Some roofs amongst my mock-ups (category geometry). I will update the file (with proper cut-outs). Models made by me CC-BY. Basic roofs are fairly straightforward. Making a rounded roof, took less than 5 minutes:

• Draw a wall the shape of your roof (as if the roof stands on its side).
• Select wall and export selection to .obj format
• Import and rotate.

See images below.

Your challenge in this case is making end walls with arched tops. (In the mock-up models above, you will find a half circle which could be used to cover the ends of your he roof when you first draw it.)