Asphalt wouldn't be my first choice, but if that's what you got, well , that's what ya' got.
Besides the usual assortment of dust, dirt, leaves, bird droppings in the first wash off the roof, you'll have granules. You can filter thru a first stage sieve to get rid of most of the larger debris. I have done some reading about harvesting for potable water and some seem to think there may be some dangers from the water leaching minerals and toxic chemicals from the asphalt composition. And some don't.
Will the Berkey do anything for the dissolved minerals or chemicals leached from the asphalt base?
A metal roof will give a greater yield especially with rain falls of shorter duration.
Here's a link
that may help