The invention relates to a permanent waterproof layer of a building roof. The invention adopts ultra-high-performance concrete, and the integral roof waterproof layer is laid in one or more pouring constructions. The obtained waterproof layer body and seams have reliable functions of anti-crack, anti-seepage, waterproof and thermal insulation layer protection, and can simultaneously improve the strength and rigidity of the roof structure. The waterproof layer is durable and long-lasting, and the working life is the same as that of the building. The construction joint bonding joint and strengthening method of the invention can ensure the crack resistance, seepage resistance and waterproof performance of the joint interface. The permanent waterproof layer of the present invention has relatively simple laying and construction, and is suitable for waterproofing and maintenance of new building roofs and old building roofs of different structures, especially for flat roofs with relatively high loads such as planting, water storage, and parking. The waterproof layer of the cement-based composite material with both toughness and rigidity has no aging problem, and the waterproof and seepage-proof reliability of construction joints is high, which can permanently solve the problem of easy leakage of building roofs.