A modular greenhouse construction in which panels are prefabricated from a universal beam or mullion and the panels are interlocked to form walls and a roof. The universal beam has a spline receiving groove in one surface and a ribbed channel in the surface opposite with C-shaped slots on either side of the beam for interlocking the two panels. Joints are formed by L-shaped inserts engaging an interior channel in the beam with the particular angle of the joint being determined by the shape of the insert. For 90.degree. corner joints, the insert would, of course, be a 90.degree. L-shape. An enclosure is formed by securing adjacent panels with an H-shaped insert engaging abutting C-shaped slots on the periphery of each panel. Caps which snap on the frames are provided to form corners for the building and a gabled roof.