Final answer:
Roofing felt is applied to the roof sheathing before shingles are installed to provide an extra layer of protection against water penetration.
Step-by-step explanation:
The material applied to the roof sheathing before shingles are installed is called roofing felt or tarpaper.
It is a layer of asphalt-saturated felt that provides an extra layer of protection against water penetration and helps to prevent leaks.