PCB design for 13A/12V Circuit
The resistance of copper traces account for more power loss and heat generation when they carry high current.

Larger cross-sectional area have lower resistance, which reduces the amount of power lost to heat. As you mentioned you know how thick your trace should be but there are other factors also which should be considered while designing high power PCB.

The main hazard is heat, This must always be considered when the housing is made, and external vents/fans should be used. 

To reduce the amount of heat produced on the board itself, it is best to choose components with low resistance.

the next main hazard is short circuit As you are designing board with power traces, when shorted it will be able to consume considerable amount of power. The easiest way to deal with this problem is to use FUSE.the fuses should be always be rated for less current than wire can handle