Darcy Weisbach e (How to determine)


I have a watergems model wherein the client wants me to use Darcy Weisbach as a friction method. 

But the info I have is the friction factor which is at 0.15, but in the model, the data needed is the Darcy Weisbach e (ft), which is the roughness height. 

How will I be able to determine the Darcy Weisbach e (ft) if what i only have is the friction factor? Also, as an additional info, I am using an HDPE pipe.