Constructing a four post building does not need to be complex or difficult. As with any construction project, it is vital for you get off to a good start. With a four post building this means getting the corners square prior to driving the first nail or cutting the first board. Having the structure square is critical, but fortunately, it is quite easy to accomplish.
Determine where your building will be located and mark the spot for the first post. At this spot, hammer the first stake securely into the ground.
Carefully measure the width of your building and hammer a second stake into the ground at the proper location. These two stakes now mark the front two corners of your building.
Measure from the two front posts back for the depth of the building. Hammer a stake at each of the back corner locations. Then tie a string between each of the four stakes.
Measure diagonally from the first stake to the rear stake on the opposite side of the building. Record this measurement. Now measure from the second front stake to the other rear stake.
Shift the back stakes slightly, making certain the string remains tight, until the two diagonal measurements are identical. The outline is now square. Set the posts in the stake locations.