How high should the tops of concrete piers be above grade?

If your posts are pressure treated, you can set the concrete level to grade. If the posts are not pressure treated, I would set the top of concrete at 8" above grade. more would not hurt. Also use metal fasteners that hold the post up off the concrete 1" in either case.

There are two grades of pressure treated lumber: 1) Above Ground Use, and 2) Ground Contact.

1) Above Ground Use is used for deck railings, building wall sole plates, etc.

2) Ground Contact lumber can be on the ground or in the ground.

If your concrete piers are within 6” of the ground, they are considered “Ground Contact Use” by the Code. Ground contact pressure treated lumber should have a tag stamped to it. It’s best to have water drain away and not have any standing water.

I like to put bring the footers 6-8 inches above grade. I go on the higher side if its on a slope. Much more than that and you are buying a lot of concrete.