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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.

  9. Rip 4 pieces of 3/4 inch plywood 4 inches wide x 4 inches longer than the length & width of the window. Install the first piece vertically on the left side of the window by screwing it through the Sheetrock into the 2x4 house frame. Keep the screws close to the window so the screw holes will be covered by your window casing. Align the right side of the plywood with the OUTSIDE of the window jamb, and the bottom of this plywood strip with the bottom (outside) of the window jamb. The top of this piece should shoot past the top of the window by 4 inches.

    Then install the next plywood strip along the top of the window, butting the left side into the vertical piece you just installed, aligning the bottom edge with the outside of the top of the jamb. This top piece should run past the right side of the window jamb 4 inches. Install the right vertical piece and bottom piece in the same manner. You have just now trimmed around the outside of the window jamb with these plywood strips.

    Now take your router with a 3/4 inche diameter straight cutting bit. Set the router base on the plywood strip and adjust the bottom of the bit until it just barely touches the Sheetrock, or maybe 1/32 inches-1/16 inches shy.

    Now use the plywod strips as the base and run the router all around the window jamb extension. This will give you a uniform jamb depth in relation to the Sheetrock. If you use a top bearing pattern bit, it will keep you from routing into the plywood strips and help to stabilize your cuts.

    If you want to mimic a factory jamb with the step on the jamb that the casing abutts, you will have to make two passes.

    On the first pass, set the bottom of the bit 3/32 of an inch to 1/8 of an inch above (proud) of the Sheetrock. Make your first pass around the window. This will leave the jamb protruding past the Sheetrock.

    Now swap bits with a 5/8 inch diameter pattern bit and adjust the bit until it touches the Sheetrock. Make your second pass. You have now created a rabbit around the outside of the jamb that your casing will set against. You can go one step further and use an 1/8 inch radius bit to round over the inside edge of the jamb.

    If your jamb extension is not 3/4 of an inch thick, then set your plywood strips exactly 3/4 of an inch away from the INSIDE of the window jamb.

    If you go this route, you must make sure the plywood strips are FLUSH with either the inside or the outside of the jamb (depending on thickness) to allow for consistent reveals.
    Last edited by Thomas C Barron; 08-25-2019 at 1:51 AM.

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