The Metal Frame
Measure the roof. Select lengths of rebar that span the length and width of each surface on the roof area with enough length for your overhangs.
Place the rebar on the long sides, 2 feet apart and parallel. Secure the intersections between frame and building with triple twists of bailing wire.
Secure the intersections between the rebar segments with a triple twist of baling wire. Place rebar perpendicular to the last course, 2 feet apart, securing each intersection with a triple twist of bailing wire.
Your roof is now gridded into 2-foot squares.
Overlay the rebar grid with the 6-inch welded wire mesh. Secure every wire-rebar intersection with a triple twist of bailing wire.
Overlap the edges of the wire mesh, and secure them with bailing wire. Ensure the entire roof area is gridded with the mesh.
Overlay the wire mesh with chicken wire. Secure the chicken wire to the wire mesh with one wire-tie for every 6-inch length of wire mesh underlying the chicken wire.
Repeat until you have secured four layers of chicken wire.
Overlay the rebar on the underside of the roof frame with chicken wire, securing the chicken wire every 6 inches along the rebar. Wire-tie the chicken wire you just secured to a portion of the wire mesh at the center of each rebar square.
Repeat for a second layer of chicken wire.
Filling the Frame
Mix one part Portland cement to two parts sand in the wheelbarrow. Add water judiciously and fold in with the hoe until the mix is just plastic, like soft clay.
It should be wet enough to form, but if you drop a handful on a surface, it should not "melt" down, but hold its shape.
Apply the mix to the top of the roof first. With a trowel, spread on the cement, pressing the cement mortar through the framing metals until it is just thick enough to peek out of the underside of the metal frame and until the mix covers the exposed wire on the upper surface.
Cover the whole roof in a single day. Wet mix bonds more readily with wet mix, and this prevents cracks in the future.
Smooth and blend the exterior surface as you go.
Apply the mix to the underside of the roof. You will see a rough layer of concrete that squeezed through from your topside work.
Trowel the concrete onto the lower side until it closes the gap and raises the interior surface enough to conceal the chicken wire.