Tree roots will often find the flaw or crack in old sewer lines. With the water and nutrients in the affluent, the roots remain and expand until a stoppage occurs. Trees do not intentionally seek out a broken tile, but do take advantage of the situation. New sewer lines are typically plastic now with far fewer joints and those joints are chemically sealed together. As to the concrete floor, construction methods and materials in the 30's is quite different by todays standards. Questions arise as to how the roots made it up to and under the floor. Most likely loose or poor foundations allow roots to make there way inside and with moisture collecting beneath the concrete, those roots can survive and grow. Roots pushing through a modern foundation is extremely rare unless there is a structural issue.
