Young fly-half Lewis Pearce made the most of the wind at his back to kick the hosts 9-0 ahead at the end of a closely-contested first half at Riverside Park.
However any thoughts Ystalyfera, struggling in One West Central, had of getting on the front foot with the change of halves were swiftly knocked when Rumney wing Lee Bendon scorched over for the game’s first try soon after the break.
Pearce missed the conversion and Ystalyfera hit back through prop Dan White. Fellow prop Jonathan Williams then thought he had set up an equalising conversion only for the referee to controversially rule he grounded the ball short of the line.
Moments later the visitors were on the wrong side of the official who ruled their forwards had deliberately collapsed a rolling maul by Rumney and awarded a penalty try that Pearce converted for a 21-7 lead. It there was a question mark over that try, Rumney’s third was a wonderful sweeping attack finished by flanker Shane Young.
Ystalyfera back-row Ashley Carter went over for a late score but was that was too late to mount a comeback.
Trebanos won 14-0 at Talywain in mid-week thanks to three penalties from outside-half Kristian Jones and a second-half try from a lineout drive finished by hooker Rhys Thomas.
They will travel to either Kidwelly or Glamorgan Wanders, one of five games postponed this weekend.
Abercrave booked their place in the Fouth Round of the SSE Swalec Bowl in style with a 42-0 win over New Tredegar at Plas Y Ddol.
Centre Nick Davies helped himself to a brace, the forwards were rewarded for their dominance with two penalty tries while Nick Evans grabbed his first try of the season. Darren Hodge added 17 points with the boot.
