Show posts from specific roles

Code you are after:

<?php
$wp_user_search = new WP_User_Query( array( 'role' => 'administrator' ) );
$admins = $wp_user_search->get_results();
$admin_ids = array();
foreach($admins as $admin) {
    $admin_ids[] = $admin->ID;
}
$args = implode(',', $admin_ids);
query_posts("author=$args");
?>