I would like to build a 6 x 8 ft playhouse with a deck covering the top. Is there a kind of roofing material that can (1) withstand a floating deck, (2) is DIY friendly (no blowtorches, hot asphalt, or custom pouring), (3) is reasonably cheap, and (4) will keep the water out even with a completely flat roof, or with a very minimal slope?

Modified bitumen is said to be great for building deck on top of. The GAP SBS Liberty system is DIY-friendly, but the instructions call for a slope of at least 1/2" per ft. If using only the cap sheet, it's about $1/ft².

Building a deck on top of the roof is much easier if the roof is completely flat. Is that asking for trouble, or will it be OK for my application?

Are there other roofing materials I should consider?