Custom Jamb Extensions

Typically, one creates and uses jamb extensions to create a finished surface (trim) that connects a window to the inside of the house finished surface (drywall).  Doing this allows trim (casing) to be installed over the jamb extensions and drywall creating a finished look.   A custom jamb extension is constructed to fit a larger depth than usual (window sits very deep in a wall compared to the inside drywall) and/or the walls or window are out of square.  By creating a custom jamb ext, you are able to match each of the 4 window sides to the exact depth and and contour of the wall.  There is a good bit of info on the net and in several books that explains in detail how to do this.  I am no expert, but I have gone through the process and here is some evidence (of both being a newbie and the process!).  My windows were 7+ inches deep from the drywall surface and each corner was at a different depth… which means lots of careful measuring and cutting.

This is a brief summary of creating a custom jamb extension for one of my kitchen windows.  I have created many variations to these extensions for other windows in the house.    A similar method can be used for doors.   Below you will find a list of the tools I used during the process and some photos of the progress.

Tools and Materials
– Pencil (make sure it is sharpened, haha)
– Tape Measure (good quality since you don’t want to cut more material than is necessary.  This costs money, but it also costs sanity!)
– Sand Paper (medium grit)
– Square (to you know, make a line 90 degrees from an edge of the board)
– Table Saw
– Miter Saw
– Power Drill
– Countersink Bit
– Torx Bit (for GRK Fasteners)
– GRK Fasteners (Screws @ various lengths)
– Wood Glue
– Paper Towels / Rags (to wipe your tears of joy from seeing the high quality finished product… or just being done with the job)
– Nitrile / Latex Gloves
– Construction Wood Adhesive
– Router (fixed base)
– Rabbet Router Bit
– Prime Pine Board (1″x8″ nominal)
– Shims
– Scrap wood (as long as longest jamb piece)
– Wood Plug cutter (3/8)
– Hole cutting bit
– Flexible Fine-cut Saw
– Compressor (portable)
– Finishing / Brad Nailer
– Brads

Here we see an assortment of tools and materials on the only work table in the house.  In case you have not been able to tell, space is limited here!


Table saw (Dewalt 10 incher)and the miter saw (Hitachi 10 incher).  I am getting ready to order a larger Bosch 12 inch compound sliding miter saw to do a better job cutting larger pieces of trim and for some other side projects.  I am working outside because of the dust and, you will never guess it, the room needed to cut the boards.


This is what the setup looked like before the jamb extension was installed.  That’s is my dad in the background cleaning mortar and tape residue from the window pane.


After cutting each board, I scribed a line 3/4″ from the edge.  Then I placed 3 equally spaced countersunk holes along the board width.  I did this on each end of the board


Then threaded in some high quality 2″ GRK fasteners


Before attaching the pieces together via screws, I placed a good bit of wood glue on the mating surface.  Of course, you should not use as much as the picture shows or at least wipe it up!  I was working fast and admittedly I was being a bit sloppy.  Adding glue to the mating pieces creates a truly secure frame.  This helps to eliminate any bending moments you may incur during installation and therefore keeping the edges tight.


Place the mating pieces on a level surface, align edges, screw the top fastener in first, align the bottom edge, screw the middle fastener in, recheck bottom and then screw the last one in.  Wipe away excess glue with a rag using soap and water.  Allow to dry and lightly sand the edges to smooth them out.


3/4 pieces installed.  Looking good so far!  Let’s hope it fits.


Completed assembly ready for window fitment.


But… before I can check I need to rout the inside edges.  I forgot about these metal nailing fins used to hold my Pella windows to their frame.


I used a 1/2″ rabbeting bit in 4 locations (2 per vertical board) to cut the reliefs for the tabs.  These were cut on the window side of the jamb ext but on the exterior (non finished side).


It fits!  I used construction adhesive to bond the window edge of the jamb ext to the window.  Then I used shims to center the window along all four sides.   Finally, to secure it in place, I used 3-1/2″ screws and brad nails attached to the window framing.  The screws were installed in holes cut with a hole cutter so the head sits below the surface of the wood.


The screws mentioned above were hidden using plugs cut from pine scrap.  they were glued in place and then the tops were cut flush using a flexible fine-cut saw.  They will be lightly sanded after the glue dries.


A close-up of the brad nails installed through the trim. They will be filled with a high quality caulk and painted over.


All in all, this process took about 2 hours.  Mostly because the boards had to be ripped to non-uniform lengths through a fairly time consuming process.  It is very satisfying to take copious measurements, make clean gluts, plan well and have a rather complex assembly fit as it should.  The drywall edges of the board were perfectly alined with the drywall surface.  This will make a flat plane to nail the casing to; creating nice mitered corners with no gaps!

Thanks to my dad for being patient with me… Many times the thoughts in my head do not come out in coherent sentences but he sticks around and waits til I can finally come up with the appropriate words.  Without him around, aligning, cutting, and installing these jamb extensions would have been very difficult!

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