While many home improvement stores or companies can supply you with pre-made rafters for your home or building, creating your own rafters is a perfect do-it-yourself project. Not to mention, building your own rafters is often less expensive than buying already-made sets. You'll need to have an idea of the desired design of the roof before measuring and cutting any rafters. Once this is determined, you can make strong rafters for your building project.
Take the first 2-inch by 4-inch board and mark it so it is perpendicularly lined with the 3-inch by 6-inch edge board. This is the board that the rafters will sit on and is usually the top frame of the structure. Line up the framing square according to both the pitch and run of the roof in order to make your rafter mark with a pencil. The pitch and run will depend upon the size of the building and can easily be calculated (see Resources).
Make the plumb cut, the cut in the rafter after aligning it with the ridge board, into the first rafter with your circular saw. Fit the rafter against the ridge board to make sure the cut is precise. If the cut is uneven or if the rafter does not fit properly, you will have to saw the board again. It is best to use another 2-inch by 4-inch board.
Use a tape measure to figure the length of the rafter. Use the framing square again to mark the outside edge of the board. Make the birdsmouth cut with the circular saw. This will allow the rafter to fit over a stud in the wall. Make the cut about 4 inches in length so the rafter has plenty of room to fit.
Determine how much overhang you want from the rafter. Keep in mind any guttering you are placing around the eave of the roof or even windows on the wall. Most overhangs are around 8 to 10 inches in length. The amount is not really a problem unless you end up with too little overhang, which may cause leaks in your roof over time.
Make the tail cut on the rafter. Mark this cut with the framing square. Make sure the top part of the cut is aligned with the plumb cut in Step 2 and the bottom is the same angle as the birdsmouth cut in Step 3. You can cut the very end of the rafter at an acute or a right angle depending on how you want the overhang to look.
Create the other rafters using the common rafter as long as it fits properly. Make sure everything lines up as square as possible. You may have to adjust your measurements if your building frame is not completely square. Repeat until all the rafters are created and in place then place the roof on top of the constructed rafters.