Installing a Prehung Exterior Door

Check the floor for level and shim if needed. Place a pair of shims -- one facing each way, so the surface will be level -- every 8 inches or so and nail them in place.

Uncrate the door and set it in the opening. Check for level and plumb, and shim as needed. Tack (partially drive) nails or screws to hold it in place temporarily.

Check that the door operates smoothly and that the gap between the door and the jambs is consistent all around. To be sure, you may want to install the lockset at this time.

Mark around the door's brick molding for cutting the siding. Or hold the molding you will use in place and mark. Use a handsaw to finish the cuts at the bottom if a circular saw can't complete the cut. Use a chisel at the corners.

Slightly pry away the siding all around and use a reciprocating saw to cut through any nails within 6 inches of the opening. Slip in strips of roofing felt or building paper, wrap them around the framing, and attach with staples.

Cut a piece of metal drip edge flashing and slip it up behind the roofing felt. Use a scrap of trim to confirm that you've allowed enough space. The drip edge should incline slightly to direct water away from the house.

Apply three beads of caulk at the sill plate before you set the door into place. If you have installed shims, apply extra-thick beads to ensure that the threshold will be sealed at all points.

With a helper lift the door's threshold onto the caulk (avoid sliding, which would weaken the caulk seal) and tilt the door into the opening. Make sure the brick molding seats against the sheathing.

Tap in shims as needed all around and check the sides for plumb. Also check that the door operates smoothly and there is a consistent gap between door and jambs all around.

Attach the hinge-side jamb first, using 10d casing nails. If possible, drive the nails where they will be covered with stop molding. Drive nails near the shims and check that the jamb has not become bent.

Nail the latch side in the same way, constantly checking for a consistent gap between door and jamb -- a piece of cardboard from a note pad makes a good gauge. Nail the header jamb as well.

At each hinge remove two screws and drive in 3-inch screws with the same size heads. These screws add considerable strength and stability to a door.

Attach the threshold by driving screws into the sheathing. Wipe away any caulking that oozes out. If the threshold has a rubber gasket, install it.

Drive casing nails to attach the brick molding. Use a nail set to drive the heads slightly below the wood surface. Fill the holes with wood filler and sand smooth.

Cut the shims inside. You can often score them with a utility knife and break them off. Or cut them with a handsaw. Cut them flush with the jamb edges, but take care not to mar the jambs.

Spray nonexpanding foam or gently insert pieces of fiberglass insulation between the jambs and the framing.

If siding is exposed below the door, it will be vulnerable to damage by foot traffic. Cut out and replace the siding with a 1x kick board or install the kick board over the siding.