WPC is made of a wood fiber-plastic mixture, right? I’m not exactly sure.
WPC consists of wood fibers/wood flour, which are embedded in plastic, basically as a filler. With plastics, there is also fiberglass (but not in the construction/furniture sector). Basically similar to particleboard or MDF, but with a thermoplastic plastic (PP) instead of glue. The wood content varies greatly. In technical applications, it is usually not higher than 20%, in construction often around 50%. Asian WPC products almost always have a wood content of 50%+, because there it can then be defined as wood by definition. The combination of plastic/wood is actually great: - resistant to weathering (microorganisms, animals, water, sun, etc.) except possibly fading - no risk of splinters - can look deceptively like wood - easy to work with - dimensionally stable - lighter than wood - actually a recycled material, since "wood residues" can be processed there
However, the negative aspect is the coefficient of thermal expansion, which is similar to plastic and significantly higher than wood, metal, or even stone. You have to take into account that the terrace expands in size when exposed to sunlight.
Stone is a very good heat conductor and heat storage material. At least the best. Wood isn’t.
Both at the same time usually don’t work. If something stores heat well, it no longer conducts heat well. Stone, wood, and plastic are poor heat conductors. That’s why they are also used for insulation (of course, there are better ones). Metal, on the other hand, is a perfect heat conductor. Plastic, however, is not a particularly good heat storage material either. Wood and stone are already significantly better in that respect.
As previous posters have already said, there is no general statement. It depends on the surface, but above all also on the duration of exposure. Basically, I would always choose materials with low thermal conductivity. They not only absorb heat more slowly, but also release it more slowly. A 60°C hot metal plate feels much hotter than a stone of the same temperature. However, if the sun is shining on the material all day, all materials will get very hot. I would see WPC and stone at a similar level. WPC warms up faster, but if it gets so hot that you can no longer walk on it, then it’s no fun on the stone slabs either.