float-css overlaps next to div image

My logo and navigation float to the left and right respectively when the screen gets to 40rem. However it overlaps with the next div which contains a background image. I tried setting margins but that just moved both logo/nav and the image lower on the page.

@import url('https://fonts.googleapis.com/css?family=Raleway:300,400,700,900');
* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: 'Raleway', sans-serif;
  text-align: center;
}

img {
  max-width: 100%;
  height: auto;
  width: auto;
}

.container {
  width: 95%;
  max-width: 70em;
  margin: 0 auto;
}

.clearfix::after,
section::after {
  content: '';
  display: block;
  clear: both;
}


/* typography
    ================= */

.unstyled-list {
  margin: 0;
  padding: 0;
  list-style-type: none;
}


/* header
    ================= */

header {
  position: relative;
  left: 0;
  right: 0;
  margin: 1em;
}

.logo {
  width: 200px;
  height: 40px;
}

nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
}

nav li {
  display: inline-block;
  margin: 1em;
  padding: 0.125em;
}

nav a {
  font-weight: 900;
  text-decoration: none;
  text-transform: uppercase;
  font-size: .8rem;
  padding: .75em;
  color: #DDD;
}


/* home-main
    ================= */

.home-main {
  background-image: url('../manjitcss/img/manjit-main.png');
  background-size: cover;
  background-position: center;
  padding: 10em 0;
}

@media(min-width:40rem) {
  .logo {
    float: left;
    margin-top: .5em;
  }
  nav {
    float: right;
  }
}

@media(min-width:60rem) {
  .logo {
    width: 250px;
    height: 50px;
  }
  nav {
    margin-top: .5em;
  }
  nav a {
    font-size: 1.04em;
  }
}

<header>
  <img src="../manjitcss/img/logo.jpg" alt="Foo logo" class="logo">

  <nav>
    <ul>
      <li><a href="">Home</a></li>
      <li><a href="">About Me</a></li>
      <li><a href="">Contact</a></li>
    </ul>
  </nav>
</header>

<section class="home-main">
  <div class="container">
  </div>
</section>

Solution

You have .clearfix in your css, but you have not applied them to header whose contents you are floating.

Add the class to header - see demo below:

@import url('https://fonts.googleapis.com/css?family=Raleway:300,400,700,900');
* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: 'Raleway', sans-serif;
  text-align: center;
}

img {
  max-width: 100%;
  height: auto;
  width: auto;
}

.container {
  width: 95%;
  max-width: 70em;
  margin: 0 auto;
}

.clearfix::after,
section::after {
  content: '';
  display: block;
  clear: both;
}


/* typography
================= */

.unstyled-list {
  margin: 0;
  padding: 0;
  list-style-type: none;
}


/* header
================= */

header {
  position: relative;
  left: 0;
  right: 0;
  margin: 1em;
}

.logo {
  width: 200px;
  height: 40px;
}

nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
}

nav li {
  display: inline-block;
  margin: 1em;
  padding: 0.125em;
}

nav a {
  font-weight: 900;
  text-decoration: none;
  text-transform: uppercase;
  font-size: .8rem;
  padding: .75em;
  color: #DDD;
}


/* home-main
================= */

.home-main {
  background-image: url('../manjitcss/img/manjit-main.png');
  background-size: cover;
  background-position: center;
  padding: 10em 0;
}

@media(min-width:40rem) {
  .logo {
    float: left;
    margin-top: .5em;
  }
  nav {
    float: right;
  }
}

@media(min-width:60rem) {
  .logo {
    width: 250px;
    height: 50px;
  }
  nav {
    margin-top: .5em;
  }
  nav a {
    font-size: 1.04em;
  }
}

<header class="clearfix">
  <img src="http://placehold.it/50x50" alt="Foo logo" class="logo" />
  <nav>
    <ul>
      <li><a href="">Home</a></li>
      <li><a href="">About Me</a></li>
      <li><a href="">Contact</a></li>
    </ul>
  </nav>
</header>

<section class="home-main">
  <div class="container">
   container
  </div>
</section>