Exactly! as floated elements don't take up any space(taken out of the normal flow...) so your wrapper/container div #head in this case won't act like it has something in it and wont expand with the flow.

So you clearing the floated element inside would have solved the problem. And you don't have to hard code everything(height in this case) it will just be more code and annoying edits in the end (if there will be any :D)

Block elements take up 100% of the given width, except for floats that will wrap down to the content inside. So if you are aiming for 100% just leave it out much easier and less code if dealing with padding aswell. But i assume you did know that.

I'm just adding on with some rambling someone else might find usefull. :)