My house has a roof that is about 10 years old (The house is 50 years old). We just had the house inspected, and found that there are two layers of wood underneath the shingles. The bottom layer is most probably the layer that was on the roof before the current roof was put on. From the attic, you can see sections of the older layer that are rotted. It looks like the people who installed the current roof just covered the rotted layer with a layer of plywood. And now, from the outside, you can see that the roof has sections that are sagging. We originally though that we would have to have the entire roof redone (take out both layers of wood and install a new layer). But one contractor said that they could fix the problem by just replacing plywood in sections that were bad. And that they would leave two layers where they did not find any problems in the wood. They would even out the difference in the surface of the roof by using multiple layers of shingle. I’ve included a picture of what our roof looks like from inside the attic.
He gave me two prices: one that would cover redoing everything, and one which would cover the cost of replacing shingles and included two replacement sheets of plywood. The second price would go up as they found additional sheets of plywood that would need to be replaced ($60 per additional sheet).
Another contractor just gave us a price of redoing everything. His price was about $800 less than the first guys price of redoing everything.