Fall, covered area, rainfall intensity, gutters and discharge points have to be examined before fabrication. Mistakes in these details produce overflows, standing water and unwanted loads.
The solution also has to allow the drainage paths to be cleaned, particularly in places with leaves, dust or sea salt.
The water is calculated, not estimated. The amount that has to leave follows from the covered area and the intensity of the rainfall, and from that come the section of the gutter, the number of outlets and the fall. A 30 m² pergola in heavy rain takes many litres a minute; if a single gutter cannot keep up, the water leaves on the side it should not. Failures gather at three points: where water stands and the coating begins to degrade, in the drainage paths, which have to be cleanable without dismantling, and at the junction with the existing building.