Kari, its been a long time since I had a baby, so bear with me. But personally, I don't sleep on a waterproof "rubber" sheet and I wouldn't want my baby to either. Its so hot! If I used one, I'd use it under the mattress pad. I always used a mattress pad, a fitted crib sheet and sometimes I used were these squares of thick fabric that I don't know the name of and I would put where her little diaper and legs rested on that. I sure wish I had a picture to show you. Usually, I don't think I used anything extra. Just kept lots of extras, just in case.