The answer to 'how to install underlayment' is to lay a protective barrier between your roof deck and the shingles or other roofing material. Underlayment provides an extra layer of protection against water, wind, and fire. It's an essential step in the roofing process that should not be skipped.
To install underlayment, start by preparing the roof deck by clearing away any debris or nails. Then, unroll the underlayment and lay it over the entire roof deck, overlapping the seams by several inches. Secure the underlayment with roofing nails, making sure to space them evenly across the surface. Finally, cover the underlayment with your chosen roofing material, following the manufacturer's instructions.