@phildaythe original question was "Can I reroof the house at the same time as I install the panels and add the cost of the reroofing to the tax credit calculation?" Clearly, for the reasons stated above, the answer is No.
Just above, Critter points out an exception, which is if some modifications need to be made to the roof to make it able to carry the solar panels, then that can be considered part of the solar project, but NOT reroofing the entire roof.
So to address your question, you can add to the tax credit calculation roofing costs that are directly related to the success of the solar panel installation, but not incidental costs, such as reroofing other parts of the roof that are not part of the solar installation.
As for solar installers that say otherwise, first, we here at the Community don't know what they are actually and specifically saying (are they perhaps saying the same thing that we are, that some roofing modifications are part of the tax credit or are they saying that you can roll an entire new roof into it?). Second, will they add a clause to your project contract promising to pay the penalties and interest assessed on you by the IRS when you are audited and the non-pertinent part of the roofing expenses are disallowed for the credit? I doubt it. They are roofing professionals, not tax professionals.
