Trim Carpentry:

Custom-Made Jamb Extensions For
Andersen Windows

In This Article:

The groove in the window is carefully measured and a router table is set up to make a simple rabbet cut.

Related Articles:
Skill Level: 2-3 (Basic to Moderate) Time Taken: 3 Hours

By , Editor


On our 1907 Victorian remodeling project we replaced a few old windows with new Andersen Tilt-Wash double-hung windows, about $300 for the 28" x 64" size shown here. Many types of Andersen windows have a groove around the perimeter that accepts their standard jamb extensions. When we ordered the window from Home Depot we didn't think about ordering jamb extensions, because it seemed that installing trim would be a long time away.

But when I talked to the plasterer he reminded me that I needed plaster grounds around the windows, and that I could just cut the jamb extensions a little wider. Plaster grounds are raised wood strips that assist the plasterers in getting the correct thickness at the edges of walls and openings. I didn't have enough time to order the jamb extensions, so I decided to try making my own.

The newly-installed window


Note the grooves (red arrows) around the perimeter of the window .

The grooves are tapered, as are Andersen's jamb extensions.


To accurately measure the width of the groove, I pulled out the most precise measuring tool I own, which happened to be a pair of digital calipers I had from my days as an engineer.

Now I realize that practically nobody has a pair of calipers hanging around, let alone digital calipers. But the point here is using the most accurate measuring tool available. A tape measure often isn't accurate enough for jobs like this, but a simple precision steel ruler probably is.

The groove measured 0.360" at the back and 0.390" at the front.

I also measured the depth of the groove, which was 0.170". This is just less than 3/16", which is 0.1875".


I went to the shop and set up the router table.


This is a carbide straight-flute router bit.


I bought this piece of 1x8x6' clear pine for about $14. Pretty expensive chunk of wood.


I set the height of the router bit to a tiny bit less than 3/8" (which is 0.375").

Hey wait a minute, you say, why's he using a tape measure now?

First, I verified the accuracy of the tape measure against the digital calipers, and second, there are some places where tape measures work better, and this is one of them.

I took a test scrap of wood and marked the approximate depth of the groove.


The first test piece.


It didn't fit. The tongue was too wide.

The jambs that I'm making here are not an exact replica of Andersen's jambs. They have the tongue in the middle of the small side of the board, while I just made the tongue flush with one side. I would need a special router bit to replicate Andersen's millwork, or I would have to rout a notch (called a rabbet actually) on two corners instead of one. My jambs will be a little closer inset than the manufacturer's jambs.

After about 4 iterations of adjusting the height of the router bit and checking the fit, I finally had the tongue at the correct width. But the rabbet was too deep, so I adjusted the fence for a shallower cut.


Once I was done with test pieces, I set up the router in the middle of the shop, on a small table. I clamped the back legs to the table to keep the machine from falling off when cutting.


I routed one edge of the board, and then made a second pass to give it a smoother cut.

One of the problems with an affordable power tool such as this Craftsman router table is that the workpiece is not held firmly in place, so the cutting bit can kick the board away at times, which leaves small humps in the finish. I find that I can remove most of these humps by running the board through a second or third time, while keeping the wood pressed firmly against the fence.

The routed edge.


After routing one edge, I set up the table saw to the desired width, taking into account the extra width needed so the jamb extensions would act as plaster grounds.


I ripped the board to width.


I cut the board to length. The length of the side jambs is not critical, but the top and bottom pieces are, because they fit in between the sides.


I set the first jamb in place to check its fit. I gave it a tap with a hammer and a block of wood. I was lucky to get a snug fit because the jamb stayed up by itself while I fastened it.

Fastening The Jamb Extensions:

I drilled a clearance hole in the jamb. Drilling a hole across the wide dimension of the board can be a little tricky because the hole can exit the other side in the wrong location. 

To get good drill bit alignment, I close one eye and line up the drill bit parallel to the face of the board.


I used a larger bit to drill a countersink. The screws I used weren't long enough and had to be recessed about 1/2".


I used trim head drywall screws, which have a very small head and are designed for installing wood trim onto walls that are made of steel studs and won't accept finish nails. The heads of these screws are not much bigger than some of the longer finish nails.


The trim head drywall screws I bought used a #1 square drive bit. These heads are tiny, about half the head diameter of a regular drywall screw.


The trim head screws installed like any other screw... no problem.


Except that the screwdriver bit kept getting stuck in the countersink hole, which was only a minor nuisance.


Just to be on the safe side, I used a square-drive screwdriver to give each screw its final tightening.

Since taking these pictures I have found longer trim head screws, made by GRK. They are available at woodworking supply stores and better lumberyards. 


Making A Tapered Extension Jamb:

One of the jambs needed to be tapered. I used a pencil to mark the jamb parallel to the wall surface. Since I don't have a taper-cutting fixture for the table saw, I decided to just plane it down. 

I cut this jamb wide enough to give me the desired width at the worst location. It's easy to remove material, it's hard to add it back on.


This is a basic block plane.

I keep it set to a very thin cutting depth. Keeping this tool sharp means always keeping the cutting blade away from metal, especially during storage.


I ran the plane down the jamb, pressing firmly. To cut the desired taper, I made progressively longer cuts.

When it's cutting right the plane will make nice long curled shavings.


The jamb protruded the 1/8" that I needed.

One of the reasons that I sunk the screws so deep was to allow me to plane down the jamb without hitting metal.


The side and top jambs were screwed in place as outlined above. But the bottom piece (called the sill) cannot be screwed or nailed like the others, because it would damage the thin wood behind it. The sill had to be fastened to something below, so I cut a piece of plain pine to fit beneath the sill.


I fastened the lower piece to the window frame, just like the others (the red arrow shows a screw hole).

Then I fastened the sill to the lower piece with a few small finish nails.


The finished jamb extensions.



Tools Used:

  • Router Table
  • Table Saw
  • Power Miter Saw
  • Electric Drill and Bits
  • Cordless Drill-Driver
  • Block Plane

Materials Used:

  • Clear Pine Lumber, 1x8x6'


Back To Top Of Page 


Before You Hurt Yourself,  Read our Disclaimer.

Search Page

Home  What's New  Project Archives  H.I. World

 Rants  Contact Us




Copyright © 2001, 2005

Written May 8, 2001 
Revised January 10, 2005