Q: I’m replacing an exterior vertical door casing that has rotted at the bottom, but the header casing is still in good shape. Can I replace the vertical casing with PVC and not replace the header trim?

A: Greg Burnet, a window and siding contractor based in Chicago and a presenter at JLC Live, responds: Though there’s no issue with mixing wood and PVC trim, I can think of a few reasons for replacing all the exterior trim while you are at it.

First, if the door doesn’t have a nailing fin, removing all the trim can give you access to the space between the jambs and the framing. On the doors and windows I encounter, this rough-opening space is rarely air-sealed or insulated properly. If you have access to that space, you should inject low-expanding spray foam designed specifically for use around windows and doors to insulate and create an effective air barrier around the door.

The second reason to remove all the trim is to make sure that the window or door is properly flashed. If there’s no flexible flashing bridging the gap between the jamb and opening, I’d suggest installing 4- or 6-inch-wide peel-and-stick flashing tape on both the sides and head of the door. Use either butyl or acrylic tape (asphalt-based tapes should be avoided if they’ll be in direct contact with PVC). Install the tape shingle-style, lapping it onto the jambs and rolling it to ensure a good bond.

With the door air-sealed and flashed, you can preassemble the new door trim. Preassembly makes for a faster installation, but more importantly, you can achieve tight, flush joints that can be reinforced mechanically with fasteners. Tight joints are important to the long-term success of an assembly, especially on an exterior. Trim joints that open up not only look bad but are also potential paths for water to enter the structure. With this in mind, replacing all the trim in your situation lets you glue the new PVC trim together with the proper cement. Once cured, that bond is usually stronger than the material itself, and the joints should stay tight for many years.

For fastening trim, I’ve had good luck with the Cortex system. The concealed fasteners look better and don’t require messy fillers or sanding, which speeds the installation. Also, be sure to install a rigid metal head flashing above the head trim.