pitched or sloped wouldn't make a diffence, if the roof construction was built correctly it wouldn't push the walls out, are you plan on haveing it open or are your boarding the ceiling?
built up felt is better than the rubber, better lifespan and the birds don't want to peck the felt like they do with the rubber.
have a look at my web site for the cold applied liquid plastics (triflex)
but i would consider for cost to go with felt as red cap sheet is £30 for 7.5m2 a roll
and sanded layer 4mm is £30 for 9m2 a roll
www.cdhroofing.co.uk