If you are growing sedums only and no edibles, you won't have to worry too much about heavy metals and toxicity in the growing medium and fertilizer you use. All the materials you mentioned, including packing peanuts can be used for your purpose. However, I would replace the Miracle Growing Potting with Expert All Purpose Potting Mix, for example, for quality, environmental, and value reasons. Expert potting mix should be just as accessible as MG Potting Mix. There are a few exceptions, but MG products tend to have more heavy metals in their products than national brands. I have been wondering for a while where the source of the chemical smell in some my old potting mixes (one year old) as compared to the earthy smell from newly purchased Expert potting mix. By deduction and elimination, the most sensible suspect is the MG potting mix and fertilizers I have been using; however, I can't prove it for sure. In an unrelated event where I bought Ironite fertilizes in preparation for repotting my Gardenias and having researched about the heavy metals in the Ironite products I recently purchased and used, I found the Washington State Fertilizer Database. I looked up and compared the test results for MG and Expert potting mixes and several national brand fertilizers in addition to Ironite products, and MG potting mixes have more heavy metals than Expert potting mixes just as I suspected (although the presence of higher heavy metals in MG doesn't necessary contribute to the heavy chemical smell I noted earlier).

Both Expert potting mixes and fertilizers may cost less than the MG brand if I remembered correctly.

Sedums need little watering, and your mix of Hydroton (LECA, expanded clay) 40%, perlite 40% and Potting mix 20% will drained very well in anticipation for consecutive days of rain. I didnt check and I'm not sure how long you can reuse Hydroton, but I believe it is durable for several years like perlite. The use of Hydroton seem a bit expensive for roof covering but will work well for your objective. Going a little on the dry side would be my preference because you can always water as needed due to the variations in weather and as an insurance for overwatering, but it's impossible to take water out of a saturated growing medium. It may also be a bigger emergency job to remix and repot the damaged sedums after root rot but doable.

Unless you already know by experience, you may also have to experiment a bit after the sedums have been planted in needing to add some heavier weight mulch to this porous mix later with well washed small pebbles, washed river sand/rocks, or plastic cover to counter lost growing medium from rain and wind exposures on the rooftop.

Have fun on the rooftop and good luck!

Few more options for you to consider. Replace the potting mix with either coco coir or pine bark fines. Pine bark fines are easily available and marketed in super home improvement and garden centers as soil conditioners. Coco coir is a renewable resource (will decompose just as bark fines) and very economical and less heavy and bulky to carry than pine bark fines. It also has several enhanced protective characteristics for growing healthy roots and also has higher electrical conductivity values (for minerals) than bark fines or pumice. One rose cultivation study showed that at identical pH of 6.0, more roses were harvested on coir than on pumice. The findings of this study are not directly relevant with growing sedums in your case but would be a consideration to eliminate the use of large lava rocks. The disadvantages of using coir are preparation work and added costs that may be involved in finding the right source of coir and in getting it ready for proper use. Read "How to Grow in Coco" at http://www.texashydroponics.com/shop/How-to-grow-in-Coco-p-2220.html

You can easily do a 75-80% perlite mix (3:1) with any of the 3 options mentioned above without using Hydroton or with less % of Hydroton.

Thanks Legacy, for your very detailed posts.
What kinds of edibles could I grow on a green roof? I was thinking Chives and creeping thyme.
Here's the development of my medium: I initially had planned to use 80% Perlite, but read that it will disintegrate with freeze-thaw cycles, and also blow away when dry. What do you think? (Btw, your idea of a pebble mulch is superb.)
I then went to planning a mix of crushed brick (hopefully reycled brick) and Hydroton (LECA) as they do in the U.K. to make it cheaper than Hydroton 80% mix. The brick holds on to water more than the LECA and so I could avoid any watering once the sedums were established. The roof is a little difficult to access and water. However, it has been difficult to find crushed brick.
I was able to get a quote for Hydroton in our local Hydroponics store $29.99/ 50 liter bag. Doing the math, that's 1.76 cubic feet of Hydroton, which will cover 21 sq feet area of nursery trays (13 trays) if I use one inch depth of Hydroton in every 2.5 inches of medium (40%). So for about 160 sq ft of trays (100 trays), I will need about 8 bags Hydroton. Now, I am not taking into account the fact that when the media are mixed, the total volume will be less because some of the particles will sit in the gaps between others (e.g, potting mix). I may make that up with the styrofoam peanuts, perlite, etc.
But, crushed brick has been hard to find.
So I finally decided to use Hydroton, mixed with Perlite. May work out better, since brick is heavier than the other two.
However, a final mix of 80% perlite and 20% pine bark fines or coir or Potting mix sounds great to me (and my wallet), provided the perlite does not disintegrate into dust.
I will see if I can find coir. and the 'Expert Gardener(?) Perfect Mix All Purpose Potting Soil'? and the Pine Bark Fines.
Should I mix the peanuts in or make a separate layer at the bottom?
The problem with living in a vertical city like New York, is that gardens are tiny and so are the choices in garden centers around here.

Silica stone is also a lasting soiless growing option (more expensive though), but I have not used them. See description at http://progressivegardening.com/pottingmixalternatives.html. Hydroton is also widely used in the U.S.

Perlite should last for some time, but nothing is permanent even with perlite. If it does disintegrate with freeze/thaw cycles, then have you thought of what to do with your sedums in the winter than with the durability of perlite that is supporting the sedums? Besides, vadalism and thefts (in community spaces and gardens) may be more of an issue than the durability of perlite... you think? I wouldn't use anything too expensive or fancy just to get started.

