sections 50 1 so as to tightly mate with similar but inverted rib wall sections of a contiguous tile the inner sidewalls 50 2 of each rib 50 are also shaped in plan view as an equilateral triangle having rounded interior corners so as to have a substantially identical size and shape as a corresponding inverted lug 40 the resulting open ended cavities 60 fig3 a bottoming on their respective enclosed areas of the surfaces 20 a 20 b have just slightly larger mating interiors than the lug 40 so as to receive inverted lug projections of adjoining tiles with an essentially interference fit the outer sidewalls of sections 50 1 of the ribs 50 fig3 and 3a are rounded adjacent the tile edges 22 a 22 b and otherwise substantially follow the curvature of the lug 40 sidewalls 40 2 40 3 to facilitate mating therebetween the spacing between the edges 22 a 22 b and their laterally adjacent sidewalls 40 1 of lugs 40 is substantially the width of the channel 42 thus each inverted one of the ribs 50 can be accommodated in a corresponding female channel 42 and since each inverted lug 40 can be accommodated in a rib cavity 60 the second set of male female interlocks is formed by a male rib section 50 1 and its adjoining female cavity 60 as will be apparent the rib 50 follows a substantially semisoidal course a substantial length of each support strip 19 a 19 b the rib merges into the central portion 10 adjacent the tile corners and thus the two endmost lugs 40 a.