The flat roof has evolved from early roofs covered with sheets of lead, copper or zinc that lasted for more than 100 years. The introduction of the bituminous felt roofing sheet in the 1960s provided an inexpensive and lightweight material to cover roofs, but the bit felt was prone to leaks and did not last for decades. The introduction of the flexible polymer sheet provided more longevity and leak protection than bit felt, and should last for 15 years when installed properly.

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