bp_document_get_root_parent_id( $child_id )

function bp_document_get_root_parent_id( $child_id ) {

	global $bp, $wpdb;

	$table     = $bp->document->table_name_folder;
	$parent_id = $wpdb->get_var(
		$wpdb->prepare(
			"SELECT f.id
FROM (
    SELECT @id AS _id, (SELECT @id := parent FROM {$table} WHERE id = _id)
    FROM (SELECT @id := %d) tmp1
    JOIN {$table} ON @id <> 0
    ) tmp2
JOIN {$table} f ON tmp2._id = f.id
WHERE f.parent = 0",
			$child_id
		)
	);

	return $parent_id;
}