Raised floor over existing concrete

[BillK]

I have a detached annex that was built in the 80's and this has an attached far older single story room built from stone. The walls are approximately 1.5 feet thick and in good condition and it has a recently replaced roof. The floor is concrete and looks relatively newly done and I suspect done by the last owners 5-10 years ago. There is no damp course in the walls, it's over 250 years old and no idea if there is a damp proof membrane in the floor.

I'm converting this building into a shower room and toilet.

There is a step up of 150mm from this building to the annex and I'd like to raise the floor level up 150mm and put some insulation in. 

 

I'm thinking the process is to put a damp proof membrane down first, then put 125 X 45 rafters in, insulation in-between with moisture resistant 18mm chipboard on top. I'd then put some 6mm MR MDF on top before tiling.

 

Looking for advice on whether this is the correct method or what I need to do if not.

 

Hope I haven't rambled too much on my first post?

 

Cheers,

 

Bill