Most of my ruins follow the same construction pattern. Since I'm using 1/72 scale plastics I can get away with using a single layer of corrugated cardboard. I base these ruins on another piece of cardboard so I will have place to put rubble and keep the structure sturdy if there are freestanding walls.
I've found that including doors and windows, or even partial outlines of windows, goes a long way toward making the ruin look like a building.
I use various bits to detail the ground around the ruin (see the close-up). The tiles are made out of squarish bits of card. The rubble is clean kitty litter and sand mixed together. Bits of wire represent pipes, girders, and rebar.
The one thing you may notice about the ruins, especially the ones below, is that the currugations tend to be very noticable. I normally do cover the edges with masking tape (as I did with the undamaged buildings), but on these I chose to avoid the hassle of taping the ragged edges, though I do tape the corners. That is probably the main disadvantage of using cardboard -- it's a trade off between time/cost and looks. These are meant to be cheap and fast.
The main notable thing here is the partial second floor that could be used as a sniper position
The garage door helps this piece stand out from the other buildings.
