Steve,

We are building  sort of green roof. We are using Hambro joists with a 4" slab. It will be mostly a roof top patio with planters. The admixtures can be applied 1) at the concrete plant making the entire thickness treated, 2) topical, powder spread across the wet concrete prior to power floating with only surface treament, and3) a brush on mixture after the concrete is set. In any case, the admixture can only seal very small cracks, less than 1/16". I would not trust admixtures if I could not see the cracks. There are polymeric materials made just for sealing roof top patios, "Ames Research" that we saw at the World of Concrete. I will have expansion joints and have to have a strip sealer of some sort.

If I were have 6" of soil accros the entire roof, truely green roof, I'd go with a rubber roof. I have had a water garden in side our home with 12"deep of water for many years made of roof rubber. It even had a glue joint because the pc I had was a bit small. As I recall, roof rubber comes in .035" and .045", $.50/sq-ft

As far as your insulation question; I would only add what was required for the rubber roofing. most green roofs are layered and the insulation is on top of the joists. Obviously the ICF type is insulated from below. One inch of insulation maybe required to pad the rubber against the concrete to prevent puncture.

Patrick T

To add a little more detail.....

My company has used Hydrotech for many years with good results.  We are also evaluating Hycrete, a waterproofing addmixture, for some projects, largely because of its Cradle to Cradle credentials.

The problem with addmixture products is, of course cracks.  Unless you are 1000% positive that your slab is and will continue be crack free for the life of the project, you need a redundant membrane.  Roots can be very invasive and destructive, so a robust root barrier is needed in any vegetated roof.

Hycrete advises that using their addmixture along with a less robust membrane to deal with cracks is usually cost neutral compared to a robust roofing system.  This may be OK for a normal roof, but I am very concerned about the "less robust membrane" with roots, even with a root barrier.

I hope to do something like this on our future home, but based on what I know today, I will go with a robust roof membrane below the roof insulation and a robust root barrier system.  Providing some positive slope to the membrane to prevent ponding is advisable in any case.  With roofing, the devil is in the details of penetrations and edge conditions.

If you have a flat roof with a parapet, make certain that you have overflow drains or scuppers to eliminate the possibility of a structural failure if your roof drains get clogged.

I would appreciate it if you would keep us advised of the details of your project.

Bruce