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furring out window and door jambs

ThomasBarlow's picture

I am going to furr out some window jambs and I was wondering if anyone has had a lot of experience and would be able to offer me some suggestions?

Wood windows, 11/16 jamb stock, need to furr out a further 2 inches with same fir material 11/16 of an inch in thickness. Do I create a reveal or would I have better attachment without a reveal? My concern is to make the  additional joint appearance to be tight as possible.

Thank you

(post #108858, reply #1 of 11)

Hi Thomas Barlow(you didn't happen to live on Pea[JOBSITE WORD] St. at one time? I went to school with a Thomas Barlow)

I personally wouldn't leave a reveal. You could build the extension jamb as a frame and then using  biscuits attach it to the window.  That is providing  you have enough room at the window to get the jointer in there.  If not you can build the extension as a frame or piece by piece and just attach to the jambs,shimming where needed. I would also rip a small bevel on the front edge of the extensions, about 3 degress.

(post #108858, reply #2 of 11)

Sounds like you're going stain grade? Since you can't joint the existing frame, you probably can't get an invisible joint, but you can celebrate it.

Knock the edge off the window frame. Make jamb extensions out 1 by (you'll only see 1/8 +/-). Pocket screw, or face screw behind the reveal. Make your extension frame up about 1/4" larger than existing.

We'll have a kid
Or maybe we'll rent one
He's got to be straight
We don't want a bent one
He'll drink his baby brew
From a big brass cup
Someday he may be president
If things loosen up

The Village Woodworks, Inc

Chapel Hill, NC


We'll have a kid Or maybe we'll rent one He's got to be straight We don't want a bent one He'll drink his baby brew From a big brass cup Someday he may be president If things loosen up

(post #108858, reply #3 of 11)

This is called an extension jamb.
I always leave about 1/8" reveal. If you want to make it flush, you can do that too, but that requires perfection, something I refrain from except where money is no object.

A simple reveal will look planed and a tiny variation the thicckness of a piece of paper will not show, but will be glaring on a flush extension. That would mean a lot more work sanding.



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(post #108858, reply #4 of 11)

It's always a good practice to make a small reveal whenever you install an extension jamb.

(post #108858, reply #5 of 11)

Same as the other kids. I use a small reveal, it makes the installation less problematic, plus it adds a shadow line.

There are 10 kinds of people in this world; those who understand binary and those who do not.

(post #108858, reply #6 of 11)

When I had to extend the jambs on 17 windows, I went with a 1/8" reveal, but I built the complete extension (all 4 sides) on the bench, then pocket screwed it to the window jamb. Stock was plain 1x ripped to the needed width.

In the past, I have also added the casing to the extensions where I was trimming out vinyl windows. I couldn't screw it with pocket screws so I tacked it through the casing to the studs.

(post #108858, reply #7 of 11)

we just added jamb extensions to several pella windows and one door. the general didnt want shadow lines so we had to flush them. major pita. we left them sightly proud glued and pin nailed them then sanded flush and with stainable glue they came out good but it took two of us 1 day to do8or 9 of them. if i dont forget ill try and take a photo tomoorrow and post it. good luck.

(post #108858, reply #8 of 11)

The need for a reveal depends on how true your current jambs are and what finish you intend. If you're planning to stain, that will require a pretty tight joint. If the windows aren't new, the chances of getting an invisible joint are remote. Still, you can get an acceptable joint with some skill & care. If you're painting, any imperfections can be filled and will disappear. If you think things won't go together tight enough, a reveal is called for.

When retrofitting extensions where the current jamb is not grooved to accept a jamb extension with a tongue, I typically use glue & biscuits to align and hold things a bit tighter. You'll need to remove the window to clamp the joint.

Mike Hennessy
Pittsburgh, PA
Everything fits, until you put glue on it.

Mike Hennessy
Pittsburgh, PA
Everything fits, until you put glue on it.

(post #108858, reply #9 of 11)

I'm w/ most ... a reveal hides many imperfections and adds a shadow line. Flush fit requires a much higher standard of craftsmanship that could be spent on something much more important (I'm not saying windows aren't of utmost importance) that REQUIRES your extra craftsmanship attention.

There ain't NO free lunch. Not no how, not no where!

(post #108858, reply #10 of 11)

I do it all the time. Many windows have a beveled edge which is great for keeping a little reveal. A square edge can be sanded so it isn't sharp. You don't have to use 11/16" stock, you only see the inside edge.

Beat it to fit / Paint it to match

Beat it to fit / Paint it to match

extension_jamb.JPG157.81 KB
extension_reveal.JPG184.36 KB
extension_trimmed.JPG172.16 KB

(post #108858, reply #11 of 11)

Thank you for your comments, great idea to place the stool on before placing the jamb extensions.