Just make a roof really really flat.
Hipped and half hipped seem to be the best.
I don't know how to do octagonal roofs wider than 1 tile. To get things to join up nicely, the game likes neat 90 degree angles. I actually took a few minutes to try and recreate (a bad approximation of) that house in-game because pictures are better.
It's a bit coincidental. Outside of sims, I was recently having a conversation specifically about wrap around porches with a friend. So I've been visting build mode on and off to make a house with a wrap around porch. No solution is perfect. The 3rd one here is simply deleting the roof, and using the Smooth Keeper, placing them as you would any other fence. Once you've created a "room" the floor is filled in, and a wood pattern seem the best choice. There's also more room for windows this way. If there's a way to do octagonal roofs, I'd use that often.