If downspouts are placed at the ends of the gutter, they can be connected to a buried drain line along each gable wall. If this pipe runs the length of the greenhouse, it will have to be large enough to carry the rainwater from one-half the surface area (48 ft. x 192 ft. = 9,216 ft.2). A PVC pipe installed at a slope of 8 in./100 ft. would need an 8-in. diameter.