How to Install and Shim an Exterior Door

1). Measure the old door and frame. There are four measurements that are important. First, measure the width and length of the door rounded up to whole inches. Second, measure the jamb width, which is the back side of the interior trim to the back side of the exterior trim. Third, measure the rough opening, which is measured after the interior trim is removed. Measure from side to side and from the sill to the top. Fourth, measure the exterior opening from side to side and from the sill to the top of the trim.
2). Purchase a door that will fit the measurements. Buying the wrong-sized door can lead to much frustration and will waste time.
3). Remove the old door and frame. If you will be reusing the trim, use a pair of pliers to pull nails out through the back side of the trim so it does not get damaged. Use a circular saw to cut all the way through the frame on one side to make it easier to remove it. A pry bar is also a useful tool in removing the old frame. Also make sure to repair any rotted wood under the sill before the new door is installed. Install a metal drip cap on the sill to move the water away from the door.
4). Level the sill area by placing pairs of shims underneath it and using the level.
5). Set the door in place for a test fit. Hold the level against the hinge jamb and get it plumb. If the siding prevents the door casing and frame from fitting in then mark the casing where it needs to be trimmed to fit in and remove the door and trim the casing.
6). Caulk along the sill and the casing once you are certain that the door is going to fit in the opening. Center the door in the opening and shim the sides until they are plumb according to the level.
7). Nail through the jamb into the framing at each location where there is a shim. Replace one screw on each hinge of the door with one long enough to go into the framing so as to prevent the door from sagging over time.
8). Insulate around the frame and install the interior trim and caulk the exterior. Remove the door and stain or paint the door and the jamb.