• Determine point C at which the perpendicular is needed • Use a steel tape, strike an arc 3’ from point C toward center of the room • Measure 4’ from point C toward point B along line AB (point E) • Strike an arc 5’ from point E toward the center of the room, intersecting the first arc • Strike a chalk line through points C and D. Line CD is the desired perpendicular
A simple way of checking the accuracy of the points plotted by this method is to measure 4' from point C toward point A on line AB. Using this new point as a base, strike another arc 5' out in the direction of the recently established point D. This new arc should intersect the junction of the original two arcs. If it does not, replot all points to find and correct the error. In large rooms, the 3-4-5 method may be expanded by using any multiples of these measurements, such as 6-8-10 or 9-12-15.