Mat and units thereof

A mat comprising a plurality of identical one-piece units adapted to be situated next to each other and interconnected for forming a continuous mat structure, each of said units being formed of plastic material having a continuous peripheral flange defined by a first pair of straight, parallel, opposed flange sides and a second pair of straight, opposed parallel flange sides extending transversely between said first pair of flange sides, said flange having upper and lower edges and an open network of intersecting ribs surrounded by and integral with said flange and situated at least in part between the upper and lower edges of said flange while being spaced upwardly from said lower edge of said flange, said open network of intersecting ribs including a first set of substantially equally spaced ribs extending between said second pair of flange sides parallel to said first pair of flange sides and a second set of substantially equally spaced ribs extending between said first pair of flange sides parallel to said second pair of flange sides, said first and second sets of ribs being integrally formed with and intersecting each other defining a plurality of adjacent rows of square spaces, a first set of alternate rows of spaces including elongate additional ribs extending parallel to said first pair of flange sides, substantially bisecting each of said square spaces in said first set of alternate rows and a second set of alternate rows of spaces including elongate additional ribs extending parallel to said second pair of flange sides substantially bisecting each of said square spaces in said second set of alternate rows, each unit having a plurality of short feet integral with and distributed along said flange and extending downwardly from said lower edge thereof and a plurality of long feet longer than said short feet and being integral with said open network of intersecting ribs and extending downwardly therefrom, all of said feet terminating in lower ends situated in a common plane parallel to and lower than a plane which includes said lower edge of said flange, at least one pair of flange sides comprising a pair of elongated opposed connecting regions one of which carries a plurality of integral fastener projections extending outwardly from said one flange region and each formed with a recess which is directed downwardly and formed with a lower open end, the other of said pair of flange regions being formed with notches for respectively receiving the fastener projections of an adjoining unit with said other flange region having at said notches flange portions the size and configuration of which enables them to be situated in said recesses of the fastener projections of an adjoining unit and wherein some of said ribs extend perpendicularly with respect to said other flange region respectively in line with but terminating short of said notches thereof to provide spaces for receiving said fastener projections of an adjoining unit.