We call this effect “UltraViolet Degradation” or “UVD”.
Over the years a very thin fraction (less than 1/1000 of an inch) of the material interacts with outdoor sunlight and begins to show fading or “greying” (a reduction in the appearance of the original colour).
Our preventative maintenance schedule recommends seasonal cleaning and a power-wash at least once per year. . However, these activities do not remove the UltraViolet Degradation.