If you want to minimise the risk of your garage door being damaged, you may want to heed the following advice.
Fix any damaged parts of the garage's gutters promptly
Most garages have gutters that line the bottom of their roofs. If your garage has this feature, it is very important to ensure that any damaged sections of it are fixed promptly. If the guttering on the garage becomes misshapen (as a result of, for example, being hit by a heavy object) or if it starts to sag because the brackets that keep it in place have deteriorated, your garage door could end up damaged.
For example, if your garage's gutters are sagging and it rains heavily, the rainwater that the gutters are supposed to propel away from the garage and into the drains on the ground below it will instead direct this water downwards onto both the garage's walls and its door.
If this happens once or twice, it might not cause too much damage, as most garage doors are designed to withstand periodic exposure to moisture. However, if you don't fix the gutters for many months and, as a result of this, your garage door is saturated with water on a regular basis, it will deteriorate.
The type of deterioration that will occur will depend on the material from which the door is made. A wooden door may, over time, be ruined by the wood-decay fungi that thrive on damp wooden structures, whilst a painted metal door may gradually corrode if the rainwater comes into contact with a section of chipped paint that leaves the metal underneath exposed.
Don't use a pressure washer on the door
If your garage door is very large or if you are quite short, and you need to climb up on a ladder to access the highest sections of the door, cleaning it without any special equipment could be tiring and even a bit hazardous. Because of this, when the door begins to look a bit grimy, you might feel inclined to simply point your pressure washer at it and hope for the best, rather than trying to wash it by hand.
However tempting this might be, it is not a good idea to use this equipment on your garage door. The reason for this is that this equipment ejects water out of its chamber very forcefully. The pressure of the water it releases could potentially leave indentations in your garage door (if it is made from metal) or even break the glass panes (if the door features a window).
As such, whilst it may be more laborious to clean the door by hand, it is the best approach to take if you want to keep this part of your garage intact.