# Window Jamb Extensions



## oldfrt (Oct 18, 2007)

Tom,
I think your on the right track by thinking about building the' box'first.

Are there any jambs on the windows?
Are you just trying to extend them?


Anyways, build the box ,set it in the opening,and nail in place using shims to square it up and take up slack between box and framing.


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## Scuba_Dave (Jan 16, 2009)

I cut pine to the depth needed all the way around
Then nail each piece in
My house has 3/4" sheathing outside & 3/4" combo rock/plaster like stuff inside
So standard windows/jamb extensions do not fit
Plsu they are freekin' expensive
I tend to put big window sills in to put plants on
This set (3 windows) I added some custom cut supports underneath
I then cut pine as surrounding pine - 45 angle cut to match the rest of the house

Best thing to do is buy a GOOD mitre saw


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## Tom5151 (Apr 9, 2009)

Thanks guys........I appreciate it......

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?


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## ARI001 (Jun 26, 2009)

Tom5151 said:


> Thanks guys........I appreciate it......
> 
> 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, nail through the shims. If you pre-build the jamb extension there should be no reason to nail the head jamb or sill (assumes you are going with picture frame style) unless you have to shim the jamb extension up to keep the reveals equal.


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## Tom5151 (Apr 9, 2009)

This is incredibly helpful....

I am going with picture frame style. What is an appropariate reveal for the vinyl frame? I will have to use pretty thick shims to get a small reveal. I was thinking about making the jamb extension box the exact same size as the outside dimensions of the vinyl frame and then maybe using something like a shoe molding or quarter round on the inside of the entire box to cover up as much of the vinyl as possible. Is that acceptable? I could just miter the corners of the molding to make nice corners. I had read someplace where someone did that but not sure if i should go that route or just use thicker shims to get a smaller reveal.


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## ARI001 (Jun 26, 2009)

I like to set reveals at 3/16". The purpose of the reveal is to allow the wood to expand and contract naturally without making the fluctuations obvious to the eye. If you where to mount shoe moulding flush to the edge the natural movement of the wood may cause the moulding to expand over the sash which would be somewhat unsightly.

You can pre-nail wood blocks into the opening and then adjust with standard shims as necessary.


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## Tom5151 (Apr 9, 2009)

ah okay.....that makes sense.....

so nail the wood blocking in first and then shim off of those....great idea....this is probabaly a dumb question but is there a general rule for how many shims i should use?...how far apart they should be?........the height of the window is about 60 inches and the width is about that is not a little wider....sorry i dont have exact dimensions here but just wondered how i should position and space out the shims......


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## ARI001 (Jun 26, 2009)

For the application you are doing you should be OK with shimming at the top, center, and bottom. Do not shim the head jamb, you can leave that floating.


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## Tom5151 (Apr 9, 2009)

ARI001 said:


> For the application you are doing you should be OK with shimming at the top, center, and bottom. Do not shim the head jamb, you can leave that floating.


perfect...thank you sir......i can't tell you how helpful this is......


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## ARI001 (Jun 26, 2009)

You're welcome.


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## Tom5151 (Apr 9, 2009)

ARI001 said:


> You're welcome.


oops one last question.......do you butt the joint the extension box or use rabbets?.....just nail and glue or glue and screw?


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## ARI001 (Jun 26, 2009)

Either way will work, however there is no real added benefit to cutting rabbits in this scenario. If you do use rabbits don't shim them to tight and/or make sure to slightly over cut the rabbits to allow for movement or you could end up with windows that have seasonal sticking.


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## Tom5151 (Apr 9, 2009)

got it....i think I'll just butt joint and glue and screw.....

can I ask you a similar question about interior doors?


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## ARI001 (Jun 26, 2009)

Tom5151 said:


> got it....i think I'll just butt joint and glue and screw.....
> 
> can I ask you a similar question about interior doors?


Yes, go ahead.