Think of carrying stones up/down the stairs and subsequent repotting and removing the gravel mulch. On the other hand, plastic mulch may be the most economical and easiest option to work with and for transport if you need to mulch.

A simple low tech hydroponic system will solve both the storage and water problems if you are going to grow anything else other than sedums. Self-watering and hydroponic planters can be very efficient, productive, less labor intensive, and economical ways of growing in containers. There are many commercial and DIY self-watering and hydroponic models to choose from.

Links to select DIY low-tech hydroponic ideas and success pictures at:

Grower Designs used in Canada, http://english.rooftopgardens.ca/?q=image/tid/66

The SNAP hydroponics setup (Simple Nutrient Addition program) and AVRDC system (the Asian Vegetable Research and Development Centre) used in the Philippines, Malaysia, and Taiwan: http://www.agnet.org/library/pt/2004008/ and http://www.agnet.org/library/pt/2002037/#picf2

Easy System Plans, http://www.growing*.com/basics/easyplans/plans.html
[The insertion of the * is to protect gardenweb from being spammed. To connect to the proper link to view the system plans, substitute the * with the word, edge]

There are also several homemade earthboxes and self-watering planters posted on Gardenweb.

Update everyone!
So this weekend was spent filling 35 2.5" deep, 21"x11" plastic flats with:
Hydroton 40%
Pea gravel 30%
Potting Mix wth coir as a component 20%
Mason's sand (and I hope that this is not a problem later) 10% for enhanced water retention after the potting mix decays to humic acid. Next year I will topdress with aged garden compost.
The beautiful sedum plugs arived from Greenroofplants.com nursery, but in the meantime, the canary in the coal mines have been some sedum cuttings that I had rooted before hand. Everything has been placed on the roof, and seems to be doing well, although the mix seems to drain almost too well?
Plan to substitute part or all of the potting mix with a mix of coir and worm castings available at the Hydroponics store here.

It must feel great now that you have set the wheels in motion!

If your goals are to enjoy some of the enviornmental and personal benefits of planting something green on the rooftop without investing in and maximizing all the horticultural and engineering standards of a self-sustaining living roof, then any growing medium you use for sedums would serve the purpose and may be even succeed for the long-term. However, instead of using a mix of coir and compost, a simple mix made of 75% inorganic (of what you already have) and 25% organic components (or less, 7%) may proof even more advantageous since similar growing medium recommendations (b/t 60-80% mineral materials) have been corroborated by various academic and commercial sources for growing rooftop sedums and/or building an Extensive Green Roof such as yours (with less than 6" of growing media) based on my cursory read.

Your final choice of growing medium made of all/mostly coir and worm castings are all organic and thus will compact and shrink over time unable to deliver nutrients and provide the needed aeration, drainage, and plant support while leaving the roots of plants exposed and causing plant degeneration or death. Moreover, many sedums do better in poor sandy soil and don't do well in fertile soil. A few studies also showed a few specific varieties of sedums thriving and covering a given area faster comparatively when less or a certain fertilizer was used. Any potential discharge of N, P, and other metals from green roofs is likely to be reduced considerably by using the minimal amount of fertilizer needed to maintain plant health when growing succulents such as stonecrop.

A few green roof studies (using specific modular or commercial systems) published by Michigan State and Southern Illinois University may be of interest to you. There is one more academic source with similar findings as SIU which I can't recall at the moment. You may also want to reference the only internationally recognized German FLL guidelines for building and maintaining green roofs when looking for answers or suppliers and contractors.

How high up are you? Will wind and water retention be relevant challenges in securing either your grower and potting mix in any way? You may want to consider using some type of a biodegradable, one-quarter inch mesh to cover the surface of the flat in the first year (or recycle plastic mesh you may already have). This material functions similarly to mulching and plastic cover or landscape fabric I mentioned previously and will hold the growing medium and also serve to prevent loss of water through evaporation and enhance nighttime condensation during the initial growing season.

BTW, SIU found pumice to be the top growing medium out of four other types tested for growing two varieties of sedums when using a brandname modular rooftop block.

Hope to see you succeed at the top :)

Thanks legacy. Final mix is not more than 20% organic in any tray. 50 of the 100 trays have 20% potting mix as the organic, and the other 50 trays have the coir mix (but in less than 20% proportion, since I started to run out of coir). 80% in all trays is growrocks (40-50%), sand (10-15%), pea gravel (20-30%).
I am thinking that even if the 10-20% organics decompose, that will not destroy the soil structure significantly and will be compensated for by the decomposition of the sedum roots themselves.
I had read the studies on the Michigan State U. and SIU websites, as well on the Sheffield U. in the U.K. (They use Hydroton mixed with crushed brick). The study on pumice seemed a little short duration to me, 10 weeks only, so I did not give it much weight. Pumice is also difficult to get around here. Heck, almost everything was dificult to get around here, except what I used.
I am only two stories high, and the trays and plants have been fine through two serious thunderstorms, without erosion or flooding of trays. I think the sand and the organics compact together slightly and hold everything together, but do not impede drainage significantly. I finished the last plantings yesterday. I even have some thyme and chives growing, and they seem to love it in the heat up there. There is a very noticeable difference between the temps of the exposed roof membrane and that beneath the trays, almost 10 or more degrees. I cannot imagine why greenroofs are not more universal. They would certainly prolong the roof life so much more.
The best part, I saw butterflies flitting among the sedums this am!!
Thank you for your help!! I have pics.

Coir is great for roof gardens. we make great coir products.
Pls visit our website at www.densuventures.com and hit the research tab.
Email me and let me know if I may be of further assistance to you
Jake

.... he says shamelessly as he continues to post dozens of self-serving commercials all over GW.

Al