Roof Size Estimate/Rule of Thumb

[Miller McSwain]

I'm developing repeatable processes for house flipping and was wondering if there is a quick way to estimate roof area. 

I think I've heard that the size of the roof can be estimated by doubling the 1st story sqft of the home. As an example, if a single-story house is 1300 sqft, a quick estimate for the roof size is 2600 sqft. Did I hear this correctly, and is there a better rule of thumb?

Thanks!