What is the term for the outer layer of a roof that provides waterproofing?

The outer layer of a roof that provides waterproofing is referred to as a roofing membrane. This membrane is essential as it serves as the first line of defense against water infiltration and helps protect the underlying structural components of the roof. Roofing membranes are specifically designed to be durable and effective against various weather conditions, ensuring that moisture does not penetrate into the building.

In contrast, sheathing refers to the structural panels that provide support to the roof system but do not have waterproofing capabilities. Underlayment is a protective layer placed between the roofing material and the roof deck, providing an additional barrier against moisture but is not typically considered the outer waterproofing layer. Flashing is used to direct water away from critical areas of the roof, such as joints or transitions, enhancing the waterproofing performance but again, is not the primary waterproofing layer.