Would you recommend re-roofing over a single layer of asphalt shingles?

Answer: Never
Explanation: we don't cut corners

Answer: In some cases
Explanation: Recommended by ARMA as a green solution

Answer: Never
Explanation: Of 800 installations, I have only done three shingle over projects.

Answer: In some cases
Explanation: LIMITED FUNDS AVAILABLE, EXISTING NOT SHOT YET.

Answer: Never
Explanation: A new roof is an expensive undertaking. The life of the new shingles installed over the old ones will be cut in half or more. The next time the roof fails you'll have to pay to strip off two layers instead of one. Also, by not removing the old shingles you won't know if rotted wood needs replacing, flashing needs replacing or ice and water barrier needs to be installed not to mention warranty issues. The smart way is to strip the roof down to the decking, make repairs if needed and install all components fresh and new.

Answer: In most cases
Explanation: in most cases I would always roof over a single layer unless there is dryrott issues with the sheeting under roof

Answer: In most cases
Explanation: Code says you are allowed to

Answer: Never
Explanation: It has been proven that a double layer on a roof is a bad thing and will decrease the life of the roof sheathing (plywood). In most cases when we remove a double layer roof, there is almost always an abundance of plywood to be replaced. In addition nowadays home inspectors know a double layer is a bad thing and if you go to sell your home, chances are they will be asking for a credit to replace the roof regardless of how old the top layer is.