I have a big two car garage detatched from the house. the roof does
not have a gutter system, and I want to put a simple gutter system
up.

The eaves have the support joists exposed, and run to the drip edge of
the roof.

I had a gutter company come out just to see how much they would charge
to do the job, and they quoted me $1,200.. and, they said that it is a
simple job I could do myself. Which, I knew ahead of time. But, also
to get some information on doing the job properly myself.. on the
sly..

The guy said that a fascia board would need to put up to cover up the
joist..

Now, what would the proper thickness be for this? How thick should the
wood be, so when I put the gutter up, it is not sticking too far out
from the drip edge..