roofing pricing question

[Nathan Letourneau]

Roofers price their work by the square.  Is that based on the exact house size, the house size plus 10% for waste or the amount of shingles used.

I haven't had to hire a roofer in a while, but when I have in the past it's always been priced based on the size of the house.  I recently hired one for a house project and got his price per square, but when the roof was finished, he billed me for the total number of shingles used not the size of the house.  In my case it was a 19 Square house and garage to do. I bought  materials 22.66 Square worth of shingles (22 regular and 0.66 ridge cap) and he billed me for 24.66 square ( I had to buy some more shingles before he finished so he wouldn't run out).  I did find some on you shingles in the dumpster but most had small imperfections or other things. So I don't think he was trying to be sneaky.

Is this common to price this way?

I didn't make a big fuss because in the scheme of things it wasn't that much more money. But I was going to use them on a 55 square roof and that would add up.