I got nice solid core prehung interior door. It seems to have pretty weak frame, though and I see that hinge screws are protruding through it and the door is attached to the frame with nails on opposite side to prevent swinging. When I removed those nails, door weight immediately skewed the frame. So I put those nails back for time being but I will have to take them out before install and likely unscrew hinges at least partially.
In addition I have 1+" wider opening than this frame and I would like hinge side to go as close as possible to stud. This leaves me with 1" gap on the lock side and I plan to put 1×4 there.
All in all, it seems that a lot of things that need to happen to install it right, so I am wondering if it's advisable to just remove door from the frame and then install frame level, plumb and square where I need it and then reattach the door with 3" hinge screws. That would make my life lot easier but I've read in some places that it's difficult to install the frame right without a door. Not sure why.
Best Answer
In response to your question about a door being difficult to install the frame right without a door, getting the door frame square, following the walls, besides plumb on the sides and level at the head, which should result in a square frame, can be difficult. The small gap that is typical of doors will reveal a lot if there is any minor discrepancy in the jamb when it does not have a door to follow. The door itself acts as a big square to gauge the jamb by. Also, some doors are not perfectly flat. Having the door in place at the time of setting the jamb allows the jamb to be tweaked a little here and there to get the door to set properly to the door stop. Using my method below is how I get past issues of weight, easier centering of a door when I have a specific spot I want the jamb in the opening, and aligning it within a wall width with allowances for tweaking.
In cases when a door is very heavy or even a lighter door that is a 36" leaf or 8' tall, I will remove it and install the jamb only to a point. After the top corners of the jamb are shimmed tight enough to keep from shifting around in the opening, and set with one finish nail so it doesn't fall out by the top and set bottom hinge plumb from the top corner and set with one finish nail also. This way if the door is light enough or at least not too heavy, the door could be set back in its place. After it is set back in place I will remove the middle screw of all three hinges and that's where the shims will go. I will rip my shims unless they're already about an inch and a half wide. I will typically rip a wider shim to no wider than 2 inches so it fits between the two remaining screws that holds the hinges in place. I failed to mention this first... the floor is checked for level at the opening so if one leg of the door has to be longer than the other that is done before the door jamb is even set in the opening so there's no gap at the bottom of the jamb, where they meet the floor.
Now back at securing the hinges. With the middle screw still out and gauging the shims so the gap is adjusted evenly, set a screw that matches the color of your hinges, but is 2 1/2" long so it will tie the hinge through the jamb and into the rough framing. The sequence are used for this is to shim the top hinge as close as can be figured and set the screw in the top hinge, all while reading the gap from the top hinge to the corner and get it even with the rest of the gap along the hinge side. You will need to confirm that there will be the same size gap on the latch side as well. With the bottom hinge presumably plumb to the top corner that could be set also with its screw in place. This leaves the middle hinge to be done, this I typically remove the hinge pin, tap the shim in or out as needed to get both knuckles to line up perfectly and so the pin drops in after I set the screw. This should get the jamb perfectly straight according to the door.
After the hinge side is squared away and presumably the gap is even across the top of the door since the jamb has been corrected to hit the floor, if necessary is no more than setting the final shims in 3 if not 4 different places on the strike side after you apply your 1X4 and finish off the set of your door making sure that the door stop is hitting the door evenly in all places along it's length.