how to include file from root dir into header.php in subdir

hey everyone,

i have the latest copy of wp installed in /blog

our client runs a website in the root directory

my problem is that, i am trying to include a file from the root directory into a header template file in /blog/wp-content/themes/default/header.php

this is the code i am using

<?php include("/core_menu.php"); ?>

which is inside of /blog/wp-content/themes/default/header.php

is there any way to include files from the root dir?

thanks for your time everyone.