One of the most common reasons tires needs to be replaced is not usually because of the tread thickness. It is actually because of what is called, “dry rot”. As seen on the Michelin Tire on the left, dry rot are the cracks that occur from multiple reasons such as the age of the tire, constant exposure to the sun with a lack of use will drastically increase the chances of cracks in the tires.
There are ways to prevent it, have the tires covered if left outdoors, do not leave the tires resting in mud that dries up later and do not leave the tires resting on hot concrete during the summers without driving every so often.
What should you do if you noticed that your tires have dry rot? You should have them inspected by a service technician immediately as it is in most cases not safe to drive on. Once the cracks reach the cords, the heat over time from driving will cause the tread of the tire to separate and have a blowout.