opening up exterior walls

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I have a 16 x 20 x 16 addition on the back of my house. It has windows all the way around, 3 or 4 per side. It used to be an open patio, but the previous owners added a room with a nice l-gable roof connecting to the main house. That's the only nice thing about the addition though - the floor still slopes away from the house and I don't know if the slab was even meant to support an addition. I want to replace the walls with columns and make it into a covered patio.

I would need to have headers built, wouldn't I?
Where can I get info on number of columns required, dimensions of headers required, etc?

thanks!