How to Make Window Jamb Extensions

Details on jamb extensions for windows and doors seems to be one of those areas that gets glossed over in the DIY how-to arena.

There’s lots of information online about how to install windows and doors. Much of it showing a pre-hung door or window with a jamb that is just the right depth to be flush with the drywall when it’s installed. Believe me, it ain’t always like that.

And if you’re upgrading an old house – all bets are off.

What’s a jamb extension?

A jamb extension does precisely what the name suggests. It’s a frame that fills the depth of the wall space from the inside face of the window frame or ” jamb” to the face of the plaster or gypsum wall. Jamb extensions are typically 3/4″ boards, or MDF and create a clean edge for the the casing trim to be nailed to.

Here’s a section view of a typical wood window, and casing.

I’ve seen all kinds of variations which may look something like this or nothing like this, but the principles are the same. The jamb extensions for the sides and top match the depth of the opening to make them flush to the wall face. Unless you’re making a “picture frame” casing, the sill or stool needs to be deep enough to allow the side casings to land on the top, with about a 1/2″ in front and 1″ or so on the wall side of the casing.

The sill also needs to be notched at each end to fit into the rough opening. The depth of the cuts will be almost the same as the width of the side and top extensions. It’s best to cut them a bit shallow and then trim them to the right depth. This creates nice tight joints – both where it meets the window frame and where it meets the wall. You can be a little less precise on the width because the side jambs will cover the cut.

Build the frame and install it

If you’re extending a jamb that butts to a flat window jamb (like the illustration) it’s easier to assemble the frame on a flat surface and then install it in the opening. The extension frame will be smaller than the rough opening so it will need to be shimmed to get it centred and keep it square.

This video from FCCOH gives a good overview of the installation process. It’s not a great camera view for details, but you’ll get the idea.

Reveals make the difference

Getting a consistent reveal where the casing meets the jamb makes for a professional looking job. 1/8″ to 1/4″ reveals are typical for window and door casings.

In the video, the installer used a “reveal block” which is an easy jig to make in the shop.

The shop made guide will do the job, but you can buy a Trim Gauge (at amazon) which is adjustable and looks rather handy for doing more than just trim work.

You can see the trim gauge in action in this window trim installation video. Although I’m not crazy about the way he builds and installs the trim, it is a great demonstration of this nifty little tool and there are a few useful tips in it as well.

I’ll follow up on this in a future post and share my experience “getting out of jambs” while trimming out the vinyl window and doors in our mudroom.