Window Jamb Extension

Old 06-03-05, 07:27 PM
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Question Window Jamb Extension

Hi - I'm working on finishing a room in my basement - the room has a 48x60 vinyl window existing in it. The problem I'm running into is I need a jamb extension, but I'm not really sure where or what to look for.

There is a gap of about 1 1/2 inches between the end of the window sill and the edge of the 2x6 sill plates. Does anyone know the best way to fill this gap and where would I be able to buy the supplies.

I took a couple pictures of the window - hopefully they do a better job of explaining...

Thanks for any help you can provide!


Last edited by 99gixxer; 06-03-05 at 07:41 PM.
Old 06-04-05, 07:18 AM
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First advise would be to get wall covering installed so you know exactly how wide the jamb extension needs to be, not to mention installing wall covering will be much easier since the jamb extension will not protrude out in your way.

If you are going to paint the jamb extension, it's alot easier to get some spruce, popular, or aspen and rip it yourself on the table saw to get it perfect. Just be sure what ever you use it knot free, otherwise even when painted it looks like back woods red neck sheeot.

Even if you bought pre made jamb extensions, many times even those need a trip through the table saw to get them exact, so they;re not always the cure all.
Old 06-04-05, 07:39 AM
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Thanks for the advice!
Old 06-05-05, 06:43 AM
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I also prefer to have the wall finished (drywall hung and painted) before putting up trim. What I like to do is make the outside dimensions of the extension jamb exactly as big as the window is. This helps keep the trim symetrical once it's installed. It also can make the jamb easier to install.

I shim all my windows in place with cedar shims- a window that big would have at least 3 shims per side. I shim the window snug, making sure the shims are laying flat (one tapered shim one way, the other tapered shim the opposite way) and then I cut the shims off even with the framing so that they stick out way past the window.

Then take measurements for your jamb. If the measurements are different, (2" in one corner, 1 7/8 in the others) I rip the jamb the smallest of them. Cut the top and bottom pieces as long as the window is, cut the sides 1 1/2" smaller than the window is. Nail it all together at the corners. (or if you don't have air tools, predrill and screw it) Once it's assembled, it should just fit in the preshimmed opening. I use a straightedge across the corners and pull the jamb out so that it's flush with the drywall, then nail it through the shims. Having the jamb flush with the drywall is important so that the miters of your casing fit at perfect 45's. Then I install a baseshoe around the inside of the jamb that is pushed tight against the window, and nail it to the extension jamb.

It might seem like an extra step installing the base shoe, but I like the way it looks, and it does a couple things- it eliminates the need to taper jambs, it covers any gaps that might be between the jamb and window (eliminating the need to caulk around the interior of the trim), it makes the thick frame of the replacement window appear a little smaller, it visually breaks up the trim and gives it a softer appearance.

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