CSS: I need some basic layouting tips
===

Well, to make a long story short, this is my website: www.tong-il-neeroeteren.be
You might notice when visiting my website, that the header is partially hidden behind the body. If not, please zoom in (ctrl + scroll up or +) and you will see that the header disappears behind the body as the site becomes larger.

What are the most common mistakes I could make that makes this happen?
Here's a snippet from my CSS file:

    html {
	height: 100%;
    }
    body {
	margin: 0;
	padding: 0;
	border: 0;
	float: center;
	height: 50%;
	width: 100%;
	font: normal small Georgia, "Times New Roman", Times, serif;
	color: #999999;
	background: url(images/back.png) no-repeat;
	background-position: top;
	background-size: 50%;
	z-index: -1;
    }
    
    #menu {
	height: 70px;
	width: 500px;
	margin-top: auto;
	padding-left: 60px;
	clear: right;
	background: #999888;
    }

    #menu ul {
	margin: 0;
	list-style: none;
    }

    #menu li {
	display: inline;
	list-style: none;
    }

    #menu a {
	display: block;
	float: left;
	width: 140px;
	height: 45px;
	padding-top: 25px;
	margin-top: -15px;
	text-transform: lowercase;
	text-decoration: none;
	text-align: center;
	letter-spacing: -2px;
	font-size: 30px;
	color: #B8DD90;
    }

    #menu a:hover {
	color: #1A1A1A;
    }

    #menu .active a {
	background: url(images/img03.jpg) no-repeat;
	background-position: center;
	background-size: 90%;
	color: #FFFFFF;
    }

    #header {
	background: url(images/banner.png) no-repeat;
	background-size: 100%;
	position: relative;
	margin: 0;
	padding: 0;
	padding-bottom: 60px;
	width: 100%;
	height: 115px;
	z-index: 1;
    }

Any help is appreciated.