The roof material shows up well - and did not warp 
The roof material, and indeed for most of the buildings, the base and wall material is pretty good. We buy cat food in pouches sold in large cartons. The card is two thin layers sandwiching a sheet of corrugated card. The outside is glossy and printed and thus water resistant. In inner is a plain brown card, but still moderately water resistant.
It cuts easily and glues well with Evo-stik woodworker's PVA. When painted it does not seem to warp at all unless absolutely drenched.
I'd recommend it to anyone for small, well braced structures such as these little buildings. For larger structures I'd use Foamcard.