Most wall framing is done with 2x4 lumber, but a new interior wall may also be built using 2x3's instead. This is a common practice when a new partition is added or a new closet is built in an existing room. Working with 2x3's is easier and will cost less than 2x4's because of the size difference, and the smaller size takes up less space which can be important in smaller rooms. The only drawback to using this smaller dimension lumber comes when installing a door or an electrical box. A door jamb and most outlet boxes will usually be too deep to fit flush in the available space and will stick out on one side or the other. This might not be a problem with a closet where the overhang can be concealed inside, but with a room partition this effect would not be acceptable.
A basic wall frame includes the soleplate, the top plate and the studs. Normally one 2x4 is used to form the soleplate, and in the case of load bearing walls, two are stacked to form the top plate. In existing construction, interior walls that are not part of the support system will have a one-piece top plate. Because a new partition is just a divider and not a load bearer, it can be built using just one piece of lumber to form the top plate as well.
The spacing of the stud in any particular wall construction will vary, but the accepted standard is "16 inches on center". This means that the studs are spaced 16 inches apart along the two plates with the center of each stud placed at each 16 inch mark. This arrangement makes it convenient to install four foot sheets of building materials like drywall, paneling and plywood.
Because of the desire to save on the cost of new construction, many home builders will fudge these numbers, in some cases spacing studs as much as 24 inches apart. The obvious disadvantages of this are a weaker structure with less support for, and more difficulty in, hanging finishing materials. For a new partition wall we recommend you use the 16 inch rule for simplicity and easier finishing.
Measure the space, from end to end, where the new wall will be built. Also measure the height of the ceiling in several places along the length of this space. In most cases the ceiling height will vary slightly along the length of a room. Find the shortest measurement from floor to ceiling and build your new frame to this height.
Subtract 3 inches, for the thickness of the two plates, from the ceiling measurement to get the actual length of the studs. Because the height of most existing ceilings will be over 8 feet, you'll probably need to use 10 ft lumber to get wall studs that are long enough.
If you have a large space to cover, build your new frame in small, manageable modules. For instance, for the 16 foot structure here we can build two 8 foot sections and join them together to form the whole wall. Begin your construction by dividing the wall length by 16 inches to determine the number of studs you'll need. For this example we'll need fourteen studs, six for each 8 foot module and two extra as the starting studs.
Determine how many 2by's you will need for the top and soleplates using the length for the new structure. If your measurement is twelve feet or a little under, which is a common room size, you can use 12 foot 2by's for your plates to avoid breaking up the structure into more than one module. If your space is longer than 12 feet, use 8 foot 2by's and build each section before joining them together as illustrated in this example.
Measure and mark the ceiling where the new wall will be erected. Use a framing square set against an adjacent wall to form a 90° angle with the new structure. Mark the ceiling with a straight line using a chalked string and then hold a plumb bob on the line at several places to find the corresponding points on the floor. Mark the floor at these points and use the marks to layout the position of the new frame. Lay a 2by on the floor next to the marks and use the framing square against the adjacent wall to get it straight. Draw a pencil line along the 2by and use the line when setting the new structure in place.
Build the frame with the lumber laying on edge, on the floor next to where it will be erected. Lay the top and soleplates side by side and measure starting from one end. Mark both boards simultaneously, every 16 inches along their length. Separate the plates and lay the studs between them so the centers align with each mark. Nail the plates to the studs using two 16d or 20d nails, driven through both plates and into each stud.
At the floor mark, nail a 2by block to the soleplates of the adjacent walls to act as a brace. Or if the floor is unfinished, you can nail a couple of blocks along the line where the soleplate will fall. Use the blocks to hold the bottom of the new frame in place while lifting the top plate to the ceiling. Wedge the structure between the ceiling and floor using a small sledge hammer to tap the plates as needed to draw it up plumb.
Set the new frame plumb and square using a level held against the edge of the studs. Tap the plates one way or the other until they aline with the marks and all your studs read plumb by the centering of the bubble in the level. Check to be sure the frame is square at adjacent walls using the framing square and then secure it in place with 16d or 20d nails. If you don't want to use nails you can also use 3 inch deck screws, driven through the soleplate and into the flooring. Finish the installation by securing the top plate to all ceiling joists running perpendicular to the new wall. Finish by fastening the end studs to the top and soleplates of the adjacent walls using a toenailing technique.
If the new wall is parallel to the ceiling joists and not perpendicular, try to plan the placement so it falls directly below one of the joists. The top plate can then be secured to that joist all the way across the room. Otherwise, 2by spacers will have to be added inside the ceiling, between the joists, to stabilize the wall. Fasten the spacers perpendicular to the new frame below at several points along the length of the room, and then fasten the top plate to them.
Beyond the basic structure of studs and plates, a door may also be built into an interior wall. A door frame will include king and jack studs to support the sides, and a header and cripple studs to support the weight over the opening. The frame should be built to accommodate a door with the soleplate in place for stability, the plate can then be cut out after construction is complete. See this link for more about framing for a new door.