Shims are time-consuming to use and tend to fall out as wood shrinks. You can eliminate shims if you use the clipping technique to hold trimmers securely in place (see p. 130). Clipping will also allow you to nail the jamb directly to the trimmer.
For many years, trim carpenters have been using drywall screws in place of shims. Screws work as adjustable shims. They can be driven into the bottom plate, for example, to hold baseboard plumb and square. You can also use screws to help level, plumb, and square cabinets as you install them.
On the rare occasion when I need a shim (as when I want to pry two boards apart), I can quickly make one by ripping a slice off the edge of a 2x.
If you set the trimmers plumb so that the rough opening is 1A in. larger than the door-frame assembly, you should have enough room to move and adjust the frame slightly. If the trimmers weren’t set plumb, you have to plumb and shim every jamb, making sure they are straight and true.
The door has to be open when nails are driven through the jambs. Usually I nail or screw the hinge side of the jamb to the trimmer stud first with an 8d nail or screw gun, sinking the fasteners behind the weatherstrip near the hinges (see the photo on the facing page). Later, these holes can be filled and painted. Nailing through the exterior casing (just like with wood windows) further stabilizes the door frame.
There will be about a Win. gap on the lock side of the door between the framing and the jamb, so this side needs to be shimmed to keep it straight and in place for the life of the house. I carry З-in. by З-in. blocks of Win., 3/i6-in., or Win. plywood to use for shims. If you use shingles for shims, be sure to push them in from both sides so you have a level bed to nail the jamb against (for more on shimming, see the sidebar above). The thin plywood blocks seem more solid to me, and I place them about 6 in. from the top and bottom, as well as one above and one below the door latch. These too can be secured with nails or screws hidden behind the weatherstripping.
Keep closing the door to make sure the Vs-in. gap is maintained between the jamb and the door. Once the frame is nailed securely in place, check once again to make sure that the door opens and closes with ease.
While much the same process is used to set interior prehung doors, they don’t have a sill, which makes them floppy and harder to handle. Again, I like to set the hinge side first, hard against the trimmer. Jambs should be flush with the drywall on each side of the opening, because any irregularity here makes it harder to install door casing.
Nail these jambs off by driving five sets of 6d finish nails toward the edges of the jambs, one close to the top, one close to the bottom, and three sets spaced evenly between. I also replace one screw from each hinge going into the jamb with a longer 2-in. screw so that the trimmer supports the door and not just the jamb. End by cross-sighting the jambs to ensure that they are parallel (for more on cross-sighting, see the side – bar on p. 180).