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## Tom5151 (Apr 9, 2009)

Thank you so much.....i really appreciate your patience with a rokkie here....

actually the doors are already installed. They are pine prehung doors. Menard's had a great deal on them and they have a standard 4-9/16 in. 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 finish 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 on the inside surface where the extension meets the existing jamb?


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## ARI001 (Jun 26, 2009)

If they are pre-hung doors your best bet is to make the slip part of the jamb out of stock that is wide enough for the intended application cut the tongue (for this application it is technically just a rabbit cut) and slide the frame into the existing groove of the other part of the jamb. 

If you have already installed both pieces of the jamb then yes you could glue and clamp the 3/8" scabs to the existing jamb. if you do this make sure you let the scabs hang over slightly (1/32" to 1/16" max) on the finished side of the casing. Then after the glue dries you can run a router around the jamb with a flushing bit. Finish flushing the corners and the last few inches at the bottom with a cranked-neck chisel or scraper and lightly sand the casing. If you don't have a router you can hand plane the edges.


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## Tom5151 (Apr 9, 2009)

dang...this is like going to school......

yes the door jambs are already totally installed so i will need to follow your second option......i have a router and flush trim bit but it's a pretty big router...2.25 HP.......might get a little awkward trying to handle that.......i am wondering if it may serve me better to get a palm router or laminate router with a flush trim bit to do that.........

thanks once again......you have made something that seemed so complex to me previously seem like it would be, while not easy, something i can reasonably tackle on my own.


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## Tom5151 (Apr 9, 2009)

Tom5151 said:


> dang...this is like going to school......
> 
> yes the door jambs are already totally installed so i will need to follow your second option......i have a router and flush trim bit but it's a pretty big router...2.25 HP.......might get a little awkward trying to handle that.......i am wondering if it may serve me better to get a palm router or laminate router with a flush trim bit to do that.........
> 
> thanks once again......you have made something that seemed so complex to me previously seem like it would be, while not easy, something i can reasonably tackle on my own.


 
If I route everything but the corners and the bottom couple of inches, would a powered hand plane get me up all the way up into those corners and to the bottom of the jambs? I don't have a powered hand planer but the ones i have looked at don't look like they would get all the way up into the corner...the guides on the seem to stick out about 2 inches in front of the blade which wouod not allow getting in corners etc. ..............I have something called a "bullnose plane" I think. That has the blade riding out at the leading edge of the plane...that may work.....


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## ARI001 (Jun 26, 2009)

No, don't waste your money on a power planer. The router will get you much closer to the edges then the power planer. Most people end up butchering the work piece with the hand held power planers. The power planers work more like a hand held joiner than a plane. A bullnose hand planer will get you pretty close but you will still have to finish with a chisel. There are rabbiting planes available that will take you all the way to the edge and into the corner.

You should be OK with your router depending on the base. If the base is really large and you can't modify it then you could try a laminate router or picking up a smaller router. I have several routers that vary from 1.75 hp to 3.25 hp and typically use 1.75 & 2.25 hp routers in the field and have performed similar tasks with them. The laminate trimmer may be a good option for you. Repeated use will probably burn out the motor but it may hold up OK for what you are doing. They are relatively inexpensive so you probably wouldn't be out much if you did burn it up. Make sure your bit is sharp and don't be surprised if it stalls out or bogs down some.


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## Tom5151 (Apr 9, 2009)

ARI001 said:


> No, don't waste your money on a power planer. The router will get you much closer to the edges then the power planer. Most people end up butchering the work piece with the hand held power planers. The power planers work more like a hand held joiner than a plane. A bullnose hand planer will get you pretty close but you will still have to finish with a chisel. There are rabbiting planes available that will take you all the way to the edge and into the corner.
> 
> You should be OK with your router depending on the base. If the base is really large and you can't modify it then you could try a laminate router or picking up a smaller router. I have several routers that vary from 1.75 hp to 3.25 hp and typically use 1.75 & 2.25 hp routers in the field and have performed similar tasks with them. The laminate trimmer may be a good option for you. Repeated use will probably burn out the motor but it may hold up OK for what you are doing. They are relatively inexpensive so you probably wouldn't be out much if you did burn it up. Make sure your bit is sharp and don't be surprised if it stalls out or bogs down some.


 
thank you...i would have wasted money on one of those.....so a rabbeting plane...okay......i have seen them...if that's the right tool to finish the corners and bottom edges then that's what I'll do......thank you.


