Will this mount to my factory bed rails, or do I need aftermarket bars?

This tonneau mounts at factory attachment points — no bed rails or clamps needed. Installation uses your truck's existing bed structure.

Can I open the cover partially, or does it have to roll all the way?

Roll-up design means you roll it back toward the cab. The farther you need bed access, the more you unroll it. Stops where you position it.

What happens if water pools on top when it's closed?

The hard panel and seal are built to shed water, but tonneau covers are not completely waterproof at all seams — minor moisture intrusion around edges is normal. Slope your truck bed slightly if standing water is a concern, or inspect seals after heavy rain.