The rough opening for a 36-inch pre-hung door is is 38.5 inches by 82 inches. The rough opening should be one inch wider than the frame and 3/4 of an inch higher than the door.
A pre-hung door has a frame attached to it. Many carpenters prefer to use pre-hung doors to save installation time. They are slightly more expensive than door slabs that do not have the frame pre-attached. A rough opening is the opening into which the door and frame are installed. It is composed of a vertical wall stud on each side of the door and a header, a wider board that spans the top of the opening.
To hang the door, a carpenter places the frame in the rough opening and inserts shims – thin strips of wood – between the door frame and the wall studs to ensure that the door is placed evenly. Shims are necessary because framed opening are not usually perfectly square. Without shims, the door would not close properly. The door is then checked with a level and nailed into place with thin finishing nails.
Installing a pre-hung door requires commonly available tools and a few basic supplies. This do-it-yourself project can be completed by anyone with basic carpentry skills.