Asbestos is 100% safe as long as it remains in one solid piece. However abrading it leads to small particles finding their way into the air. Just one of those (nearly invisible) particles entering your lungs could result in asbestos-related disease (possibly many years later). So you should take great care not to knock, crack or abrade the surface of the asbestos and, of course, wear a mask (preferably with an overall which can't harbour small particles of asbestos, which might later be released into the air).
Also remember that asbestos roofs aren't load-bearing. Don't try standing, or kneeling, directly on the roof.
Having got that lot out of the way, see Luke Smith's post here for an answer to your question:
http://www.howtomendit.com/answers.php?id=3504
Chris
PS: Since you only require the roof to be waterproof for a short period, a large plastic sheet on the roof (weighted down with a few bricks) might be a simpler solution.