I didn't have the heat stuff with mine- just bought some paint on tar type adhesive that was recommended to seal the edges.
The roof is approx 16 x 8 feet or so but like already said re the slope - it does have a very small slope on it so I suspect that helps. It had an asbestos roof on orignally that had gone porous - yuk!
I used this Ruberoid stuff
http://www.ruberoid.co.uk/index.php?page=49 Cold Self Adhesive stuff.
Not much help if you gotta do it again though - sorry.
I did try some of that paint on stuff on the asbestos thats supposed to stop leaks but it didn't work for me no matter how thick I painted it on.
Tarpaulin til summer ?