Built up roofs are the oldest form of Bituminous Roofing membranes. They are constructed of lapped asphalt-impregnated (cellulose, glass, or synthetic) felt installed by hot-mopping coal tar pitch (level or very low slope) or liquid asphalt product which adhere to eachother to forma durable, multi-ply roofing layer.
An underlayment may be necessary and/or required by local building code and is often perlite, gypsum board, or tongue and groove wood.
Also a vapor retarder may be necessary under the underlayment (or base sheet if no underlayment exists). They are commonly constructed of a base sheet of glass fiber typically lapped 2″ or a base roofing felt with 4″ laps. Then layers of the asphalt-impregnated ply-sheets (432SF or 216SF; 15 or 30lb rolls) are installed with 12″ then 24″ starter courses before applying the 36″ plies, with hot asphalt binder in between each ply. The mopping between felts typically requires 25-30 lbs per square. For 2-ply roofing, lap is typically 19″; 3-ply is 24-2/3″; and 4-ply is 27-1/2″. Finally a specialized/coated cap sheet is applied in rolls with the final coat of hot-mopped asphalt applied on top, often at a higher rate of 65-75lbs per square. Pipe boots for built-up-roofs are typically lead or copper and installed by the roofing contractor (except for non-MEP penetrations). Galvanized pipe boots will not adhere to asphalt membrane roofing.