回複：請教： 更換屋頂的合同糾紛

Most states allow shingle on top shingle. It is a normal practice, as long as total layer of shingle is less than 4. Most roofer will do that, it has better protection and less chance of leak. See the reference.

http://www.roofer911.com/roofing-articles/roof-shingles-layers.htm