A method is described for manufacturing helically formed pipe from strip material. One side of the strip material is formed to provide a first side flange that projects substantially 90.degree. relative to the plane of the strip. The opposite side of the strip is formed with a channel that includes a second side flange that also projects substantially 90.degree. relative to the plane of the strip but in the opposite direction from the first side flange. The strip material is then helically coiled while the first and second side flanges are moved into abutting contact. Pressure is then applied radially of the pipe axis, forcing the contacting edges of the flanges together while the flanges are welded, thus forming a rib with a helical weld seam located at or near the line of neutral forces in the rib.