Three sheeting materials are shown. The gray material is by Plaststrut #91630 "asphalt shingles" Item PS-112.  With Black mortar lines this makes a very convincing concrete block material. Not usually seen on top of a building, concrete block goes well along the exterior wall of an industrial building.
Also shown in tan color is a sheet of stone material. This was used for the chimney in the lead photo. This looks good on top of rustic buildings and even  townhouses in the city. The stone is Slater's Plastikard item #419. The brick red material is what I will use in this clinic. It is Slater's Plastikard Item #0401 4mm brick red. Plastikard is available from Railway Models, Box 871 Edgewood MD 21040. There are other manufacturers of this type of material including Holgate and Reynolds.  I chose these because they are pre-colored, making it easier to add mortar lines without taking off the brick color.
I also prefer to buy this in sheets about 7 1/2" x 12" rather than in long strips a few inches high. This dimension allows about a dozen chimneys to be made at once.