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Flat Roofs Some buildings have flat roofs. You often see these on large buildings like shops or offices, and sometimes on modern houses. A flat roof is not completely flat. It has a very small slope so water can drain. The roof is covered with a special waterproof material. This is often called a membrane. The membrane can be made of rubber or plastic. Flat roofs can be used as a terrace or a place for air conditioning units. CONVERSATION 2: A Problem with a Flat Roof A building manager is