A strip assembly helically windable to form a pipe is disclosed. The strip assembly includes a pair of joined elongate strips, each strip having a strip width and a strip height and each strip including: a base; at least one longitudinal rib upstanding from the base; a pair of spaced apart longitudinal parallel edges; and an angled joining end surface. Each joining end surface has a joining width, the joining width at least 5% greater than the strip width. Also each joining end surface has a joining height, the joining height at least 10% greater than the strip height. The additional joint area provided by the angled joining end surfaces provides a significantly stronger joint.