An improved greenhouse structure having sides and a roof to define at least a partially enclosed space, with an interior ceiling section and the interior ceiling section being at least substantially transparent. An air channel is formed between the roof and the interior ceiling section with an air input device in fluid communication with the air channel and the interior ceiling section has one or more openings through it to allow air in the air channel to pass through and into the enclosed growing space to provide a top-down air conditioned effect.

A roof for a building comprises a ridge beam for defining an upper edge of the roof, and a first eaves beam for defining a lower edge of the roof. The roof further comprises a first panel connectable to both the ridge beam and the eaves beam, being capable of spanning the distance therebetween and extending over at least a first portion of the area of the roof. In addition, the roof comprises one or more retention structures positionable relative to the first panel and one of said beams and configured to clamp said panel against the other of said beams.