It doesn't specifically say what it's made out of. Just that the second layer was filled with "Masonry and wooden construction"
But I wouldn't think a wooden floor 2 miles across would be stable enough to hold buildings. The would would start rotting and stuff and constantly have to be replaced, and the supports would have to be huge. I'm assuming it's made out of some kind of stone
There is no stone material on Earth that could make an unsupported floor 2 miles across either. Stone/concrete has great compressive strength, but very low shear strength and tensile strength. It is also very heavy. You would have to have arches and lots of columns along with steel beams and supports. Wood is actually much better at tensile strength than stone or masonry.
That said, wood also couldn't support a 2 mile floor without lots of supports, and concrete's great compressive strength would be more suited to having things piled on top of it (like large buildings).
In reality, it's probably best not to think about the details of this, because the engineering details are probably impossible to work out as Jim writes about it. At least not without inventing some super-material like spirestone, or some other "magical" means. Maybe they were able to put lift crystals in the floor?