Rough Framing for a Window
Most people do not think of a window as a hole in their wall, but it is. It lets in light and creates ventilation. To prevent it from being a weak spot, especially in a load-bearing wall, it must be properly framed in. This is important in both new construction and if you just happen to be adding a window to a blank wall.
In your framing in general, the studs will be placed 16-inch on center. This is to accommodate sheathing, drywall, etc. Your window's placement should leave this pattern intact.
It is most important to know how big your rough opening must be. This is a factor whether you are putting in a replacement window or putting up a new building. This specification will be provided with your new windows. It is important to get the rough opening as close as possible to the required rough opening. However, if you must err, make your error slightly on the bigger side as it is easier to shim your window in than trim your opening to make room for the window. Therefor, for the beginner, the best approach might be to begin with the wood directly surrounding the rough opening.
The header is going to be the support that spreads any of the weight that the wall bears. To build this, get a piece of the appropriate dimension lumber (4-foot openings require a minimum 4" dimension lumber, 4 to 6-foot requires 6" dimension lumber, 6 to 8-foot openings require 8" dimension lumber). Cut two 2Xs to the width of your windows rough opening requirement PLUS the width of two 2x4s (which will form the trimmer studs). Sandwich a piece of plywood between the two boards and knock them together with 10D nails.
You may cut the two trimmer studs to the prescribed height of the rough opening. Then cut a sill using a 2X4 the same length as your header. Nailing from the bottom of the sill, attach your trimmers and then your header by toe-nailing. (Toe-nailing is nailing a board in at an angle.) Attach the king studs (cut to the height of the other studs in the wall) to the sides of this opening using 10D nails. Now that you have your rough opening, you must place it within the frame. It is not required in most local codes, but it is a good idea also to block the king stud to the next stud in the wall. This will add strength to the whole assembly. To test the opening for squareness, you can measure the diagonals, if the measurements are within 1/8 of an inch, you are probably close enough.
At this point, you probably have your frame half built and lying on the floor. Simply set your rough opening where you want it to go. Position your king studs; attach them to the top and bottom of the frame using two 10D nails on each end. Now cut your cripple studs to length and attach them to the frame and the rough opening.
If you are working on an existing wall, you may have to tear out drywall or siding and sheathing in order to frame the opening properly. If you are replacing a window, be sure that any rotted or weak wood is torn out and replaced. When done framing, be sure to caulk all seams where pieces of wood come together. This will add to the insulation of the structure.