Has anyone applied a roof with 2" - 3" river rock ballast over a membrane on a low-pitched roof? Does it look good? Is it a pain to maintain (I suppose I don't mind moss -- could become a green roof here in misty Seattle). Do the stones gunk up the downspouts?

Here is the background:

We have a vintage 50s's house with a single layer torch down roof well-maintained with silver paint (probably asbestos). The original blue prints called for "marble chip" roof, so presumably this roof is not the original roof. We are doing a remodel and would like to add insulation to our roof (our roof is also our ceiling, which is exposed cardecking). There is no attic. We also live in a neighborhood with view covenants, so that 6 inches of rigid insulation had to be approved by the neighborhood board. Our pitch is just a bit more shallow than what you need for a standing seam metal roof, which I think is our first choice (looks nice and our roof is what you see the most of coming down the hill approaching our low-slung house. Perhaps metal is still an option, but either it would go over a membrane roof (paying for two roofs), or the metal would need to be folded over at every seam very tightly.

I don't like the look of the light grey TPO. If we had to do a TPO or PVC, I would want it to be very dark. In Seattle that extra absorption of heat from a dark roof might work to our benefit for most of the year.

So, should we even consider a ballast river rock roofing system?

I love this forum, thanks for your input!!