I was wondering if anybody has done, or has recommendations about, a green roof over a SIP roof.

We're planning to use steel SIPs to put a low-pitch flat roof on an existing brick 2-story building (25'x30').  Green roof would be ~4 inches of soil / plants.  Load would be ~25 lbs per sq.ft.  We're debating between using modular trays to contain the soil or putting it straight on the roof (with membrane or metal roofing over SIP surface).

loweep;

we just completed a large sundeck over steel SIPs, it sounds like you will need a surface for foot traffic as well as the loading, we laminated 5/16" hardi to the steel skin then subcontracted for a fiberglass deck / with tan gel coat. It turned out very nice

I'm not an engineer and I don't know your location's/region's weather characteristics, but 25#/SF sounds a little low for the load your roof will be resisting. Best to think worse case with something out of the ordinary like this.

Tell us where your project is.

25 lbs/sq.ft is a typical number I see given for the weight of a 4in green roof. I will be having an engineer look at everything.

The house is in Houston. Anybody else built with steel SIPs in the area?

My thought is that you don't want any soil or runoff from soil coming in contact with the metal SIPs or any metal roofing. Soil is corrosive and will definitely corrode your roof fasteners and then eventually your metal roofing/SIPs. A membrane that covers over everything, fasteners & all, seems like the durable way to go in my opinion. But anchoring your foliage and not compromising the membrane is the tricky part.

Don't know if you get heavy wind/rain storms in Houston, but they may suck plants off your roof or hold too much water on the roof. Good drainage and securing the foliage are definitely design priorities.

Here in MN we are 45 -50# sf just for snow. You need strong heavy walls to support a heavy roof.

Being that most SIP walls are 2.5 to 3 times stronger than frame.. no problem!

There is a lot of information on green roofs on the Web.  Doing a search on "green roof details" or just "green roof" will give you a lot information.

One example is http://www.greenroofs.com/

Green roofs are general made up of a number of layers between the planting media and the underlying roof surface.  A typical sequence can be found at:
http://www.hadj.net/green-roofs/assemblies.html

The figure of 25 psf appears to be commonly used for green roofs of about 4" depth when using light weight growing media and lightweight components under the soil.  However, when figuring roof loading, you will also need snow loading and/or live load.  In our area (north central Oregon) snow load is 25 psf for a total design requirement of 50 psf with the four inches of soil.  Your structural engineer will know (or find out) these figures, including wind loading and other factors.