In a game that saw two players red carded and a further two sent to the sin-bin, the Roberts boot made the difference as he kicked seven penalties and dropped a goal to bag all his side's points.
Llangennech's triumph was one of three on the road in Swalec Division 1 West as Tata Steel won 19-8 at new boys Tondu and Carmarthen Athletic snatched victory from the jaws of defeat at Llanharan.
A fifth penalty goal from scrum half Chris Williams gave Llanharan, playing their first fixture in the West divisions, hoisted the home side into a 15-10 lead with only five minutes to go.
But wing Dan Gravell was worked over for a try that Stuart Morris converted to grab a sensational victory for the visitors. Carwyn Edwards had earlier scored Athletic's opening try and Morris added a penalty and conversion before striking the winning points.
Newly promoted Ammanford, who only lost twice last season as they took the Division 2 West title by a massive 19 point margin, took the lead in their clash at Whitland through a Carwyn Richards penalty, but had to settle for a 3-3 draw as the weather got the better of both teams.
John Thurston helped himself to 16 points as Builth Wells beat Crymych 26-16 in Division 3 West, while Nicky Giles scored 13 points in Kenfig Hill's 23-12 home win over Kidwelly. Craig Edwards crossed for two of Felinfoel's five tries at Maesteg, where they won 30-3, and Jack O'Reilly provided 16 points to Glynneath's 31-6 home win over Loughor.
Three penalties and a conversion of Derek Lewis's second half try earned Steve Hewitt all the plaudits as Tumble beat Newcastle Emlyn 21-16 in Division 3 West and there were four penalties and two conversions from Jamie More in Penclawdd's 23-3 triumph over Pondarddulais.
A Tom James interception try in the first-half proved enough to steer Tenby to a 10-3 win at Gorseinon, while a last minute try from Ian Harries made him the toast of Pontyberem as they squeezed past Dunvant 14-13 on home soil.
Tries from Mike Davies, Jamie Jennings, Mark Davies and Rhys Evans were enough to steer Trimsaran to the top of the table at the end of week one in Division 4 West as they picked up a winning bonus point at home with a 28-14 triumph over Aberaeron. Four of the other five games in the division ended with away wins