Size of the roof (area) is only 1 of about 7 major costing factors. The material cost for the mix (SH+Surfactant) is the only thing directly impacted by sq ft and it's only about 10% of my price. I can eyeball (estimate) the size (how much roof wash mix) from a bing birdseye view just based on the size of known objects in the image (cars, doors, streets, sidewalks, etc).
We've been estimating all roof cleans and house washes from aerial view for two plus years now. For me this would be waste of money and time. I'm usually not off by more than 10% on costs and time (over/under) at the curb as it is.
For someone new at roof cleaning, this might give them some comfort in pricing a job. Over time, you'll start seeing the same basic floor plans over and over and eventually, you'll be able to spit out a price and have an idea of gallons within just a few seconds of looking at a roof.
As an example, and a caution to anyone pricing "only" by sq ft, The pricing factors of cleaning half of roof with 2500 sq ft is not half of the price of cleaning the entire roof. The travel costs, mobilization, demobilization are the same for 1250 sq ft as for 2500 sq ft. You save ladder movements and material but since the ladder is already off the truck it's not a doubling of labor to double ladder moves. Other costing factors that will eventually get you if you cost by the foot are dormers, pitch, height, access, vegetation, downspout discharge, and terrain slope. Also, usually, the half they want cleaned is the worst side, so you are spending almost the same time for half the price.
I used a half roof in that example but the points hold true on any "area" pricing model.
There is also a an app for Ipad users called Measure Map. I thinks it like 2$ or something that someone told me they use. Can't remember who. I also saw it on another BBS talking about it recently. I agree with Cyclone as far as bidding. I can generally tell how much mix I need by seeing it on google maps or visiting the house. Square footage is often not the #1 thing that goes into my estimates. But it is at or near the top.
Your local property appraisal web site should also have the sq ft listed as well. This is how I price. I also do an aerial view through google and bing satellites to see the property. I normally only go out on bigger projects to look at them in person.
I havent measured a roof yet , I dont look at google maps either . I go to every estimate and talk with the customer and personally sell the job. The sq footage is a minor factor in my pricing . But ,I am going about it a little different than most . I own an irrigation company , and 80% of my roof cleaning work is from existing customers . I show up for their spring irrigation service , and tell them I can get those stains off thier roof and I basically sell the job every time .I also cold call , I went out Saturday to yard sales , I stopped at 8 different houses ( only the ones with dirty roofs ) bought some odds and ends , but I sold 5 jobs . I have no reservations in pulling my truck over and knocking on a door , cold calling ,when I see a dirty roof . I do hear no , but I hear yes more often . Weekends are great selling days , pick a neighborhood or 2 and start walking , approaching people doing yard work is easy , someone with a nice lawn , manicured bushes , and a paver driveway is an easy sell for cleaning a roof , they want their place looking good . I like the old fashion method of selling face to face . So far this season I have 35 roofs completed and 20 more to do . All within 20 minutes of my shop .
Still to this day , I dont have a web page , only a small listing in the yellow pages (just name and number ) And I am keeping extreemly busy .
Here in Fl. with all the vegetation and no gutters the extra time spent watering, covering and moving tarps is always a consideration. When I look at a roof, the landscaping is one of the first things I consider.