Elon Musk says the solar roof that will be sold under a combined Tesla-SolarCity will likely cost less than a normal roof to install

Check out any number of headline’s on this topic. I love that Musk hardly does any advertising for his companies, because he doesn't want to mislead anyone, so news outlets take his quotes out of context to mislead people instead.
Musk appears to be referencing comparable slate, concrete, and tile roofs, not necessarily asphalt or sheet metal roofs. The former are more expensive to ship, and suffer more breakage. He says as much in the Q&A

If you want to see the solar roof demonstration check it out

Musk unveiled four solar shingle options for a solar roof on October 28. The solar roof will incorporate glass developed by Tesla’s new glass division. Tesla will produce the solar cells for the roof with Panasonic at a manufacturing facility in Buffalo, New York. The carmaker anticipates that its roofing would be lighter and easier to ship, thus reducing costs on breakage and transportation. It's looking quite promising that a solar roof will actually cost less than a normal roof to install, before you even take the value of electricity into account. “So the basic proposition would be, Would you like a roof that looks better than a normal roof, last twice as long, cost less and generates electricity?” Of course you would.

AlthoughTesla’s solar shingles are expected to still be considered a premium product at rollout, it would look just like any other premium roof tiles, while doubling as solar panels for power generation. The Tesla Solar Roof will also work like any rooftop solar system, connecting to your home’s electric panel through an inverter. You could stop there, but the system is being packaged alongside Tesla’s forthcoming Powerwall 2.0, a battery storage device with a built-in inverter and an installed cost of $6,500. Combining Solar Roof and Powerwall 2.0, Musk promises, will power an entire home with 100 percent renewable energy.

To be fair, the target market of solar roofs are probably not areas like Minnesota, Wisconsin, or Michigan, where you can get tons of snow and ice piled up on your roof. But, how would this solar roof handle lighter amounts of snow? Here is a tweet where Musk addresses the question: https://twitter.com/elonmusk/status/792218248917811204

So how much overall will Tesla's Solar Shingles cost overall to buy and install? About $70,000 - $100,000. That cost will be because consumers will essentially be paying for long-term electricity costs up front. Tesla says the life expectancy of its tiles will be 30 years. So that adds $60,000 to the value of the roof.