The roof of our 2000 sq. ft. bungalow (straightforward gable, 4/12
slope, no skylights) needs to be redone.

We've asked 5 contractors to give us an estimate. 4 are in the same
ballpark (i.e. $9,000 CDN), one is asking for TWICE that amount. He
says that the roof "felt soft" when he walked on it, so he's added
4700 sq.ft. worth of 1/2" plywood (to put on top of the existing one)
to his estimate. No other roofer mentioned "softness", but they did
indicate that, should they discover rotten wood underneath the
shingles, they would then charge us for the replacement (at $2.00 per
sq. ft.).

Well, I don't like surprises. Nor do I want to replace wood that
could still be in good condition.

Is there a way that I can assess the condition of the wood under the
shingles WITHOUT removing them? What signs should I be looking for in
the attic?

Also, is there a magic formula to calculate the size of a roof? None
of the roofers have quoted us the same size.

Any and all information relating to having one's roof redone is
appreciated.