A polymeric riser cap member includes a central tubular section defining a generally cylindrical bore for receiving a sprinkler riser, the central tubular section having open top and bottom ends, and an annular skirt coaxial with the central tubular section and having an inner diameter larger than an inner diameter of the central tubular section. The annular skirt has an inner surface having internal threads threadably connectable with the externally threaded surface of a tubular riser adapter member of a tubular coupler. The bottom end of the central tubular section has an annular projection disposed opposite and spaced from a portion of the annular skirt. The riser cap member includes an annular recess adapted to receive a seal located in a wall extending laterally between the annular skirt and the annular projection.