A decking board, comprising: a first longitudinal side having an extension member (1314, 1314a), said extension member (1314, 1314a) including a first surface and an opposing second surface, the second surface having a recess (1337, 1337a) formed therein; a second longitudinal side including a first portion (1320, 1320b) defining a tongue and a second portion; and, a main central body disposed intermediate to the first longitudinal side and second longitudinal side; wherein the first portion (1320, 1320b) and second portion of the second longitudinal side define a cavity (1362, 1362b) therebetween to receive an extension member (1314, 1314a) of an associated decking board therein; wherein the tongue of the second longitudinal side includes a first flexible seal member (1328, 1328b) extending generally upward from the first portion (1320, 1320b), and the second portion includes a second flexible seal member (1324, 1324b) extending generally downward from the second portion; wherein the second flexible seal member (1324, 1324b) is configured to operatively engage a first surface of an associated extension member (1314, 1314a) when an extension member (1314, 1314a) is inserted in the cavity (1362, 1362b); characterized in that: the first surface includes a first lip (1316, 1316a), the second portion includes a second lip, and the second flexible seal member (1324, 1324b) includes a first prong extending generally downward from the second portion of the second longitudinal side and a second prong defining a third flexible seal member extending generally downward from the second portion of the second longitudinal side. a first longitudinal side having an extension member (1314, 1314a), said extension member (1314, 1314a) including a first surface and an opposing second surface, the second surface having a recess (1337, 1337a) formed therein; a second longitudinal side including a first portion (1320, 1320b) defining a tongue and a second portion; and, a main central body disposed intermediate to the first longitudinal side and second longitudinal side; wherein the first portion (1320, 1320b) and second portion of the second longitudinal side define a cavity (1362, 1362b) therebetween to receive an extension member (1314, 1314a) of an associated decking board therein; wherein the tongue of the second longitudinal side includes a first flexible seal member (1328, 1328b) extending generally upward from the first portion (1320, 1320b), and the second portion includes a second flexible seal member (1324, 1324b) extending generally downward from the second portion; wherein the second flexible seal member (1324, 1324b) is configured to operatively engage a first surface of an associated extension member (1314, 1314a) when an extension member (1314, 1314a) is inserted in the cavity (1362, 1362b); characterized in that: the first surface includes a first lip (1316, 1316a), the second portion includes a second lip, and the second flexible seal member (1324, 1324b) includes a first prong extending generally downward from the second portion of the second longitudinal side and a second prong defining a third flexible seal member extending generally downward from the second portion of the second longitudinal side. the first surface includes a first lip (1316, 1316a), the second portion includes a second lip, and the second flexible seal member (1324, 1324b) includes a first prong extending generally downward from the second portion of the second longitudinal side and a second prong defining a third flexible seal member extending generally downward from the second portion of the second longitudinal side.