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While you technically can add a roof over shakes or shingles, it’s not a good idea. Roofing over existing roofing creates a moisture barrier underneath the new roof and since the trapped material is wood, you can expect a moisture problem. While you could get away with installing a new roof over asphalt or steel, wooden roofs should be removed, new underlayment installed, and the new roof installed over that. Also, the extra weight would add strain to your roof structure.

While you can technically install TPO roofing over an existing roof, it isn't always wise to do so. The more layers of material you add to your roof, the more weight it must support. You may also develop moisture issues by covering your existing roof with TPO. Your local building codes and regulations may actually require you to remove the existing roof before adding TPO, so check with your local municipality.

While there are many benefits to installing a metal roof on your home, there are some downsides, too. Zinc roofs, in particular, are susceptible to corrosion from overly salty air directly near coastal areas. Another potential downside of zinc roofing is that it must be allowed to expand and contract with the weather. This means special attention is necessary to how it's fastened to the home to avoid issues. 

Metal roofs have a reputation for being noisy, but with proper installation and adequate insulation, they are not significantly noisier than shingle roofs. The type of metal used can affect the sound level; for instance, steel roofs are typically noisier than slate or asphalt tiles but quieter than aluminum. While some people find the sound of rain on a metal roof pleasant, you can take steps to reduce the noise. Installing a thicker underlayment, adding insulation between the roof and the house, or using acoustic tiles can effectively muffle the sound. The gauge (thickness) of the metal roofing can also contribute to sound dampening.