I would look into a 100% silicone waterproofing system for ease of install and longevity of the material. Make sure to follow manufacture spec as many times you will be required to use a bonding primer over the concrete in order to achieve manufacture warranty. There are now many options in regards to manufactures of these 100% silicone systems such as Gaco (Firestone), Mulehide, Henry, GE, Gardener, and others. Do your research, follow the specs and it is a fairly easy install process very similar to painting.
I must say I’m a little confused. ICF (insulated concrete forms) are not something I have ever seen as part of a deck above a living space.
But let’s take out the ICF part and address the deck in question. There are a lot of builders not wanting to address this type of build in their projects because they fear the potential of failure down the road and let’s be honest who wants water pouring into their newly finished home and who can blame them. But if you take the time and address each layer of the project properly you can have a nice water tight concrete deck that is over an existing living space for many of years.
The first part is the framing. I would suggest using TGI’s because of their adherent straight and I would also suggest a placement of 12”-16” on center, as opposed to 24” on center or larger. If you cannot install the floor joist framing members on a slight decline then you will need to install sleepers on top of the floor joists to create this slope. The slope does not need to be dramatic, just enough to allow run-off if there is penetration. The sub-deck is next and this is done in the typical fashion with a ¾” T&G board. I would also use an OSB as opposed to a CDX board.
From there you are installing your EPDM rubber roofing membrane. Just remember to run it up your side walls approximately 12”-24” and turn it down on the outside edge(s). If your deck is larger than the EPDM membrane make sure you follow the manufactures recommendations on adhering the pieces together. You really don’t want the water to get to this level but if it ever does this is your last lines of defense so don’t cut corners.
On the EPDM goes 2” of polystyrene. I’ll also install this same material, in strips, around the perimeter of any walls coming in contact with the deck and I do this because I don’t want the next material puncturing the EPDM. And the next layer is corrugated metal decking. Install this as you typically would be installing it on a roof. Let it hang over the outside edge just a little and that is it.
Now you are ready for your concrete deck. I use #4 (1/2”) rebar in a grid pattern for my tensile strength and I’ll pour a good 4” 3500 psi concrete slab and finish with a nice brush finish. And of course you will need to form your ends, but you already knew that didn’t you.
Good luck and remember what your father use to tell you “A job worth doing is a job worth doing well”.
I would recommend fiberglass with a gel pay finish. If you look at concrete after 28 days under a microscope it has many holes due to shrinking during curing. This is why concrete decks don’t leak for weeks when installed
fiberglass with a gel coat finish. It will last for ever