How to change rotation angle? (Easy question?)

This is my first post, and though I've had a fair amount of Rhino experience I am completely new to Grasshopper. I'd imagine this is an easy question for anyone with any Grasshopper experience, but I could be wrong on that...

I have a submission next week and I'm desperately trying to model some roof tiles on a Rhino building model. My tutor's colleague kindly wrote me a script to cover a single sloped surface with overlapping roof tiles (file attached).

It works a treat and I understand how to 'bake' something now, but the only problem is I cannot work out how to change the angle at which the tiles run down the sloped roof surface.

I have tried all of the sliders but I still can’t work out which slider changes the direction of the tiles going down the slope of the roof i.e. aligning the tiles ‘downhill’ so they look correct and rain would drain off properly.

Because my roof has many different planes above each room, I need to create a slider that changes the rotation of the tiles so I can get it right for each roof plane.

Hope this makes some sense. My tutor's colleague is out of contact and I'm desperate to work the issue out ASAP so I can produce a section drawing from a Make2D for Monday afternoon!

Huge thanks in advance! Sorry for the lengthy description.

Barney

Attached:

1. Grasshopper script at the moment

2. Rhino file with a blank surface to test it on