The answer to 'difference between underlayment' is that underlayment is a layer of material installed between the roof deck and the roof covering (such as shingles or tiles). There are two main types of underlayment: felt paper and synthetic underlayment. Felt paper is the traditional and most common type, made from asphalt-impregnated organic fibers. Synthetic underlayment is a more modern option made from materials like polypropylene or polyethylene, which offer improved durability, water resistance, and ease of installation.
The key differences are that synthetic underlayment is generally more expensive but longer-lasting, more slip-resistant, and better at protecting against moisture and leaks. Felt paper is a more budget-friendly option but is more susceptible to tearing and deterioration over time. The choice between the two often comes down to the specific roofing project, budget, and the homeowner's preferences.