Redirect in header for everyone except admin
 

I need to know how to redirect everybody from my site to a 'under construction' page while I do a massive upgrade. I do not want the 'site closed' message because it will show the new header.

So I was thinking about a conditional in the header or maybe headerinclude template that would check if not a member OR if member does not equal admin* then redirect using exec_header_redirect possibly.

Would this work? What code should I use exactly?
Will I still be able to log in to admin panel if I log out?

Thanks in advance