with the jambs applied,they'll most likely be stapled on and you'll have a helluva time trying to pull them off. On some windows, they're also T&G'd into the frame, so you'd have to rip the exposed edge, plane, sand, ease the edge, etc.
    And Walter's advice is also a good method, and probably the easiest.

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