Tires should be stored in a dry, cool place, away from sunlight and sources of ozone, such as electric motors.
If you must store tires flat (one on top of the other), make sure you don’t stack too many on top of each other. Too much weight can damage the bottom tire.
Also be sure to allow air to circulate around all sides of the tires, including underneath, to prevent moisture damage.
If storing tires outdoors, protect them with an opaque waterproof covering and elevate them from the ground. Do not store tires on or over black asphalt or other heat-absorbent or reflective surfaces, such as snow-covered ground or sand. Solvents, fuels, lubricants and chemicals should be kept out of contact with tires.
Spare tire carriers on your vehicle are not intended to be used for long term tire storage. If your vehicle has a full size spare (same size and type tire recommended for use by the vehicle manufacturer, not temporary use spares), it should be included in the tire rotation pattern.