Tzeentch laughs at likely
Join Date: Dec 2009
Location: Inside my Head, Bristol, UK
Based on my experience of scratch-building 40K terrain in the 80's:
(1) most rigid plasticard is strong enough well before it looks realistically thick and can be heavy;
(2) Expanded polystyrene is light and shape-able but easy to damage.
One way to get light robust ruins is to make two identical plasticard pieces and fill a gap between with expanded polystyrene.
When my friends and I chipped in to buy the massive Cities of Death bundle just after release we built some 1' buildings and mounted them on 2mm plasticard, which is still working well. Unless you are thinking of 2' entirely open buildings it would probably work for the scale-up; otherwise you could use 5mm.