When you overlay a new roof you attach the new shingles or other roofing material to the existing roofing material.
Overlay shingles roof.
However this can only be done once.
Metal roofs offer energy savings but do cost more upfront than traditional asphalt shingles.
That said overlaying asphalt shingles with a metal roof could be an option provided there is no underlying damage.
They also last longer and stand up to foul weather better than shingles.
In general overlaying a new roof takes less time less labor and thus less money.
It s good to know that you can only choose this option if your roof has not been overlaid previously.