Ask Mr. Smarty Plants

Let’s start with the question of soil first.  It should be as light as possible so you can make it as deep as possible without stressing your support system. The maximum amount of native soil that is recommended is 1/3 clean topsoil,  mixed with 1/3 compost, and 1/3 perlite or other inorganic material.  This was discussed in the FAQ’s on greenroofs.You may even want to reduce the amount of compost in the mix since the natives you'll be using don't require much organic materials in the soil.  Also it will break down over time and you will end up with less total soil. Finally, the organic material can pollute the run-off.  In this case, you would probably want more perlite and less compost. This was a recommedation of  Mark Simmons Ph.D., who is the head of green roof research at the Wildflower Center.

In a Houston, Texas experiment, the Schlindler Company’s landscape architect used a mix of 70% perlite and 30% packing peanuts and says she thinks a 50/50 mix would work just as well.  She just put the peanuts on the bottom and the perlite on the top. She added a soluble organic fertilizer once a week and got dramatically heavy growth on her plants. And the plants made a strong matrix of roots which prevented any of them from being blown over. This is  also the very lightest  soil solution.

You will have to do your own research and match the engineered  soil to your load limits. Also consider if you will have a drip irrigation system that makes it easy to add a liquid fertilizer or not, and consider the cost of various  materials.  From personal experience, I can recommend that you could also add the packing peanuts to a mix of topsoil, perlite, and compost.    I used to use peanuts in a mix to start tomatoes and the peanuts encouraged many more roots to form which helped the plants resist stress better. The peanuts seemed to work that way in this study as well. 

The Ladybird Johnson Wildflower Center is involved with research on the best plants for people in Texas to use. Sedums don’t do well here, although they are the plants of choice in other places. Here is the list of plants  recommended for green roofs. You can use various combinations of them whether you plan to have an extensive or intensive greenroof.   Click on each plant to get a full description of  it.  But be sure and use several different species to keep the entire roof  healthier and to prevent the lost of all your plants from some disease. This  information was also in the FAQ’s referenced above. You mentioned using grasses so you might want to narrow your search - see the side pannel to grasses/grass-like in the habit selection.

Here are some greenroof projects from Texas.You will have to use the search engiine to look for Texas projects as I don't get a new URL when I search. 

And for a complete discussion of building, planting, and maintainig greenroofs, see Green Roof Construction and Maintenance.  Luckett, Kelly. GreenSource Books. Mcgraw-Hill's Greensource.