Modified bitumen membranes are hybrids of the built up system, only pre-manufactured in the factory. The products come in rolls, and are modified asphalt or coal tar systems with a rubber added for low temperature and elongation characteristics. These products typically use a built-up membrane underlayment before application of the final modified bitumen membrane. Modified bitumen membranes can be torch-applied, self-adhered or mopped in place with hot bitumen. They always are surfaced with some type of topping system.