Abstract: A channel adapted to be attached to a building structure comprises a pair of lips defining a slot therebetween and a back portion having a rounded cross-sectional profile. Preferably, the back portion has an half-elliptical profile. This channel is adapted to form a channel assembly with a fixing co-operable therewith. In one embodiment, the fixing comprises a dome-shaped head constructed from a circular profile with flats on opposite sides thereof. Preferably, the dome-shaped head is made up of a series of radii that define part of an ellipse. Preferably also or in a another embodiment, the fixing comprises profiles that are stepped inwardly of opposite edges of the head between the flats. These profiles locate and bear against opposite edges of the lips when the head is rotated within the channel.