How To Install An Exterior DoorThe question of how to install an exterior door is a something I hear at least once a week. My first response is to explain that there are two basic things you want to accomplish.If you want to understand how to install an exterior door correctly first remember that there is more than just putting some caulk around the edges and nailing the frame in a hole. It is important to have to correct tools and installation supplies ready when you begin the process. 
I am going to assume that you already have a correctly sized hole in your wall. This will typically be 1/2 inch wider and 1/2 – ¾ inch taller than your door frame, but I recommend that you confirm the correct size as recommended by your door supplier.
To understand how to correctly install an exterior door I will first cover the need to understand how to keep the weather to the outside. First I encourage you to install some type of a sill pan system. This can be either a plastic preformed unit or a flex tape system. The most important thing is that it wrap up the sides of the opening at least a couple of inches and wrap out and around the exterior of the opening at least an inch at the bottom and each side. (see example)
In our discussion on how to install an exterior door it is time to dry fit your door into the opening to confirm opening clearance on both sides and head jamb. Once you have confirmed this remove the door from opening and place an even bead of approved caulk around the exterior perimeter placing a bead up both sides and across the head jamb exterior face. Place these beads to that the nailing flange or brick mold of the door frame will bed into this caulk.
You also need to place one single heavy caulk bead across the sill area along the interior edge of sill pan and up each edge. Do not over do the sill caulk, we have seen over the years an excessive use of caulk under the sill can lead to premature sill rot or even door frame rot and this can be avoided. (see example) Now set the door frame into the caulk and set a temp screw in the upper hinge side corner to hold the frame in place. Move to the interior area so you can begin the process of shimming and plumbing the door in the opening. Once again I will mention that your priority needs to be plumbing, leveling, and truing the door even if that means its not exactly lined up with an out of level wall (Correct the wall if possible).
First plumb and level the hinge side and insert shims to snug the space behind each hinge location. You can anchor this side by either installing a 2-1/2 inch long screw into one screw space on each hinge or by inserting a trim head screw of the same length through the jamb at a point that it will be hidden under the door jamb weather strip. When explaining how to install an exterior door most installers recommend you place five screws in this fashion on the hinge side and eventually at least three more on the latch side before completion. Remember to install proper shims directly behind every screw location so you don’t create any pull on the jamb side.
Now make sure the door frame is pressed securely in the hole on the latch side and release the door to check that it swings and closes without binding. Attach the latch side by placing shims behind the jamb at each attachment point and secure with a 2-1/2 inch trim head screw through the jamb in at least three points so the screw will be concealed under the jamb weather strip. Check the door swing once again to make sure there is no binding, adjust as needed. If your door frame has a nailing flange or brick mold on the exterior you can now place nails or screws here. These flanges are not designed to install the door but to help make the exterior weather tight. 
A couple of last things when covering how to install an exterior door would be to caulk along the interior edge of the sill all along the bottom and up the two sides about four inches. The important thing to understand is that modern sills will weep moisture and we want that moisture to drain or evaporate towards the exterior so we don’t want anything to trap it under the sill. Also check the outer edge of your sill and make sure if it extends more than a half inch beyond any support under it you need to add a support to it. If you do not support this sill extension the constant movement of the sill will cause a breakdown in the caulk between the sill and jamb sides promoting premature rot.
Final steps on how to install an exterior door are to adjust the sill adjuster strip if this is a part of your door, caulk the edge of this sill strip and install the wedge strips that came with your door down along the sill strip. Now install the door hardware and do a final inspection of your door system.
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