Underlayment for Roof Deck

Hi all:

I want to use these tiles from Greatmats on my flat asphalt rolled roof. The people at Greatmats assure me these will not damage the asphalt:


The roofer who fixed some damage to our roof said whatever I put down I should use an underlayment, but he was not specific about what. I would like to consider an underlayment even with the Greatmats tiles just to be ion the safe side. Any recommendations?