The pilot reported that while cruising at 800 ft he experienced severe buffeting with a subsequent loss of directional control. He was losing altitude so he increased engine power to reduce his sink rate. The passenger reported to the pilot that the night sign seemed to be coming loose. The airship came to rest draped over a seven story building. A witness reported seeing the airship heading south. He reported that it seemed normal; however, shortly thereafter he noticed a hole on the underside which kept getting larger. Prior to the tear the witness reported seeing a small dark mass with a white flexible material fall from the airship. Examination of the airship revealed that a section of the right banner, which was secured to the envelope by Velcro tape, and a projector, which was laced to a lacing strip glued to the envelope, had separated tearing the envelope. The envelope did not contain seams to prevent tears nor was the fabric tear resistant. However the fabric met or exceeded design and FAA regulatory strength limits.