Hey Frex
I like this work in progress shot. I don't know how what your paint shop or gmax skills are, nor am I familiar with any previous work of yours. So I'll make a couple of observations at the risk of saying something you already know or had planned to do.
Two things immediatly caught my eye.
The parapet needs more detail. The coping can be made of different things. Precast stone or concrete. Sometimes the coping is obmitted in favor of metal flashing. It would be difficult to find a good metal texture so I recommend a coping similar to the picture below. Roofing material usually extends some distance above the roof deck. Don't worry about the build up of material at the joint but it would be nice if the roof texture is extended upwards. If you wish to mimic that 45 degree slope you could use a line, set the mesh to have 4 sides, and trace along the roof geometry.

The roof material is good but I think it needs darkening. You can either simultaneously decrease the brightness and contrast in your paint program, or apply a solid black layer. Adjusting the opacity adjusting until you're happy. Alternately, a light roof is usually a stone material ... so you can always find a good pebble stone texture and apply it as a layer and maybe using dodge, burn, or other blending technique.
New batters tend to be lacking in paint shop skills at the start, but get better as time goes by. You should add some roof drains, vent pipes, and off air antennas that will cast shadows and add interest to the roof surface. These buildings do not lend themselves to roof top HVAC as they are somewhat narrow. .. perhaps window units placed at random.
I'm not familar with your previous work ... so please forgive me if I am suggesting something that you normally do.
I like the wrought iron fencing and you have nice detailing around the windows. The head and sill play against the light to cast subtle shadows. The doors have good textures as I can see brass name plates, knobs, etc. . .
Good Luck
- Jim