This isn't like roof underlayment, or horizontal siding, it's "Header Wrap".
It's FABRIC. Shedding water, and road grim will NOT be any different, regardless of which way you lap the stuff. It's tighter than a ducks a$$ when it comes to water/debris, except for the fact that it absorbs water like a sponge.
I've only had mine for about five years now, but there's been no issues with water intrusion, road grim, or rotting/rusted pipes.
