Style breaks when i add a banner

Hi,

Having a wierd issue when I add a top banner. When I add a banner it somehow breaks the style. If you look here you can see that the area were the post appear, which is white, does not extend to the bottom of the page. It seems to be cut off. Now when I take the banner out its fine. I racked my brain on this one. I know its something simple. Any Ideas?
Thanks in advanced.

Relevant Link: https://www.cuff.org/blog/