French, garden and other double interior doors can open up your space by increasing access between rooms. Double interior doors provide privacy and traffic control without blocking the view or preventing light from coming through. Installing double doors requires similar tools, techniques and materials to hanging single passage doors. The main key is levelling the two outside jambs to ensure proper operation of the doors. Use a prehung unit with the hinges and lock bores in place to save time and money.
Set the double prehung door into the opening of the wall frame and centre it. Wedge shims between the top of the jamb header, or horizontal top piece, in the centre of the door. Nail through the jamb and the shims into the frame header above the door jamb.
Level one side jamb, which is the vertical frame member at the side of the interior jamb frame. Wedge shims behind the jamb at the top of the frame to hold it steady. Set the level against the edge of the jamb and adjust the bottom of the jamb in or out to centre the bubble in the level indicator. Add shims behind the bottom of the jamb and nail through the jamb and shims at the top and bottom.
Wedge shims behind the top of the opposite jamb and place the level against the edge of the jamb. Move the bottom of the jamb in or out until the bubble is centred in the indicator. Nail through the jamb and shims into the frame behind as you did for the first side.
Add shims above the top corners of the jamb header and nail up through the jamb header into the frame above. Test the doors for proper operation. If the door function is off, recheck the level on both side jambs and adjust as needed.
Cut door casing. Mitre cut four side pieces at 45 degrees on the top end. The short side of the mitre should hit ¼-inch above the bottom edge of the jamb header. Cut one right and one left hand piece of casing for each face of the frame. Nail these in place on each side of the door unit on both faces of the door frame. Use finish nails and a hammer to nail them into the edge of the jambs, ¼-inch out from the inside edge of each jamb.
Measure between the top outside corners of the side casing on each face of the door frame. Cut one piece of casing with both ends mitred at 45 degrees, one right, one left, so that it fits snugly between the top corners of the side casing pieces. Cut one piece of casing for each face of the frame and nail them as the side pieces to the edge of the jamb header.