Abstract: It is known in structural curtainwall systems to have a series of rafters and purlins supporting glass panes. It known also to provide drainage panels in the supporting elements to drain away any water which seeps past the external surfaces of the glass panes. In this invention, separate panels are formed comprising purlins and sub-frame members and supporting glass panes. These panels will be formed off-site in a factory. The sub-frames of the panels are received directly into channels in the rafters thus enhancing the resistance of the system to leakage particularly where the panels intersect the rafters. The joint between adjacent panels spanning the same pair of rafters is sealed by providing tongue members on the purlins which are resiliently deformable. The biasing of the tongue members provides a seal between adjacent panels. Furthermore, the tongue members form drainage channels to drain water into the drainage channels of the rafters.