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!

[Richard Helppie-Schmieder]

@Miller McSwain You can order a quick estimate from Roof Scope and they cost $15.00. This will give you the size. You can measure the ridges and valleys on Google Maps and get within $100 of your cost for a new roof. This is how I run my roofing business here in Dallas.