Installing Windows with Deep Jamb Extension
Installing windows with jamb extensions that do not exactly match wall thickness:
Hello,
Seeking advice/opinions on relative merits of ordering and installing windows with factory-installed jamb extensions IN THE CASE WHERE the jamb extension will protrude 1/4″ – 1/2″ beyond surface of GWB.
My wall stack is:
– GWB (1/2″)
– Stud wall (3 3/4″) (This is a 100-year old house)
– Wood plank sheathing (3/4″ – 1″)
– EPS foam board insulation (1″)
– WRB (Barricade)
– 1×4 furring strips for rain screen (3/4″)
– Hardi plank siding
I am planning on installing new vinyl windows (Harvey) with the nailing fin laying on the WRB. Adding up the components above (GWB+Stud+Sheathing+Foam) gives a jamb depth of 6″ to 6 1/4″, with the variability due to the thickness of the (100-year old) wood plank sheathing.
I am tempted to buy the windows with a 6 9/16″ jamb extension installed, which will result in the jamb extending beyond the GWB by anywhere from 5/16″ to 9/16″.
Questions are:
1) For the $60/window the jamb extensions cost, is it a good way to go, even if I have to do some finessing at the end? Here finessing would include measures like rabbeting the window trim to fit over the protrusion or routing out the protrusion. Is the effort/cost savings on the overall window trimming worth it despite the need for such measures?
2)) Does the deep jamb extension make the window harder to install, as in level and plumb and air and water seal?
3) Would a shorter jamb extension (4 9/16″) possibly be a good compromise measure to jump start the window trim, to which I could just add a piece (with some reveal to the factory jamb) that would be exactly flush with the GWB?
Details of my wall and window install flashing plan are shown in pictures at my previous post; link is below. At that time I was planning on re-installing finless replacement windows. I’ve subsequently decided to buy new new-construction windows with nailing fins.
Thanks.
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Replies
longrun_23,
Three things are harder with attached jamb extensions:
- Predicting the total depth needed
- Installing and shimming the windows
- Air-sealing the window perimeter
Thanks for reply Malcolm. Is it then your opinion/advice to not get jamb extensions at all?
longRun-23,
Not necessarily - I just wanted to point out some of the difficulties they create. I've never used them. Maybe the problems I envisage aren't that difficult to surmount, but I'd have to find new ways of doing all those tasks. My own preference is to make the casing, and include a stool that extends out past the trim.
Can you speak to the general question of dealing with the jambs coming out proud to the GWB surface? Is this considered to be fairly straightforward to correct? What specific measures are favored?
Thanks.
longRun_23,
It depends what you have for tools. A hand planer can take the excess down flush with the wall surface pretty quickly.
One problem with that is you don't really want the casing different depths to conform with what may be an uneven wall surface. That makes trimming them out much harder.
Very good, thanks for your advice!
I like #3 -- shorter jamb extension with a site-built extension over it. That way you can scribe the extension to the wall and then take it off to shape it on a workbench or table saw. Your basic problem is the walls are inconsistent thickness, and you need to figure out a way to make up the difference, I think that's the easiest way.
The tricky part is attaching the two pieces together. They're only about 1-1/2" to 2" so you might be able to just go through the face with a 2-1/2" screw.
DCcontrarian,
Can you elaborate on that last bit about attaching the two pieces? I assume that the trim on the factory extensions is at most 3/4" thick. I can't picture what you are referring to about the 1-1/2" to 2" aspect. Thanks.
I meant to say that the jamb extensions themselves are, I assume, constructed from 3/4" thick stock.
The factory jamb extensions are 4-9/16", the walls are 6 to 6-1/4. So the site-built jamb extension needs to be a piece of wood that is the size of the window frame in one dimension and somewhere between 1-7/16" and 1-11/16" in another. The third dimension would be the thickness of the stock, presumably 3/4". I'm trying not to use words like "width, depth and height" because they depend on how the piece is oriented. Let's call the difference between the size of the factory jamb and the wall thickness -- the 1-7/16" to 1-11/16" -- the "second dimension."
The site-built extension is attached to the factory extension edge to edge -- which is generally the toughest way to attach two pieces of wood. Since the "second dimension" is relatively small you could put a 2-1/2" fastener through that dimension and into the edge of the factory jamb extension to join the two extensions. The head of the fastener would be covered by the window trim. You could also do biscuits or pocket screws and I'm sure you could think of other methods.
Thanks for clarifying, I get it now. And I guess one would want to leave a small (1/8" to 1/4") reveal between the two jamb pieces.
