Whatever you do, don’t use corrugated roll pipe. It’ll plug with debris hanging up on corrugations or bc it’s too easy to get bellies in it. Use sdr35 (green pipe usually) and it comes gasketed or solvent weld. Schedule 40 is outrageous right now. All you need is a foot of minimum cover with the sdr unless it’s absolute mud. If its cold enough to freeze in the pipe, it’s frozen on the roof or gutter and not getting into pipe anyway. And all you need is 1% slope. 2% is for turds inside a building. We lay flatter all the time. I’m installing a storm system at .25% right now. It’s not desirable but works if sized properly.