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## Tom5151 (Apr 9, 2009)

I probabaly should have asked this question about window jamb extensions earlier but forgot........is it better to have the finish wall material in place before making and installing jamb extensions or can it be done prior to that?

It will probably be a little while yet before we are ready to finish the walls where the windows are. So far we've pulled off all the old drywall and have reinsultaed everything and have plastic stapled to the studs. So we are basically looking at open wall framing right now. My thought was to get ahead of the game a little bit and build the extensions now so we have that out of the way when we are ready to finish the walls. 

Intuitively i can see the benefit of having the finish walls in place when you go to build the extensions but is there any reason that you have to wait until then? I have a bunch of scraps of the finish wall material (3/4 inch #2 pine tongue and groove) from the rooms we already finished, so i could use that as a template to build the extensions now before the finish walls go up....

Thoughts?

Thanks.


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## ARI001 (Jun 26, 2009)

Just make sure you allow for the finish surface when making the extensions. The casing moulding will accommodate for slight variations in the finish surface. Pre-assemble your casings on a bench for better results. It doesn't hurt to reinforce the joints with biscuits or splines either. The finish wall surface would be in place before the finish carpenters started work.


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## oh'mike (Sep 18, 2009)

It's always best to have the wall finish up before making your jamb extenders.

Often the window is cocked in the hole. Always take measurements at all four corners.
The jamb extenders often need to be tapered when corner measurements differ.

Slight differences can be cured by"beating in"'the drywall .Crush the offending drywall with a hammer,

Brutal, but standard practice.


Door Jamb extenders are frequently set back1/8 of an inch. The lead edge of the jamb is radius ed.
If you butt your extender flush (without a small reveal ) that radius will leave an ugly seam.

Check measurements in all corners. Good luck!--MIKE


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## ARI001 (Jun 26, 2009)

oh'mike said:


> It's always best to have the wall finish up before making your jamb extenders.
> 
> Often the window is cocked in the hole. Always take measurements at all four corners.
> The jamb extenders often need to be tapered when corner measurements differ.
> ...


I agree but it can and has been done without the finish wall in place. 

The OP is pre-assembling the jamb extenders and should be able to shim them equal to the window as he has stated there is a lot of room in the RI opening.

No, not standard practice :no:. Would have gotten you thrown off any job I have ever been on. Regardless OP is using T&G boards as finish wall cover.

See smiley :no:

The OP ic choosing method #2 as he has already installed the jamb and does not want to remove them. Glueing and clamping the scabs to the existing jamb will be fine as long as the OP remembers to match the grain. The same process is commonly used to join boards together in furniture work. The process is known as edge glueing. After planing, scraping, and sanding the there is no visibe seam. 

It should go without saying to double check all measurements, hence the saying; measure twice cut once.

To illustrate my point about seams here is a picture of a cherry bench seat I did a while back for somebody. The top is edge glued and dowel reinforced. The faces are cope and sticked. The panels where all edge glued from smaller pieces.


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## Tom5151 (Apr 9, 2009)

ARI001 said:


> I agree but it can and has been done without the finish wall in place.
> 
> The OP is pre-assembling the jamb extenders and should be able to shim them equal to the window as he has stated there is a lot of room in the RI opening.
> 
> ...


May I ask what an OP is?....lol......sorry I am such a rookie.....


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## Scuba_Dave (Jan 16, 2009)

OP = original poster


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## Tom5151 (Apr 9, 2009)

Scuba_Dave said:


> OP = original poster


ah ok........got it.......lol....thank you


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