I just started a bathroom renovation for a 2nd Floor Master bath and found a 3-4" slab of concrete below the original floor tiles and joists. Was this a common approach used on homes built in the 20's? I'm worried about the extra weight of the new bathroom if I leave it in place and also concerned about opening a can of worms if I remove it. Would you recommend I leave this slab in-place or start with a clean slate and rebuild the joists and sub-floor?

 

You're right about the worms & yes, it was common at one time. Leave it there.