The answer to 'underlayment sagging' is that sagging or uneven underlayment is a common issue that can be caused by a variety of factors. Underlayment is the layer of material installed between the roof deck and the shingles, and it plays a crucial role in providing a smooth, stable surface for the shingles and protecting the roof deck from moisture.
Causes of underlayment sagging can include poor installation, damage from foot traffic during the roofing process, or simply the natural aging and deterioration of the underlayment material over time. Symptoms of sagging underlayment may include visible dips or uneven areas on the roof surface, which can lead to issues like leaks, shingle damage, and even structural problems if left unaddressed.