Believe it or not the best way to protect your roof from dry rot is to have ventilation in the attic. Proper ventilation ensures that your roof will live out its intended life span. When your attic space gets hot and humid, the warm moist air rises, unless it has a way to escape, the plywood (decking) can’t breathe and this will cause your decking to start decaying. The shingles are attached to that plywood and if that plywood starts to decay then the shingles could loosen and even blow away leaving your home vulnerable to leaks. The failure of your plywood may also damage your insulation. Your attic needs to stay cool and have an exchange of air to prevent the dry rot.
A common misconception is that having ventilation is a bad idea in the winter and any ventilation should be covered or sealed during the cold months. Ventilation is important in the winter too because you don’t want condensation to form on the decking. A great way to visualize this is the example of a cold soda can just out of the fridge. When you take the can out of the cold as it starts to warm up condensation forms on the outside of the can. Imagine that on the inside of your roof. Without ventilation that condensation will stay on the wood until it is absorbed. Over time this will cause dry rot and your roof and insulation will fail causing hundreds of dollars in repair costs.