Like many common home improvement questions, the answer is you can, but you need to decide if layering shingles is the best course of action for your home. It isn't uncommon to layer a second roof right over the first and then tear everything off before you put a third layer down. Building codes and most municipalities will usually allow you to overlay a second (though never a third) layer of asphalt shingles over an existing shingled roof. You will need to check with your building inspections department to be sure.
Common Problems with Layering Shingles
Layering shingles has a few drawbacks that you should take into consideration before you decide to lay new shingles over the old.
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Re-Shingle Roof as Part of a Larger Plan
Despite these drawbacks, layering shingles can still be the best plan for your roof. One of the situations, where this option can make the most sense is for a temporary fix as part of a long term plan. Many of the common problems listed above can be avoided if you end up replacing your new roof in the next five or ten years. This sounds like an unwarranted expense, but simply layering shingles can make for a reasonably priced project. You can then invest your money and invest in a high-performance, energy-efficient roof in ten years that will last for the rest of your life.