Final answer:
Most jurisdictions only permit up to two layers of asphalt shingles on a roof to comply with building codes and ensure the roof's structural integrity.
Step-by-step explanation:
Most jurisdictions allow no more than two layers of asphalt shingles on a roof. When installing a new roof, it's important to adhere to local building codes, which often restrict the number of shingle layers to prevent excessive load on the roof structure. If a roof already has two layers, it is typically required to remove the existing layers before applying new shingles to ensure the roof's integrity and compliance with fire ratings and building codes.