I built mine with 2x12's. My ceilings in my basement are not that high. I have a 106" screen and anything lower than 11.5" the back row could not see the bottom of the screen. In my theater build thread I have some pics of my riser.
Thanks guys. The reason I'm asking is because I'm thinking of trying to stuff some subs in horn arrangements under the riser as has been done by some of the other guys here. I'll check the calculators. Thanks!