Ventilation is so key to making a roof stay sound. Especially if you have a plywood deck. When you breath, shower and cook in your home you create moisture. That moisture rises into your addict and condensation occurs. As a result the insulation gets wet and actually does the opposite of what It is supposed to do. In the summer wet insulation absorbs heat into the house and in the winter the heat gets absorbed into the addict. It is like putting a wet oven glove onto a hot plate.

With high moisture levels in an addict mould, rot can occur. Make sure your bathroom fan runs to an outlet vent. Most people do this but  always make sure you have B vent.

The second problem is the temperature below the roof is much more extreme. In the winter ice forms and the summer turns the addict into a sauna. These extreme temperatures deteriorate the shingles faster. Rot occurs for two reasons. One for the reason I said before. But for this to occur the addict has to be air tight so that under 10% is ever released.  You should know something is wrong when your ceiling changes colour. The second reason is the roof is leaking from above. This permanganate leak makes rot occur and sometime a leak can occur for a period of time and you are not even aware of the problem.

Rot does stop in it's track if it's source of liquid is stopped. But the disintegrated wood is not strong at all and needs to be changed or you jeopardise the integrity of the roof. This must be checked when getting an estimate