A decking board , comprising: a first longitudinal side having an extension member , said extension member including a first surface and an opposing second surface , the second surface having a recess formed therein; a second longitudinal side including a first portion 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 and second portion of the second longitudinal side define a cavity therebetween to receive an extension member of an associated decking board therein; wherein the tongue of the second longitudinal side includes a first flexible seal member extending generally upward from the first portion , and the second portion includes a second flexible seal member extending generally downward from the second portion ; wherein the second flexible seal member is configured to operatively engage a first surface of an associated extension member when an extension member is inserted in the cavity ; characterized in that : the first surface includes a first lip , the second portion includes a second lip , and the second flexible seal member 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 , said extension member including a first surface and an opposing second surface , the second surface having a recess formed therein; a second longitudinal side including a first portion 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 and second portion of the second longitudinal side define a cavity therebetween to receive an extension member of an associated decking board therein; wherein the tongue of the second longitudinal side includes a first flexible seal member extending generally upward from the first portion , and the second portion includes a second flexible seal member extending generally downward from the second portion ; wherein the second flexible seal member is configured to operatively engage a first surface of an associated extension member when an extension member is inserted in the cavity ; characterized in that : the first surface includes a first lip , the second portion includes a second lip , and the second flexible seal member 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 , the second portion includes a second lip , and the second flexible seal member 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 . <EOS>