Measure the distance along the roof surface from the roof hip to the fascia board at the base of the roof span. Record the value. If the building is at the planning stage, ascertain the roof's height from the eaves to the hip, and its width from wall to wall. The length of the roof slope equals the square root of half the width squared plus the height squared.
Add the length of the overhang at the fascia board to the previous measured length. This is determined by the width of the guttering used, and its distance from the fascia board.
Consult the technical data for the metal ridge plate you intend to use. Ascertain the distance under the edges of the plate that the roof sheeting must extend. If the technical data calls for a ventilation gap to be left between the top of the sheeting and the ridge, deduct this distance from the sheet length. If there are no specific requirements, deduct 1 or 2 inches from the overall length. This creates a hidden space and allows adjustment of the sheet to achieve a straight edge along the roof's bottom.