If not replace bad wood with new plywood sheathing or 1 x 6 sheathing boards whichever is applicable to your type of roof.
Replacing entire roof.
Replacing shingles due to wind damage or a fallen limb is a relatively easy and inexpensive.
The downside is that.
Well it depends on the level and expanse of the damage.
The shingles are missing pieces if a piece of a shingle is missing then the entire shingle will need to be replaced.
The average according to 2014 cost data collected by home improvement site angie s list is around 11 000 for that kind of money you want to make sure job is done right.
So can you replace just part of your roof or is at an all or nothing deal.
Torn or damaged shingles can be removed and new ones can be slipped in place.
Make minor repairs on the roof if it is in good condition.
This can happen if a piece of debris has been blown into your roof thereby causing a piece of shingle to break off.