I'd like a concrete beam with a sloped top.

In the process of trying to do it I realize I don't understand the beam family. When I edit the concrete beam family and go the left view I see the reference plane in the middle. I'd think that it should be up top because thats where it attaches from. There is an equal on the left and the height control on the right. So when I make it bigger it makes it larger based on this middle reference. That just doesn't make sense to me. I would have guessed that it would have made it larger based off the top only.