Estimating roof replacement by J Scott

[David Hodge]

I just started J Scott's book on Estimating Rehab Costs. I love how detailed it is! Can someone help me understand a few things regarding how he calculates the cost of a new roof? On page 29 it says that Sheathing ranges from $30-45 per sheet then below that it says Roof Replacement for asphalt shingles is $150-400 per square. Does the $150-400 include the sheathing or do you need to add the sheathing estimate on top of the roof replacement estimate? Also, it seems like the numbers get skewed on larger homes. I calculated the cost of replacing the roof on my primary residence (just for practice) and ended up getting $21,000 using the formula in the book. I live near Denver and believe most in my neighborhood have been paying closer to $10k. It makes sense that a formula like this wouldn't work well on really small or large homes due to fixed costs. Does anyone know the range of homes his formula works well on? Thanks in advance!