stoner

Hello, hope someone can help me. I have got a job (12m2) of limestone and heating to lay on a chipboard floor. Floor is already tiled with porcelain which i'm going to pull up which has been down a few years with no visible problems. Can i tile straight on top of the chipboard (providing it does'nt pull to bits) or do i really need to put something on top first?

Great site by the way. I have been tiling for 5 years for mainly tile showrooms and just decided to go on my own so heating is a little new to me. Better safe than sorry.