I think this approach has a lot of appeal. Thanks again for your feedback!
One last question: would the recommendation be to do a test fit to get the right "2nd dimension" for that particular R.O. and then pull out and assemble on the bench? Or attach the supplementary jamb extension piece after the window is installed?
(Response to #17)
In #19 Michael lays out how to do it. If the wall is uneven you have to make a judgement whether you want the frame to be straight or follow the wall.
I never order windows with factory-applied jamb extensions, as in my experience they never perfectly match the wall thickness. It's not hard to make your own.
Hey Michael,
I have to confess I've never trimmed a window before. I have appreciable framing experience but essentially no trim experience. That's why the idea of some help from a factory solution has appeal to me. I'm imagining it may take me hours to do---at least for the first window, to work it all out---compared to paying $60 for a, say, 90% solution.
I understand. I've trimmed hundreds of windows and when I started out I used factory extensions but I got tired of fighting with them on every opening, on every project. If you're not picky about how the finished product looks, you should be fine with factory extensions. But if you want clean, flat, tight-looking trim, if you can frame a house I'm sure you will quickly learn how to make your own extension jambs.
Copy that; and thanks for the vote of confidence!
Keep in mind that Michael was a furniture maker in another life, his idea of "not hard" may be different from yours.
Haha, you have a point there. But now that I almost always have walls deeper than 6 9/16", all of the builders I work with make their own extension jambs as well, if they aren't using drywall returns.
It can be as easy as measuring the reveal at various locations around the window, using a number on the high side of average, ripping boards to that dimension, screwing them together to make a box, slipping the box into place and securing it with finish nails. Sometimes you don't even need shims.
Or, sometimes every piece needs to be custom-cut with a taper, and the stool cap (i.e., interior finish sill) is traditional it can be more complicated.
LoL. Thanks for the warning! I think I'm leaning towards the idea of a custom supplement to the factory 4-9/16" extension. Seems to make a lot of sense.
If you have a track saw, it's easy to scribe a line from the GWB, then cut to the line (no need for it to be parallel with the window side of the extension, unlike with a table saw). Even without a tracksaw it's not that big of a deal to rip to a line with a circ saw, then a couple swipes with a plane to clean.
One tip for scribing: Get a nice flat piece of wood maybe 8" to 10" long (like some hardwood flooring scrap) and cut a 45 on one end. Ink the sharp of that cut with black marker, then slide the large flat surface of the board against the GWB while the inked cut marks the board exactly in plane with the wall. You'll need to reink occasionally, but in my experience it leaves a pretty good mark.
If doing a traditional stool, you'll probably need to scribe the boards before assembling into the box.
Having done quite a few of these, this is the method that worked best for me.
Rip your jamb extensions so that they project a bit beyond your deepest measurement. Assemble the frame this way and set in window opening with the bottom shimmed flat at the correct reveal height for the bottom of the window frame. Now hold the frame in place and have a helper trace the jamb frame along the walls. Now you can disassemble the frame and use a track saw or jig saw to cut to your scribe lines. Cut a little big. Now use an electric planer or preferably a jointer to clean up the trimmed sides of the jamb extensions.
Now you can reassemble the jamb frame. Set screws around the sides and top of your rough openings at the proper depth to keep the jambs from bowing when you nail them in. You can also use more shims to get the sides plumb, but wafer head screws are easier.
Now you’re ready for trim. I like to picture frame my trim and pocket screw it together in advance. Now you can just set it up to your flush jambs and nail it in. When all four corners of your jamb extensions are different, pre-assembling the window trim eliminates having a corner “pop” on you like would happen if you were nailing individual trim pieces in place.
Thank you, this is very useful information.
longrun_23,
Remember that rocket190's excellent advice presupposes the window is already installed, shimmed, and air-sealed - and doesn't have the extensions attached.
It’s very common in my Ontario market to have to option to order vinyl windows with a “wood/drywall” return channel pre-installed on the window frame. Basically this is 3/4” wide by 3/4” deep groove around the perimeter of the window that is designed to accept either a wood or drywall jamb extension. We have used these on many new and retrofit projects and it really simplifies the buildout process. The 3/4” depth provides plenty of adjustment and depending on the wonkyness of the wall essentially eliminates the need to scribe.
Here is an example of site built jamb extensions, the window stool, and the casing being constructed as an assembly that is then slid into place: https://www.instagram.com/reel/CwSMAVkAEEs/?igshid=Y2NkYjk0MDhjYg==
Not my work, but it is my home. The text has the description of the process. Carpentry by Nielsen Crist (@nielsencristcarpentry on Instagram).