PURPOSE: To keep off any rupture due to flection by forming a U-shaped slit being intermittently continued to a thin steel sheet, and installing a rib lath, elongating this split to some extent, and having it formed into a vertical strand and a horizontal strand either.

CONSTITUTION: A slit of slit sheet steel is elongated in both directions and formed into a reticulation made up of a vertical strand 11 and a horizontal strand 12, while an intermediate part is bent in a U-shaped form as a vertical rib 10, through which a rib lath if formed up. Both the horizontal strands 12 at the right and left of the vertical rib 10 are formed after being symmetrically crossed with each other. With this constitution, any possible rupture in the vertical rib 10 is thus preventable.