No, the 18 deg North facing roof is not a big problem (unless you live in Tassie or somewhere very far south and use a lot of energy in winter?)
In summer the lower pitch will actually slightly boost total power but in winter when the sun is lower on the horizon it will reduce slightly compared to 30 deg pitch but only by 15% or so.
There is more information and diagrams in the discussion…How to find the best solar panel tilt angle?