seems like we have more sunshine than ever during springs and summers because of the global warming. Couldn't we build new types of roofs forming solar cells? at least for the sunny countries, it's a big waste of energy.

IIRC, there are technologies out there already that can pretty much make your entire roof one giant solar collector. Unfortunately, they're ridiculously expensive for the average consumer. I almost think that the US should start legislating federally that all buildings of a certain size or cost (warehouses, industrial facilities, office parks, malls, etc.) should have to generate X% of their energy from renewable sources or use X% of their budget towards solar/wind/geothermal usage.

Even better would to have all new construction using solar power roofing, but good luck getting that one passed.