How to build a waterproof roof out of LEGO blocks?
Since you are willing to use a non-LEGO frame to hold the roof and glue to fix all together, I would recommend buying baseplates in bulk and gluing them onto a wooden frame as if they were shingles. If the gradient and the overlap (vertical between rows and horizontal between individual plates) are correctly matched, the rain won't get inside.

For more resources on professional shingle roofing see these links:

https://www.iko.com/na/pro/building-professional-tools/roofing-101/how-to-install-shingles/
https://www.wikihow.com/Lay-Shingles