What is the most common underlayment used for a built-up roof?

The most common underlayment used for a built-up roof is rigid foam board. Built-up roofing systems typically consist of multiple layers of roofing felt and a protective coating or cap sheet on top. The inclusion of rigid foam board as underlayment serves multiple purposes.

First, rigid foam board provides excellent thermal insulation, which is crucial in reducing heat transfer into the building. This insulation helps to maintain a more consistent indoor temperature and can contribute to energy efficiency.

Second, the rigid structure offers a solid substrate that can help in evenly distributing weight and providing additional support to the layers above. This stability is particularly important in built-up roofing systems, where the weight of multiple layers can place stress on the underlying structure.

Additionally, rigid foam board has good moisture resistance properties, which protect the integrity of the roofing system and help prevent water infiltration that can lead to further issues such as mold or structural damage.

Other options, while useful in different contexts, do not serve the same purpose as rigid foam board in the context of built-up roofs.