Final answer:
Blisters and bubbles on a built-up roof are an indication of moisture trapped between the felt-plies. These roofing issues should be addressed quickly to prevent further damage and deterioration of the roof materials. A roofing professional should be consulted if the problem is widespread or severe.
Step-by-step explanation:
Blisters and bubbles on a built-up roof usually indicate moisture trapped between the felt-plies. This is a common issue that can arise when moisture penetrates the roofing materials or when it becomes entrapped during the installation process. The presence of trapped moisture can lead to the deterioration of the roofing materials over time and may require repairs or replacement of the affected sections.
Proper roof maintenance and addressing moisture issues promptly are essential in extending the life of a built-up roof. If the blisters and bubbles are widespread or severe, it may indicate a systemic problem that needs to be resolved by a roofing professional. Neglecting such issues can lead to further damage, such as leaks or structural problems, which can be far more costly to repair.