Corrugated steel is an often overlooked building material, particularly for tiny houses:

http://2.bp.blogspot.com/-Z4MwyfNR-Ek/TbNYZHvul_I/AAAAAAAAY5...

Photo taken in Chloride, AZ. Many of the small homes in the town have their original iron siding and are well over 100 years old.

This book has more information: http://www.amazon.com/Corrugated-Iron-Simon-Holloway-Morneme...