Trim Carpentry 101:

Installing Andersen Window
Extension Jambs

In This Article:

Spacer blocks are cut to support the sill jamb, the side extension jambs were nailed in place, then the top. 

Related Articles:
Skill Level: 2 (Basic) Time Taken: 1 Hour

By , Editor


The window to be trimmed: a twin Andersen Tilt-Wash double-hung unit.

The window jamb does not reach all the way through the 2x6 framing.

Andersen is pretty clever about this. They make a series of extension jambs that are designed for just this application: 2x6 stud walls with 7/16" OSB sheathing and 1/2" drywall. The homeowner ordered the jambs through Home Depot, which cost about $230 for 8 windows, 3 of which were big twin units like the one shown above.

Note how the twin window unit is connected with metal plates. The design of this window leaves a wide gap at the bottom.

A view of a top corner.


The Andersen extension jambs come in pairs, wrapped in plastic. Note how the lower piece has no holes and is marked "Use On Sill Only".


The extensions have a tongue that fits into a matching groove in Andersen's windows.

First I tested the sill to check how well it fit. Then I measured the gap between the jamb and the framing.


I cut a series of blocks to act as spacers for the sill.

I tested each spacer to make sure it made a tight fit. I ended up with  3 or 4 different thicknesses.

The spacers were the main reason we needed a table saw. A miter saw can also be used to cut precise thickness blocks of wood.

The spacer blocks were attached with drywall screws. (I pre-drilled the holes.)

Then the sill was nailed to the spacer blocks.


The drywall on the sides was sticking out too far and had to be trimmed back. There was no need to use spacer blocks on the sides and top, because those jambs are nailed into the main window jambs.


The side jamb is positioned and attached with 3" finish nails.

On the right hand side, the extension jamb was a little warped and stuck out about 1/8" in the middle.


I drilled a hole big enough for a drywall screw to pass through...

... and drilled a countersink for the screw head.

But... the screw hole was too close to the edge, and the screw split the wood. So I had to drill another hole, closer to the center of the board. Using trim head screws (which have a tiny head) would have been smarter.

The installation sequence is pretty simple:

  • Install sill
  • Install side jambs
  • Install top jamb

The installation of the last three sections takes about 5 minutes, if the boards are straight and nothing goes wrong.

Note the gap visible between the ganged window units. 

I caulked the gap with Alex Plus.


Then I nailed the trim strip (supplied with the window) using 1" brad nails.

The finished extension jambs.

See the installation of the window casing.



Tools Used:

  • Pneumatic Finish Nailer
  • Cordless Drill/Driver
  • Table Saw
  • Basic Carpentry Tools

Materials Used:

  • Andersen Window Jamb Extensions
  • 3" Finishing Nails


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Copyright © 1999, 2005

Written October 31, 1999 
Revised January 10, 2005