It’s time to consider re-roofing when numerous shingles are missing or damaged.
If the shingles are not old and worn out, this would be indicative of shingle failure and a sure sign that the shingles have reached the end of their life. If the roof is only a couple of years old and the fiberglass matrix (backing) is visible in areas of each tab or along the edges of the tabs, it’s possibly a manufacturing defect.
Loss of granules (gritty finish) is evident. The shingles will have a dead look to them; this indicates that the shingles are at the end of their useful life. They will start to deteriorate quickly now that the grit surface that protects them has worn off.
Buckling, curling or blistering is present. These are the signs of a roof that should have been replaced a year or two ago. If the roof looks this bad, the decking underneath could have sections that have rotten.
Also, look at the space between the tabs on the shingles. As these age and reach the end of their useful life, the space between the tabs increases from what looks like the thickness of a pencil to that of your little finger.
In short, it is time to re-roof when the roof still looks to be in reasonable condition!
A bad looking roof is a roof has been on too long!