Appendix 3 - The 3: 4: 5 Method
The 3:4:5 method is a simple way to ensure that corners are right angles or 90° and can be used when laying out a building or field kiln.
The 3:4:5 rule states that when a triangle has one side (AB) 3 units long, another side (BC) 4 units long, and the hypotenuse (AC) is 5 units long, the angle ABC will be 90° or a right angle.
This method requires that you use some unit of measurement. This unit could be the length of your foot, the distance between two knots tied in a string, or a length of one metre as measured on a tape measure. The length of the unit can be anything you choose but you will find that a unit measurement of one metre is ideal for marking the corners of a field kiln.
For example, if a triangle has one side 3 metres long, another 4 metres long and a hypotenuse 5 metres long, the angle ABC will be 90° or a right angle. This rule also holds true if the measurements are multiples of 3:4:5, such as 6 metres: 8 metres: 10 metres or 9:12:15.
Example: Laying out a corner of the field kiln
1. Select your unit of measurement which should be one metre or more in length. The most accurate method is to use a 5 metre tape measure. Note: If you do not have a tape measure, make one with a long piece of cord or string. First tie a knot close to one end of the cord and holding the knot in one hand, stretch your other arm out as far as possible as shown in the diagram. Tie a second knot at that point on the cord. For most people this distance is about one metre. Hold the second knot and measure that same distance again and tie a third knot. Continue until you have six knots with equal spaces between them. The distance between the first and last knot will be 5 units or approximately 5 metres.
2. Mark the first side of the kiln. Drive a large nail or spike into the ground where the comer of the kiln will be. Tie a string to the spike and stretch the string in the same direction as the length of the kiln (the side with the tunnel openings). Fix the string in this position. Using a tape measure (or the cord that was made to measure units), measure four metres from the comer along the string and drive another spike into the ground at that point.
3. Mark the second side of the kiln. Tie another string to the first spike (where the corner will be) and stretch it out in the direction of the kiln width (the side without tunnel openings). Using a tape measure or measuring cord, measure 3 metres from the corner along the string and tie a knot in the string at that point.
4. Mark the hypotenuse. Attach another string to the spike that marks the 4 metre point on the first side. Measure 5 metres on this third string and tie a knot. This string will form the hypotenuse of the triangle.
Stretch the knot on the 5 metre string towards the knot on the 3 metre string. Move both strings until the 5 metre knot and the 3 mete e knot are exactly on top of each other. Both strings should be taut.
5. Fix the 3 metre string in this position and the 5 metre string can be removed.
If you have followed all the steps and measured accurately, the angle or corner formed by the 4 metre string and the 3 metre string should be 90° or a right angle.