Most homeowners are installing a new door to replace an old door that's been damaged or cut short to fit existing flooring-a simple enough fix if you take the time to measure.
If the door is short because you've changed flooring, measure from the door top to the new floor height. Write down the height for later reference. Push out the hinge pins with a punch, tapping up from the bottom. Have a helper hold the door as the pins come out. Lay the door on sawhorses or a work table. Remove the door knob assembly and hinge leafs.
A new core-a door without holes, hinges, or lock mechanism-can be ordered from a lumberyard or home center. Just measure the existing door for the right measurements. When you have your new core, lay it on top of the old one. Check the tops of both doors to see if they're grooved or chamfered a bit. Doors are usually beveled from three to five degrees on the “strike” side of the door where it hits the door stops. Make sure both doors have the bevel running in the same direction because it's imperative that your new door face the same direction as your old door. The hinge mortises must be cut on the correct side and face of the door to achieve this.
CUTTING TO FIT
Align the doors perfectly to see if the new one will fit into the jamb. If it's larger than the old door by about an eighth of an inch on the sides, it will have to be cut down using a circular saw and a straightedge - don't worry about the top and bottom height just yet. If it only needs about an eighth of an inch taken off the sides to make it fit, use either a jack plane or a belt sander to size it. A belt sander is useful to knock the sides down but will make a lot of dust. The jack plane will also trim it to fit and isn't as messy. Place the door on edge using a portable vise or pipe clamps to act as feet and hold it vertical for planing. Once planed, put the core in the jamb to check for fit.
Top and Bottom
Once it's cut to the proper width, it's time to cut it to length or width. Cutting the bottom is much easier than cutting the sides, so try to get a width that fits to within an eighth of an inch or so on each side. When cutting the length, make sure you know where that beveled top edge is - you don't want to disturb the bevel that helps the door close properly, so you'll only want to cut it to size from the bottom. Now refer back to that door height you wrote down before you removed the existing door. The new height is the measurement from the top of the jamb down to the floor minus half to three-quarters of of an inch. You usually want about a half-inch opening at the bottom of the door so it glides over obstructions like carpet. The three-quarter-inch opening gives more clearance if the floor isn't level.
Before you trim the bottom, wrap masking tape around the door where you'll cut to keep the surface veneer from splitting. Mark the cut line, then score it with a sharp utility knife. Use a straightedge or a straight 1×4 clamped to the door to guide the cut with the circular saw. Check for equidistance from the edge of the saw guide to the mark across the entire width of the door bottom.
MARKING FOR HINGES AND HARDWARE
Place the new door on top of the old door again and mark the hinge locations. Use a combination square to connect the vertical lines for the top and bottom of the hinge. Also mark the knob holes and the backset hole. Use a utility blade to score where the backset face plate and the hinges will go. Score down as deeply as the hinge. Use a chisel to cut a number of lines in the mortised area. Then chisel carefully to remove the waste, one section at a time, keeping the mortise at the proper depth.
Place the hinge in the mortise, mark holes for a center punch and carefully drill small pilot holes for the hinge screws. Be sure to check packaging first before setting the holes. Most standard door knob assemblies use a 2-1/8-inch hole saw to cut the door knob hole and a 7/8-inch spade bit to drill out the hole for the backset. Place the backset into the hole and trace around the front plate for the striker, then mortise it in with a utility knife and chisel the same way you did it for the hinge leaf. Install the knob and you're ready to hang the new door.
Enlist your helper again to mount the door, putting the top hinge in first and sliding the hinge pin down part way so it holds. Slide the bottom hinges together and put that pin in. Tap them both into place.
If you're planning on painting rather than staining the door, use automotive body filler. Use this two-part product to fill the trim nail or trim screw holes and any other imperfections in the door or jambs. It's quick, easy to work with, sets up fast, and sands beautifully for an extremely fine finish.
If you go the pre-hung route, the steps are the same but much simpler. Carefully remove the trim or casing around the door. If you're hanging a new jamb, you'll need to remove all framing parts. The pre-hung door comes with all the hardware and hinges attached.
Open the package but do not remove the cross bracing that holds the door jambs in place. Set the complete door in the framed opening and shim the header jamb and side jambs with wooden shims. Make sure that you place shims behind the hinges. Don't force the shims in or you'll bow out the jambs. Slide the shims in gently until they stop.
Check to make sure the door is square in the frame and check both jamb sides for plumb. Check the top jamb for level. If everything is square in the frame, you can nail the jambs to the frame using finishing nails. Countersink the nails after they're driven in and fill the holes with putty. An alternate method is to attach the door frame with trim screws. The head is a bit larger than a trim nail, but is still easily covered with putty once countersunk. An added benefit is that you can take them out if the door frame shifts during installation. Cut off the excess shim material, and install the trim around the door.