Regardless of what you might think about the integrity of your space, I'd have a structural engineer dope out exactly what load bearing capacity you've got. Sounds like you want to add some real tonnage.

I wouldn't proceed with the detail work until I had some assurance my rooftop garden didn't become a livingroom courtyard.

I'm with duluth. The weight of the retaining wall + soil + plants + water will be in the tons and I wouldn't guess at whether my roof would hold that up.

I'd agree with the others here, you may be flirting with disaster here, and having a structural engineer do some quick calculations on what your roof can bear is well worth doing first. Most roof top planters will try to minimize weight by using light weight soil mixes that may not have any soil at all, but are often a mix of sand, perlite, scoria, and some organic content from bark or coconut coir. It is important not to use actual dirt for containers, as the drainage will be too slow. Also, avoid artificial mixes that are high in organic content as they will lose too much depth over time as the humus content is lost.

You may find it makes more sense to build planters that are "false fronts" of lighter weight materials, and use individual large pots hidden behind it, so that you can more easily rotate out plants or root prune/divide them as necessary over time.

You'll want to read up quite a bit more about roof planters and their issues before you do this, if you want to avoid creating problems down the road.

We're notorious here for not quite answering the OP question.... sometimes this is useful, we hope. Of course, perhaps you have had this sussed out by a P.Eng.

I'm wondering whether the brick wall will have structural integrity against sideways forces... it's not brick's strongest angle. I also vaguely remember hearing that the plastic composite does not have that much inherent strength; can't recall in which direction.

I think Bahia is onto something with soilless mixes. I think he is also onto something with the issue of mobile plantings. I suspect nothing can stop determined roots and particulates (even airborne, not from your planters) from getting into that space below. Even a metal pan leading directly to drainage would eventually degrade. As such, I wonder if it would make sense not just to try to make it impermeable, but also to make it so that you can clean it. For my containers that are on decks and patios, I am a huge fan of casters - everything rolls. You might consider some kind of a system on rails.

Perhaps the most important factor to recognize is that nothing in landscaping is permanent. Trees will, for instance, eventually outgrow those planters and that roof. So you need to be prepared - or you need to prepare for a future owner - to empty, renovate, and refill the space. Making the whole installation flexible is one way of doing this.

KarinL