The answer to 'what is underlayment' is underlayment is a water-resistant barrier that is installed between the roof deck and the roof covering, such as shingles or tiles. Underlayment provides an extra layer of protection against water infiltration and helps extend the life of the roof system.
Underlayment is typically made of asphalt-saturated felt, synthetic materials, or rubberized asphalt. It is an essential component of a roof system, as it helps prevent leaks and moisture damage to the home's interior. Underlayment is installed before the roof covering is applied and helps create a smooth, even surface for the roof shingles or tiles to be installed on.