What is true regarding the installation of new asphalt shingles over an existing layer of shingles?

The practice of installing new asphalt shingles over an existing layer is permitted under certain conditions, which makes the chosen answer accurate. Specifically, the International Building Code (IBC) allows for the installation of new shingles directly over one existing layer when certain criteria are met, such as ensuring that the roof structure can support the additional weight and that the existing shingles are not severely damaged.

This approach can save time and labor costs since it eliminates the need for removal, provided the underlying shingles are in good condition and not causing any moisture retention or structural issues. It's crucial, however, to check local building codes as they may have specific rules regarding how many layers are allowed.

Other options present considerations that do not universally apply to all scenarios. For instance, removing all existing shingles is often recommended for optimal results, but it is not strictly necessary if there is only one layer that is intact. Installing shingles on a flat roof is generally not advisable due to drainage issues. Lastly, while dry weather is preferable for roofing installations to ensure proper adhesion and prevent moisture-related problems, it is not a rule that governs whether or not the installation can occur.