I found this simple alternative to adding a maintenance plugin. Add this to functions.php of your child theme.
function maintenace_mode() {
if ( !current_user_can( 'edit_themes' ) || !is_user_logged_in() ) {wp_die('Sorry, We are performing routine Maintenance. We will be back up shortly.');}
}
add_action('get_header', 'maintenace_mode');