Iws rdwc getting started!

Hi, friend of mine just got a iwc rdwc but is yet to start growing. Question is, when you first put the plant in the clay pebbles the roots don't hang out the bottom and because its an rdwc with a control unit that sets the water level in each bucket which i would think is just below the net. How does the roots get any of the water? will he need to top feed at first?
 
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