Abstract: Structural panel unit (10) comprising a first plate (11) and continuous web members (13, 14) extending along intersecting directions so as to support the plate (11), the web members (13, 14) being each slotted (141, 131) partway through at their intersections so that they fit into one another, the web members comprising first web members (13) and second web members (14), wherein the first web members (13) are so disposed as to have slots (131) open towards the first plate (11) in which slots (131) fit the second web members (14), wherein the first web members (13) are united to the first plate (11) thereby interlocking the second web members (14), and wherein the second web members (14) are linked to the first plate (11) by no other means than by interlocking with the first members (13).