Flex is great but tables are also a natural solution for anything grid-based, which of course layouts almost always are. The whole internet used to do stuff like this before the structural flow of content became important:

    <table width="100%" height="100%">
      <tr>
        <td colspan="2" height="50">  header  </td>
      </tr>
      <tr>
        <td width="200" valign="top">  sidebar  </td>
        <td valign="middle">  content  </td>
      </tr>
    </table>