PENRHYNCOCH crashed to their first defeat in five games as Porthmadog prevailed at the Traeth.
Joe Chaplin headed wide and Sion Bradley tested goalkeeper Lee Jenkins before the hosts led on 21 minutes with Rob Evans bursting into the area to set up Bradley to lash home.
Bradley went close to doubling his tally moments later only to drag his shot wide of goal before the Roosters were galvanised.
The Roosters ended the half strongly with Jonny Evans forcing an outstanding save from goalkeeper Richard Harvey before Jon Foligno failed to find the target with a header from Antonio Corbisiero’s free-kick.
Penrhyncoch maintained their momentum after the break with Liam Lewis denied by a goal line clearance from Ifan Emlyn before Evans tested Harvey.
The home side rode out the storm and Porthmadog sealed the points with two goals in five minutes.
On 72 minutes Chaplin and substitute Julian Williams linked to tee up Emlyn to drill home from 20 yards and it was all over five minutes later when Jenkins spilled a header from Williams into the path of Iwan Lewis who gleefully pounced.
PENRHYNCOCH: L Jenkins, E Evans, Davies, S James, Foligno, O James, C Jones (Warren), Corbisiero, Mousley (Meredith), J Evans, L Lewis. Subs: G Lewis, Reynolds, K Jenkins.