taken from greenbuilder.com
3.2 Conveyance Subsystem
If a straight run of gutter exceeds 60 feet, use an expansion joint.
Keep the front of the gutter one half inch lower than the back.
Provide a gutter slope of 1/16 inch per foot minimum.
Provide gutter hangars at 3 feet O.C.(on center).
Gutter should be a minimum of 26 gauge galvanized steel or 0.025 inch aluminum.
Downspouts should provide 1 square inch of downspout opening for every 100 square feet of roof area.
The maximum run of gutter for one downspout is 50 feet.