More than a dozen wildfires killed at least 23 people and engulfed more than 3,500 homes, buildings, and other structures across Northern California in one of the worst firestorms in state history. …In the US, homes are typically made of two basic materials: wood and concrete. The former is very flammable, which could be why only certain parts of houses — like brick fireplaces and concrete floors — survived the [Santa Rosa] fires. The surface of concrete can burn as well if it’s covered in flammable materials, like varnish or plastic. GigaCrete, a homebuilding firm based in Las Vegas, aims to make homes more fireproof. …GigaCrete also makes a non-flammable, proprietary varnish that is painted on the walls. When the material is layered on, the home becomes fireproof, bulletproof, waterproof, and super insulated, according to the company. The coating, called PlasterMax, is fire-rated, meaning it passed combustion and room fire tests.