The seal between the cap and polycarbonate panel doesn't look weather tight, why are there no gaskets?
Properly installed, the cap provides an excellent seal, even on a 1/12 (5%) pitch. Glass and acrylic are hard materials, requiring a gasket type seal on the glazing system cap. Polycarbonate is a resilient material. When the cap is "snugly" installed, the pressure of the legs of the cap causes the polycarbonate to give slightly, creating a watertight seal. One of the great advantages to polycarbonate is that gaskets are not required to achieve a watertight seal. The more parts of a glazing system, the more likelihood for leakage. Gaskets in glazing system caps will often shrink, creating a gap for water to enter. Also, because one sheet of polycarbonate runs from the ridge of a greenhouse to the overhanging eave, there are no horizontal joints, no impediments to water shedding off the glazing area, no water damming up and, consequently, leakage rarely occurs. The most often reported cause for leakage is loose caps, (not tight enough when installed).