In order to permanently divide a large room into two smaller ones, it's necessary to know how to frame a wall. Fortunately, this is not as difficult as it may seem. Required supplies include 2x4 studs, a box of 12D nails, a measuring tape, a pencil, a level, and a hammer or nail gun.
Begin by measuring the width of the space the new wall will occupy. Cut two 2x4 studs the same length as that measurement. Place them side by side on the floor and use a pencil to mark off 16 inch (40 cm) intervals across the width of the studs. If there is to be a doorway in the wall, determine the width of the door and where it will be placed, add two inches to the door width, and clearly mark the opening on the two boards. When the two boards are separated, there will be matching markings on each board.
Determine where in the room the new wall will be placed. Cut a third 2x4 the same length as the other two and nail it to the ceiling joists, keeping the corners square in relation to the adjacent walls. This board will form the top of the new wall.
Now measure the height of the wall at both ends where the new wall will form a corner with the existing one. Novice carpenters are often surprised, as they frame a wall for the first time, that their house walls are not the same height on both sides of the room, but few houses are absolutely plumb. Subtract 4½ in. (11 cm) from each wall height and cut two 2x4s to equal these new measurements. These will become the vertical studs at each end of the new wall.
The preliminaries are over, and it's time to frame a wall. Lay all four studs on edge on the floor and nail them into a rectangle using 12D nails, making sure the boards that span the width of the room are on the top and bottom of the frame with the pencil marks facing inward. The boards that extend from the floor to the ceiling should be sandwiched in between the two long studs, forming the sides of the rectangular frame.
Unless the existing walls adjoining the new partition are exactly the same height, it will be necessary to cut each of the other studs individually. Starting at the end of the partition furthest from the anticipated doorway, measure the height between the marks on the inside top and bottom of the frame, and cut a 2x4 to correspond to each measurement. Nail these in place parallel to the two end studs.
Presumably a pre-hung door assembly was purchased at the same time as the other materials needed to frame a wall. Check the assembly's instructions to determine the doorway dimensions it will require, and cut a shorter 2x4 the same width as the recommended doorway opening. Nail this crosspiece horizontally at the appropriate height between the two studs that form the sides of the doorway. Once this horizontal piece has been attached, nail two more vertical studs flat against the doorway frame to reinforce the opening.
Now lift the entire frame into place and wedge it beneath the stud screwed to the ceiling. Use a level to make sure the vertical studs are perpendicular to the floor and as plumb as possible. Nail the bottom of the frame to the floor – avoiding the spot right under the doorway – and secure the top rail to the stud in the ceiling. If electrical service will be required in the wall, this is the time to add it.
Finally, carefully cut out the section of 2x4 under the doorway, insert the pre-hung door assembly according to the manufacturer's directions, and the wall is ready for drywall or paneling. Learning to frame a wall is a basic carpentry skill that will last a lifetime.