A couple a weeks ago I wrote about the basics of how to make window jamb extensions.
Today, in part 2, I’m going to illustrate 3 three different real life door and window “jambs” that I had to deal with when I trimmed out a mudroom addition a few years ago.
I’ve included a section drawing and before & after photos for each situation.
1. Triple Vinyl Window
I’m not sure if this is the case for all or most vinyl windows, but mine had a 3/4″ wide moulded channel around the edge, designed to accept jam extensions. Too easy. 
I measured the depth from the bottom of the channel to the face of the drywall and ripped the (select pine) boards slightly narrower to make sure I could get them flush to the drywall if the depth varied around the perimeter. I cut the top jamb extension the full length of the channel as well as the sill* (*see footnote) and cut the side jambs to fit between them (the ends butt into the top and sill).
All sides were primed and the exposed surfaces painted before installation.
To install this type of extension, you should insert the jamb extensions most of the way into the channels, leaving them slightly proud of the drywall and use a (straight) 16″ long 1×3 board as a guide to make sure the jambs are flush to the drywall surface. Just hold the board diagonally across the corners where the jamb extensions meet with both ends overhanging the drywall.
A gentle tap on the board with a hammer will set the jamb flush to the drywall. Re-check all the corners before nailing. Use shims and a square to support the front edge of the jamb extensions where you’ll be nailing it.
Cut and nail the casings, allowing for a 1/8 – 1/4″ reveal on the edge of the jamb extension.
2. Exterior Door
The jamb for the pre-hung exterior door was correctly sized to suit the depth of the wall framing and sheathing. But I added 1″ thick rigid insulation to the exterior of the wall. That left the jamb about an inch short of the drywall.
The remedy for this was to add a short extension to reach the face of the drywall The first step was to .measure the depth at various places around the opening to determine how wide the extensions needed to be. I ripped the stock to width and cut the pieces to proper length.
I marked a 1/4″ reveal line on the pre-hung jamb. The extensions were shimmed and nailed to the rough opening. I primed and painted the jamb and extensions before installing the casings and corner blocks, which were also pre-painted. 
You can see in the picture that there are two reveals – one at the extension and another at the casing. If you’re installing corner blocks, you need at least a 1/4″ reveal in order to centre the block on the casing.
3. Kitchen Door
The door between the kitchen and the mudroom was installed several years prior to the mudroom addition. It’s a pre-hung exterior door with vinyl brickmold on the mudroom side.
The vinyl brickmold was slightly proud of the original door casing before adding the rigid foam. After the insulation and drywall were installed, the brickmold was recessed 7/8″ from the wall face.
You can see in the picture that this wall was originally an exterior wall. It’s full-dimension 2×4 studs, two layers of 1″ thick boards, and faced with clapboards. The mudroom space is unheated so I added 1″ rigid foam insulation. The strapping was located for attaching the drywall and casings.
Since I couldn’t nail the extension to the foam and drywall, the only option was to attach it to the vinyl brickmold. I ripped 7/8 x 1″ strips from 2 x 4 stock. After marking a 1/4″ reveal line on the vinyl brickmold, countersunk holes were drilled in the 1″ wide face of the jamb extensions.
These were offset enough to be covered by the casing and leave a 1/4″ reveal. The pre-painted extensions were installed with 2″ #8 screws, on the assumption the brickmold just had airspace in the centre.
The casing and corner block installation was the same as the window and exterior door.
The end result
Although each situation was slightly different, the overall impact of the door and window casings is consistent and gives the impression that the mudroom is part of the original 100 year-old house.
The multiple reveals add to that sense of sophistication and “mass”, reminiscent of the grand old houses of the past.
*Footnote:
I initially planned to do a “picture frame” casing (mitred at all four corners) but changed my mind to go with the corner blocks and larger casing to match most of the original casings in the house.
I had already cut and installed the sill to be flush to the drywall. Rather than tear it out, I added a 1 1/2″ wide strip to the front edge of the installed sill that also extended beyond the rough opening for the side casings to land on. I rounded off the front edge and ends of the strip, then pre-drilled several small holes for 2 1/2″ finish nails along the front edge.
I applied wood glue along the back edge, inserted nails in the holes and tacked the strip into position, making sure that the top surfaces were flush or slightly higher than the installed sill. After the glue was dry, I recessed the nails with a nail set and filled the holes as well as the joint between the two pieces that make the sill. After sanding and painting the joint was (and still is) invisible.
Part 1 – How to Make Window Jamb Extensions
