<header class="sticky"></header>

$(window).scroll(function() {
  var windowTop = $(window).scrollTop();

  //check to see if we have scrolled past the original location of the sticky element
  if (windowTop > stickyTop) {

    //if so, change the sticky element to fised positioning
    $(".sticky").css({
      "position": "fixed",
      "top": 0
    });
  } else {
    $(".sticky").css({
      "position": "static"
    });
  }