Build a framework on the roof for the concrete roof using insulated forms. These forms come in sheets which can be "locked" together to create large areas, follow the directions that are provided. These forms should be sitting on steel or wood beams to provide support. These beams should be a part of the current structure. You will want a structural engineer to determine if you have enough support for the roof. There are many companies that supply insulated forms so find what works best for your situation. These forms all do the same thing as far as providing structure for the liquid concrete. They differ slightly in thickness, size, design and cost.
Insert the steel re-bar into the insulated forms. Most forms will have holes or slots designed into the system for easy positioning of the bars. The bars are crucial in providing strength to the concrete. Without these bars, the concrete is actually fairly fragile and will not have the strength you need.
Install outer perimeter bracing around the forms. This will contain the concrete in the forms while you are pouring it. You can use either plywood or rigid plastic sheets, cut to size. These forms will be removed once the concrete has been poured and cured.
Pour the concrete roof. This concrete strength should be a minumum of 4,000 pounds per square inch (psi), which you can specify when you place your order. This just means there is a higher concentration of cement in the mixture which provides more strength. Start in the corner that is farthest away from the concrete mixer. Fill up the area and move back towards the mixer. Be sure to use a vibrating device to remove any air bubbles that may be in the concrete. If the roof is quite large, you may want to break the pour into sections so it is easier to work with, but it is critical that you do not have any seams in the concrete, as this will impact the overall strength.
Smooth the concrete out using a concrete float. It is not critical that the finished concrete be perfectly smooth but it is important that you work out any high or low spots, so the roof is even to prevent water from pooling. Cover concrete with a thin sheet of plastic to keep any moisture from getting on it.
Remove the outer perimeter bracing from the forms after the concrete has had several days in which to cure. Also, remove any temporary shoring that you may have installed. Seal with concrete sealer. You should now have a concrete roof.