To check using four reference lines, measure the two diagonals, then write down the measurements on the footprint you used for measuring the dimensions. If the corner points are set correctly, then the diagonals will be the same length. If the reference dry lines are square, then the diagonals will be the same length. (See “Start-up" example.)
If you have only two reference lines to work with, you’ll need to use a triangle to help you check for square. The two reference lines will be “square" with each other if they create a right angle (90°). You can use a 3-4-5 triangle or the Pythagorean
theorem to determine whether the two reference lines create a 90° angle. In each case, the three sides (the rise, run, and diagonal) of a triangle must have a certain length relationship for the reference lines to be “square." Since we can let the two sides of the triangle be the reference lines and make them any length, it is the third line (the diagonal), which will determine if the reference lines are at a 90° angle.
A 3-4-5 triangle works well because as long as one angle is a right angle (90°), and the lines on either side of the right angle (the rise and run) have a relationship of 3 to 4, then the third side (the diagonal) is a 5 in the same relationship.
To use the triangle, use your reference dry lines to replicate a right angle, then create a triangle using the 3-4-5 relationship for the sides. To do this, measure a distance out on each reference string line from the point where the two lines intersect. The measurements of each leg should be a multiple of 3, 4, or 5. Note that the longer the length, the better assurance you have of accuracy. So if you are using a 25′ tape, for example, measure out 20′ on the one side and 15′ on the other side. The distance between these two points—across the diagonal-should be 25′.
The Pythagorean Theorem system sounds a lot worse than it is. If you use a calculator like a
Construction MasterPro®, all you need to know is that the three sides of the triangle are represented on the calculator by a “run" button, a “rise" button, and a “diagonal" button. You’ll need to find the length of the third side of the triangle (diagonal) that is required to make the two reference lines square (90°). Enter into the calculator the lengths of the two sides of the triangle that are next to the angle that needs to be 90°. (Press the run button for the one side and the rise button for the other side.) Pressing the diagonal button will give you the length of the third side of the triangle. This length is the distance needed to have the reference lines square and the angle to be exactly 90°.
If the diagonal length is not what it is supposed to be, then write on the footprint how much over or under it is.
