Protection of Relays on the K6714 relay board

Hi Velleman Support,
I am using the relays on the K6714 to switch 12 volt DC solenoids. What is the best way to protect the relays from the back-EMF of the solenoids? Is it enough to fit a diode in parallel with the solenoid coil with the Cathode on the +ve side of the supply? If so, what would be the best type of diode to use?
Cheers,
Pete Luton