Lone-Soldier
Remember to add some "broken glass". You can use acetate for overhead projector sheets or cut up any of the clear plastic that items come in a blister pack.
Maybe add a window frame that has fallen intact with a few cracked or missing window panes.
Don't forget that all buildings have some type of wiring either in the wall or attached to it.
If it's a factory you're building. There would be some type of stairs or overhead walkway maybe. Some of this could be "in" the rubble pile.
Drain piping from the outside, gutters downspouts etc. use drinking straw or brass tubing.
Roof tile - Use the cardboard back of a pad of paper cut into squares of appropriate size.
Sheet Metal Roofing - Lead foil pressed into the corrugated pattern. Glue 6 or 8 strips of sprue with a sprue width distance between the pieces. Press the lead foil on top of the sprue jig and press the foil into the grooves to create sheet metal sections. Give them a rusty coat with a black wash. Maybe a slight coat of Mr. Surfacer or just some Baking Soda and white glue prior to painting for a rust effect.
Some "human factor" items also could be added. A few old tin cans (Article in FSM on how to do this check the index of articles) A telephone, furniture, pictures, posters, rags, old newspaper, stove, heater, kitchen sink, etc etc.
The key is to put in as much as you want. The best way to get it right is reference. If there is a building or area similar to what you are trying to depict in your diorama photograph it from all angles. Make notes of what is around the area. How much garbage is floating around the area. What is the area around look like, stains, plants, trees, animals, rodents etc etc.
That's about all I can think of right now. I hope this helps.
Cheers;
Gregory