WordNet (r) 3.0 (2006):

rooftop
    n 1: the top of a (usually flat) roof

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "rooftop":
   ceiling, clerestory, eaves, housetop, lantern, overhead, penthouse,
   plafond, ridgepole, roof, roof garden, roof-deck, roofage, roofing,
   roofpole, rooftree, shingles, skylight, slates, surface, tiles,
   top, top floor, top side, topside, topsides, upper side, upside