I have installed hundreds of doors and windows in my career as a carpenter. I’ve installed more doors than windows and probably more interior doors than exterior, but it’s close. A properly installed door is a joy to behold. It should be both functional and attractive. There should be an even gap between the door and frame around its perimeter. It should neither sag nor bind and open and close freely without rubbing. It should touch the door stop along the entire leading edge when closed and not be “wound” whereby the hinge side of the frame and the strike part of the frame are not parallel. It’s really not that difficult to install a door properly, as long as it was hung properly. I have seen some terrible jobs of doors hung onto their frames, straight from the factory– where either the hinge mortises are uneven in depth and in some cases height (or position along the jamb) in the case of double hung doors. Usually these are the budget line doors from the big orange or blue building centers. I have even seen mortises that were cut an inch bigger than their hinges. It’s always upsetting when I uncrate a door and find such inherent defects, because it the puts the burden on me to either repair the mess or return the mess for a new one. Either way, its more time and money and I HATE breaking it to my customers that the door(s) they got from the Big Box store is/are defective. Anyway, I’ll save that rant for another day.
In the case of exterior doors and windows, weatherproofing is critical, in addition to being properly hung and installed. I have seen more exterior doors that have rotted out frames or door bottoms in South Mississippi and New Orleans than I can count. Shame on me for always looking, but I can’t help it. I have replaced so many rotted exterior doors, I have noticed a trend. They are almost always not weatherproofed; that is, they are just installed in the hole and walked away from. I have also noticed that many of theses rotted out doors are installed in brick houses with a non-beveled brick threshold beneath the door’s threshold, so that water will not shed properly and just pool up under the door. Years ago, I did warranty work for a ghastly tract-style homebuilder (that shall remain unnamed, but suffice it to say they’re no longer doing work on the coast) and there was a crew of brick masons who, it seemed, installed the thresholds and window ledges so that they sloped back toward the house. I replaced so many doors and windows (and sheetrock for that matter) during my time fulfilling warranties for this company. All of this could have been avoided if the proper slope had been present from the beginning.
This is my procedure for a typical exterior door: First, I measure the new door and frame and make sure that it will fit. Next, I remove the existing door and all molding and inspect the sheathing and framework. I will then prep the opening for the new door and frame and/or the new frame and molding for the opening. I make sure the opening is somewhat square and that the floor is perfectly level side to side. I like to use a long beveled shim like a piece of clapboard to slightly cant the threshold so that it sheds water to the outside of the house. Ideally, I would install a metal or plastic pan in the opening on top of the shim, otherwise I will use copious amounts of sealant (I like elastomeric, though its a little pricey) to the bottom of the door’s threshold. I will set the frame (and usually the pre-hung door) centered in the opening, level the hinge side and shim it in place at the hinges. I will usually screw through the shims into the framing with either trim head or countersunk and plugged screws. I will then close the door and get it square in the opening and parallel with the hinge side, by making sure that the leading edge of the door fully contacts the door stop, making adjustments as necessary. Once I am sure that the door is square in its opening and that the gaps between door and frame are the same, I will secure the strike side of the frame with shims and screws, followed by the top. I will reapply sealant to the threshold where necessary and hit the area where the brick molding will lay (ideally about 3/16″ above the floor) with some sealant. Once the brick molding is on (if it isn’t already) I will apply sealant between the brick molding and the siding (usually brick) and the brick molding and the door frame. (Other types of siding such as wood, ply, cement fiber or vinyl will have drip cap flashing installed beneath the siding and over the top of the brick molding at the top.) From the inside, I will spray low expanding foam for doors and windows between the door frame and house framing, and then install the casing. Then it is a matter of filling, caulking and painting once the sealant dries, then installing the hardware. I have seen doors that I installed over ten years ago looking like they did the day I installed them (especially after a fresh coat of paint) with no frame rot, nor water incursion.