Is the slab below my bath tub supposed to be solid like what is visible elsewhere? Recently I opened the wall behind my toilet due to need for a 12" rough in distance. When I peered through the wall toward the area behind where the tub is located I found I was looking through the slab at the sand used to fill it. I do not know how far under the tub the hole extends, I was only looking at the fill and drain end.

I needed to replumb the drain from the sink in order to gain the 2" needed for the deeper rough in. I closed up the wall, as I will deal with the slab later when I have the tub replaced, but need to know why the builder would leave a hole in the slab like that.

I think this is where the subterranean termites came from. I found a root had grown along side my toilet drain and was dipping into the hole being watered and fed with each flush.

They often leave a 16”x16” hole in slab under tub. This is for access to trap, clean-out, and making connections easier. Plus remember, all tubs don't have drain holes punched in the same place so this make it possible to reposition the elbow that hooks to tub drain.

Termites and roots are caused by leaks; fix these and that should stop. I also recommend mounting a few boards on sides of hole and adding a cross member just before drain to support the tub if you replace, otherwise tub will feel spongy or flex near drain. Once its nice and dry, using a few cans of Great Stuff foam can fill the void nicely.