As MrFixIt stated, Down here most of the plumbing vents on your roof are lined with a lead flashing. When folks allow palm tree fronds or other tree branches to overhang or come close to your roof, squirrels can climb on your roof and chew on the lead flashing. In time, this puts holes in and around this flashing and eventually most all of the flashing can be gone.
Drive around the older homes here in The Villages and look at the vents. Many times you will see portions of the white PVC pipe exposed on these lead flashings.
This allows rain water to get into the space between the PVC pipe and the lead flashing. This rain water can then get into your attic and down into your interior walls causing water and mold damage. Pests can also then enter your attic.
To protect these lead flashings, some folks will install the plastic vent covers, commonly found at ACE Hardware, etc. They are secured by thumbscrews. One must be very careful when installing these that the thumbscrew has a washer type end on them so they do not puncture the lead flashing. It should be snug enough to stay on, but NOT puncture the lead flashing.....if you puncture the lead flashing you will cause the very problem you are trying to prevent.
Here is a picture of typical squirrel damage I commonly see....
I should add that if you already HAVE this damage, adding these squirrel guards will NOT prevent moisture from entering your home. Any gaps made already by the squirrels need to be sealed FIRST. If the damage is bad enough a new lead flashing may be needed to be installed by a qualified roofer. Only then can the guards be installed to prevent further damage.
I should also add that if you have an older roof that has the older aluminum ridge vents, the foam end covers need to be checked.
I typically find about half of them missing. Squirrels, rats, bats, etc can then very easily enter your attic of these are missing.
As long as they are installed as stated above, you should have no problems. The squirrel guards come in different colors and sizes.
Hope that helps!
Frank D'Angelo, ACI