When I said 4 or 5 layers...it was worse case with houses 100 years old and counting old shake singles which the COB considers one roof, then many times roll roofing is found , and other layers, sometime 1 or 2 layers of 240lb asphalt shingles...many of the new suburbs, this is not the case but I speak from experience as an owner of homes nearly 100 years old, in the city.
and yes, there are slats across the rafters, but if they are rotted as in worse case scenarios, I have seen them pulled of which is an expensive nightmare, especially where rafters are not 16" o.c., but 23 /24" o.c. and 3/4 plywood cdx, not 1/2 partcle sheathing as is usually used.