Trees are a problem. Look here at "Trees and UHF."
http://www.hdtvprimer.com/antennas/siting.html
I don't think there's much you can do if the trees stay in the way. Unlikely you'll be able to go higher than a redwood tree. Redwoods are kinda narrow and tall - any chance you can move around on your roof to skirt the blockage of the big tree?