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Installing double doors requires more plumb and level readings, shim adjustments, and—above all—more patience than hanging a single door, but the procedure is much the same. So we’ll zip through the steps covered earlier to get to the heart of the matter. figuring out why the doors don’t meet perfectly in the middle and what to do about it.

INSTALLATION: A QUICK RECAP

► Measure the RO to make sure it’s big enough to install the door frame.

► Using a 6-ft. spirit level, see if the floor is level and the sides of the RO are plumb. Also check the walls on both sides of the RO for plumb. If the bottom of the opening is more than !/i in. out of level, correct the situation. French doors are wide and heavy and need to rest solidly on a level opening. Otherwise, they can rack and bind.

There are several ways to level an exterior doorsill. After you’ve installed a floor pan, place a spirit level across the bottom of the rough opening (RO) and insert pressure – treated wood shims under the tool till it reads level. Tack shims to plywood subfloor­ing to keep them from drifting, and then install the prehung door unit. If the subfloor is concrete, use construction adhesive to spot-tack the shims. Or you can place the door frame into an unlevel opening, use a flat bar to raise the low end of the frame sill, and insert shims under the sill till it’s level. To prevent flexing between shims, space shims every 12 in. beneath aluminum/combination or oak sills, or every 6 in. beneath sliding-door sills.

The problem with any of these solutions is that many sills—especially hollow al...

To replace an exterior door, first remove the interior and exterior casing to expose the door frame and the rough opening. Proceed carefully, minimizing damage to the siding and finish surfaces whenever possible. Exposing the rough opening enables you to measure it exactly and inspect for water damage or rot, which must be repaired before installing a new unit.

To remove casing without breaking it, use a flat bar to raise it slightly; then use a reciprocating saw with a metal-cutting blade to cut through the casing nails.

(Wear safety glasses.) Cutting nails is far easier on surrounding surfaces than prying alone. However, some casing can’t be removed without destroying it—stucco molding, for example.

In many older houses, the sill will be a 2-in...