I have a Moen one handle that keeps dripping from the shower head our house was build in 2008 what could be the cause of the drip?

It could be that you have debris lodged in a seal in the cartridge or one of the "O" - rings is clipped / damaged, allowing water go get through.

Call MOEN. They will send you new cartridge for free. Once you have the cartridge in hand, come back. We will lead you through installation. Here is the info:

Call: 1-800-Buy-Moen (1-800-289-6636)
8:00 am to 8:00 pm EST Monday through Friday
9:00 am to 5:30 pm EST Saturday
Closed on Sundays and all nationally observed holidays.

To contact Moen Canada call: 905-829-3400

Good luck... Milo