Where could this root coming from?

I bought a house in March, previous owner had rose bushes everywhere, including tree of heaven trees right up to the foundation. As I’m digging up the dead rose bush root balls, I discovered what looks like a limb, it’s about 6 inches in diameter, and extend across my foundation for at least 3 feet. It’s not a limb, it’s just a big root. And it’s not dead. I have no idea what tree it belongs to. On this particular side of the house, there’s no trees, just dead rose bushes and native grass.