How to Install Door Casings in a Sliding Glass Door

Lead Image for How to Install Door Casings in a Sliding Glass Door
  • 3-4 hours
  • Intermediate
  • 25-70
What You'll Need
Measuring tape
Door Casing
Rubberized asphalt product
Red rosin paper
Galvanized finishing nails OR 2 nails from a pneumatic nailer

The installation of door casings in a sliding glass door is a moderately easy task to complete. Door casings are four-sided wood pieces of various lengths and thicknesses that are either molded or surfaced. You may need to install new casings to replace rotten ones or as a first step when installing a new sliding glass door.


The first thing you have to do is to measure for the replacement door casings. Measure the points between where the exterior material (brick, stucco, etc.) begins and the old door frame ends. Take measurements at the bottom, center, and top. Record the most detailed dimensions.

Flashing the Door Frame

Apply the flashing around the door frame from the bottom to the top before applying it across the bottom flange. Flashing is used to waterproof the flange of the door frame with a rubberized asphalt product. While some carpenters prefer not to use flashing at all, it can make a huge difference if water gets behind the siding or trim in wet weather.

Attach a piece of flashing along the sides of the door frame. Then, add flashing to the top and make sure to leave a few inches so it overlaps the side pieces. Flashing is only semi-adhesive, so it is best if you staple the flashing to the fiberboard wall sheathing.

Add Rosin Paper

Red rosin paper should be applied along the top of the door as a slip sheet. To do this, you slide it up so that it is underneath the top of the sliding door.

Add the Door Casings

In this step, get the door casings you will use that cuts to the width of the door and use either galvanized finishing nails or 2-inch nails from a pneumatic nail gun. Install the side pieces that go from the bottom to the lower parts, before installing the casing above the door frame. The top piece should run the full width of the door plus the side pieces. You will need at least three screws on each side to secure it properly.

You can use shims, these are wooden wedges, commonly used to trim and level the casings within the space. Make sure that the door casings are fit snugly and properly leveled by using a carpenter's level and a framing square.


There are two areas that you need to caulk. The first is the area between the door frame and the casing and the second is the space between the casing and the siding. After this has been completed, you will need to prime and paint the trim to the color that you desire.