Once established, this greenroof will mostly survive on rain water. That would be another reason for not using Perlite as it may all wash off with the rain.

I like this idea! Best of luck with the project.
Obviously I have no real experience in a project like this but I think there are some general guidelines you can follow.
Sedums are a good choice for this; which species did you have in mind? They are not like many other C&S and will thrive in soil that is a bit richer. The mix I am using is *very simple* and *ultra fast draining*. 50/50 haydite and organic potting soil. I believe this would give your sedums a good jump-start so they spread quickly. The organic matter may break down...but the haydite it will NOT compact. Haydite is lighter than decomposed granite. IMHO, I would completely toss the idea of using perlite, vermiculite, topsoil, sand or peat in this application. As you said, this is a one-shot deal. The organics will be replaced by leaves, birds, lizards, etc who find their way to or over your roof. The "grit" will be the main concern. Here is a site for haydite. Scroll down and you will find companies that manufacture it and they should be able to help you find a dealer in your area. My local organic gardening shop sells it in bulk. I pay $5.85 for a 40lb bag.
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I'm sure others will jump in and share their ideas; most folks love to talk about their personal soil mixes. I love the haydite. We've had nearly 3 months of non-stop rain and yet my pots are still "running thru". Again, IMHO, you'll want to give the sedums a good start so they spread quickly and there is no wash-off in a downpour. After that I dont think your soil or (lack of ferts) will matter: the plants themselves will collect materials that will compost just like it does in habitat. The main considerations would be weight and drainage. Fortunately, sedums are shallow rooted and grow on hillsides so deep soil is not required. I say, pick the "grit" you feel confident in...add some good potting soil and go for it! I wish you the best of luck in your endeavor and hope you will keep us posted (with pix) as it progresses. HTH - Jo (who is actually NOT a haydite salesperson just a satisfied client)

Here is a link that might be useful: haydite

Thanks Jo. Great idea!
Most shallow (2-3" deep) greenroofs use sedums as they are ideally suited for living on a hot roof with low water supply. Right now I am going with some low lying sedums I have in my yard, Dragon's blood, Sedum acre, etc. (or at least that's what I IDed them as). Also some Delosperma, etc. I am only becoming a sedum enthusiast as a by-product of the green roof project, so I don't know a whole lot about them, except that they are easy to root from cuttings (I have a few trays rooting right now). I also ordered some seed mix (they also sell some in Germany specifically for greenroofs, but I got one from Park seeds), but I hear that they could take about a year to get going.
Actually I think that the commercial mixes look very much like the Haydite pic you posted. And expanded shale is recommended for greenroofs (along with styrofoam peanuts and a whole host of media), so I might have something there. Permatill which I was looking to use is another expanded shale product, but expensive. I did e-mail the company, so let's see where I can find it.

Keep us posted! I'd love to know what you finally decide on and see a "before", "new" and "later" pic. Jo

Avoid grit and sand, just too heavy. Go for something "expanded" like the haydite or aquatic soil. I would avoid perlite and vermiculite for this type of installation, they will wash away, decompose, or collapse. Crushed brick would be good, it is basically a "not very expanded" clay so a little heavier than the others but really good in succulent mixes. Pumice too but might not be easily available in your area. Scoria is a good alternative. You would only want a fairly small amount of organic materials such as your potting soil. Include some loam if you can but only up to about 20%. Something like 15% loam, 15% potting soil, and the rest crushed brick would work. Too much organic material and it will just collapse and clog up.

I'm not so sure whether you need holes in the bottom of the trays. Is the roof sloped? Will the trays be raised above the roof with a gap between? Or laid directly on? Any rain is going to have to leave via the slope so holes aren't going to help unless it is raised. The installations I have seen used a waterproof membrane, a base or trays to keep the soil in place depending on your slope, another membrane to stop dirt washing through, then the soil. But on a smaller scale I've also seen trays suspended above, but not in contact with, the roof.

Make sure your roof can take the load, a typical home roof won't be able to handle several extra tons without reinforcement. Use the maximum soil depth that your structure can handle, allowing for it to be saturated with water from time to time.

Thanks Shrubs_n_Bulbs. Is aquatic soil the same as aquarium gravel? I did look up Pumice and Scoria. Sound suitable too. Need to find a supplier/ store that sells this stuff. Same for loam. What about garden compost? I have that.
I think Haydite sounds like it might be lighter and more porous than crushed brick, so I may use that. Or some mix, but I agree about less than 25-30% organic matter (potting mix is the best contender at this point).
The roof is flat and very gradually sloped maybe 10-15 degrees (it's a New York brownstone/ brick house). I was not planning on raising the trays, but there may be gaps under them due to the unevenness of the roof itself (several dips and valleys). I could raise them with small blocks of wood at the corners. I suppose water would collect in the trays if they did not have holes?
I was thinking of using a membrane/ landscape fabric to keep soil from washing out, but I have heard they, like all filters, eventually clog. The load will be about 10 lbs / sq.ft. when wet, and I read that any roof that was to accumulate snow will be able to handle this. These old houses were also quite overengineered.
Jo, I will take pics.

Jo, how many gallons of Haydite in your 40 lb bag, would you say? I found a supplier who is selling them for $24.00.
Thanks!

Aquatic soil is another one of those expanded fired clays, like Turface. For your purposes, it will do the same job as Haydite.

The trays on a sloped roof woukd need to drain at the lower end, the bottom isn't really important. Also, that means that whether the filter layer clogs or not isn't really important so long as the water can still escape from the end of the tray. I'm not sure if your slope is enough for that, it probably is.

Garden compost would be OK, but it needs to be very well aged or it will be too soggy and collapse. Again, only a small percentage should be compost.

>Also, that means that whether the filter layer clogs or >not isn't really important so long as the water can still >escape from the end of the tray.

I'm not clear. How would the water escape if the filter were to clog?
I looked at some retail Haydite at a Bonsai store and it costs a pretty penny. Anyone know where I can buy it in bulk? Or at least cheaper?

$24!!! Holy-schmoley!!! I pay **$5.85** for a 40lb sack. I'd keep looking. Or like Ian says, check into Turface if you cant find haydite reasonable in your area.
As to gals per sack...I have no idea...wouldn't even know how to calculate that. sorry. Jo

I think I am going with Hydroton (LECA), crushed brick, styrofoam peanuts and potting mix combo. Found a hydroponics supplier who sells Hydroton for $29.99 fo a 50 liter bag. One of those would cover about 26 sqft. (if I used 40% of it in the mix). Finding Turface, Haydite, etc. at reasonable rates around here has been such a failure!