Final answer:
A built-up roof with a lap joint every 12" is a 2-ply roof.
Step-by-step explanation:
A built-up roof with a lap joint every 12" is a 2-ply roof. The term "lap joint" refers to the overlap of layers in the roof construction, which helps to create a waterproof barrier. In a 2-ply roof, there are two layers of roofing material that are overlapped and sealed together to provide added protection against water infiltration.