Prefabricating Exterior Window Trim
Prefabricating exterior window trim makes installation easier and can save a lot of trips up and down the ladder.
Most windows on the market today look a lot alike. Vinyl flanges attached to sides of the jambs form the attachment point, and the jambs themselves protrude about an inch beyond the wall. It’s a good system that’s easy to install and to flash, but the narrow profile of the jambs provides a weak visual impact when compared to the wider trim found on traditional architecture. To get that old-house look, windows with wider trim are available, but they can be expensive, as well as being harder to install and flash.
An alternative is to trim around a regular flanged window. Prefabricating the trim makes installation easier and can save a lot of trips up and down the ladder. That’s a big benefit, especially if you’re working high off the ground. Also, the joints are easier to get tight, and the surfaces are easier to get flush.
Here, I use 5/4×4 PVC stock for the legs and heads, and I make a sill from 6/4 PVC. You could do the same using wood or another material. Just be sure to make the frame 1/4 in. larger than the window. This eases installation and leaves a 1/8-in. gap to allow for a good caulk joint.
Step by Step
![Bevel the sill](https://images.finehomebuilding.com/app/uploads/2016/05/05112932/021260088_02-exterior-window-trim-700x331.jpg)
1. Bevel the sill
Set the saw to a 10° bevel, and set the fence to cut the beveled section of the sill wide enough so that it will extend 3/4 in. or so beyond the face of the casing.
![Rip the sill to width](https://images.finehomebuilding.com/app/uploads/2016/05/05112933/021260088_03-exterior-window-trim-700x331.jpg)
2. Rip the sill to width
The sill should be wide enough so that the flat fits under the window and the bevel begins at the edge of the jamb.
![Rip a drip kerf](https://images.finehomebuilding.com/app/uploads/2016/05/05112935/021260088_04-exterior-window-trim-700x331.jpg)
3. Rip a drip kerf
On the sill’s bottom, cut a 1/4-in.-deep kerf 1/4 in. from the face. This helps to prevent water from following the sill back to the house by forcing it to form drips.
![Cut the sill and casing to length](https://images.finehomebuilding.com/app/uploads/2016/05/05112937/021260088_05-exterior-window-trim-700x331.jpg)
4. Cut the sill and casing to length
Make the legs the height of the window plus 1/4 in. Make the head and sill the width of the window combined with the widths of the legs, plus 1/4 in.
![Rabbet the back of the casing](https://images.finehomebuilding.com/app/uploads/2016/05/05112938/021260090_01-exterior-window-trim-700x262.jpg)
5. Rabbet the back of the casing
Cut a shallow rabbet along the inside edge to fit around the window flange. Otherwise, the trim won’t sit flat on the wall.
![Drill pocket holes from behind](https://images.finehomebuilding.com/app/uploads/2016/05/05112940/021260090_02-exterior-window-trim-700x793.jpg)
6. Drill pocket holes from behind
Use a jig and special bit to drill these stopped and concealed screw holes in the tops of the legs.
![Assemble the head to the legs](https://images.finehomebuilding.com/app/uploads/2016/05/05112941/021260090_03-exterior-window-trim-700x451.jpg)
7. Assemble the head to the legs
Clamp the joint together so that the rabbeted portions of the casing pieces are flush. Make the connection with 1-1/4-in. corrosion-resistant pan-head screws.
![Screw the sill to the legs](https://images.finehomebuilding.com/app/uploads/2016/05/05112943/021260090_04-exterior-window-trim-700x782.jpg)
8. Screw the sill to the legs
Align the back of the sill with the rabbet on the legs, and fasten the assembly with 21/2-in. corrosion-resistant screws.
![Place the frame around the window](https://images.finehomebuilding.com/app/uploads/2016/05/05112945/021260090_05-exterior-window-trim-700x1049.jpg)
9. Place the frame around the window
Maintain a 1/8-in. expansion gap when fastening the frame to the house. Use 2-in. stainless-steel or 2-in. color-matched trim screws in pairs about every 16 in.