Poster #19,
Wrong, very wrong, everything that you said is wrong.
US roof construction is not "significantly different from the UK".
More roof tiles are used in California & the SW of the USA than in the UK.
East coast housing uses, besides tiles, large amounts of slate roofing.
The average life of US asphalt shingles is about 30yrs often its much longer - up to 50yrs.
Believe me, roof framing uses some hefty sections from 6x to 12x
at 16" center's.
So, with respect, I'm not convinced that you know what you are talking about.