A channel 6 adapted to be attached to a building structure comprises a pair of lips 2 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 6 is adapted to form a channel assembly with a fixing 27 co-operable therewith. In one embodiment the fixing 27 comprises a dome-shaped head 28 constructed from a circular profile with flats 29 on opposite sides thereof. Preferably, the dome-shaped head 28 is made up of a series of radii that define part of an ellipse. Preferably also or in a another embodiment, the fixing 27 comprises profiles that are stepped inwardly of opposite edges of the head 28 between the flats 29. These profiles locate and bear against opposite edges of the lips 2 when the head is rotated within the channel.