Sticky Navigation

<script type="javascript">
(function($) {
    var $window = $(window),
    $nav = $('nav'),
    offset = $nav.offset().top;
    $window.on("scroll", function() {
        if (offset < $(this).scrollTop()) {
            $nav.addClass('fixed');
        } else {
            $nav.removeClass('fixed');
        }
    });
})(jQuery);
</script>

Demo