Mainly foamboard and joint compound.
The interior features PTE bunk beds. I also used various weapons to decorate.

The Dio features scratch built, working, doors and various graffiti and items seen in the game
The outside brickwork was done with joint compound. And as an added touch I used an Umbrella Corp advertisement for the billboard. (Thought it would be neat to tie the L4D and Resident Evil worlds together.)
I'd also like to give a huge thanks to DanOfTheDead for his help in supplying screen-caps from the Left 4 Dead game.