Step 8: Framing the overhead door header and top plate
With the other three walls framed, you will want to tackle the big overhead door opening. Since most of this wall is empty space, it is easier to build two little walls on each side with the jack studs for the main beam and then lift the overhead door header into place. You can nail the top plate in place across the king studs before lifting the main beam in order to add some rigidity to the walls before hefting the big beam. The sequence of sketches below give an idea of one way to do this.
You will want to include at least two jack studs on the ends (as pictured) in order to support the large span and mass of the header beam. You can either bond nail the two 2x12's that make up the header beam on the ground and lift them both or you can lift one in place, end nail it to the king studs, hold it in place and lift the other one up and bond nail and end nail it as well. My opinion is that nailing the header together first while heavier is easier if you can muscle it. You will want at least four people for this job. Two to lift the header from the ground, and two on ladders at each end to hold and nail the header in place. You should also toe nail the header to the jack studs for security while you finish the wall.
The final step in framing the header beam is to cut and place cripple studs between the top of the header beam and the top plate. End nail them to the top plate and toe nail them into the header beam.
Once all four walls are up, take your 2x4 bracing across the top plates off. Then measure and cut another set of 2x4's that will nail on top of the top plate to form a double top plate. At the ends, be sure to cut the 2x4's so that the topmost top plate will overlap the lower plate and connect the two walls together. While nailing on the double top plate, measure across the opposite corners of your four wall at the top. If the measurements are off, then your walls are not completely square. Get someone to help wrestle the walls and corners so that the measurements are close and the walls square before nailing on the top plate.
With the top plate on and the walls square, nail on corner braces across the top plates again. Double check that the walls are square by measuring across the garage corner to corner as in the sketch. Once you start sheathing it will be difficult to square up the building so take your time here.