how to lay floor for garden room

[Bluebaron]

I’m about to start building a Garden room. It’s 6 x 4 metres, normally I would probably build a Joist frame on piles/concrete blocks or something but this time there is a slab in situ.

the slab however is 8x5 ish so I can’t build along the edge. should just lay a DPC and fix a wall plate to slab, I’m not comfortable that the wall plate won’t rot as it’s so close to a level the water will just run over the dpc and rot the timber. 

I suggested bullding a two brick course and starting there and then using a floating floor in the middle, ( DPM, PIR, 22mm flooring)?

Other than that is laying a few blocks around the perimeter and building a joist frame, insulated between joists, flooring (ply, chipboard etc) and the wall plate of that. 
Thoughts?
Thanks.
 

[Onoff]

I'd go a couple of footing bricks high or even cast a concrete ring beam at least 150mm high. I'd liquid DPM the resultant "pond" then lay a sheet DPM. 

 

Then fill with pir and top with 22mm flooring. 

 

That's just me...I'm weird. 

[markocosic]

Single block on flat "piers" then resin fix studding through a few of them into slab for uplift?

 

Suspended floor atop that. Mineral wool fill. Slab is acting as oversite and footings.

 

Unless you're expecting flooding that brings structure more than high enough off slab for the water landing on the slab to drain of to the adjacent ground.

 

What is going to sit on top of the exposer border of the slab? How high relative to garden room floor?

 

How high is adjacent ground? If it's only damp not standing water that you're concerned about then floor could be floating independent of walls on top of some closed cell foam insulation sat in a dpc pond without risking it floating up into the room.