Ambis Engineering produce some very nice corrugated sheet in 2, 4 and 7mm scales and it's available direct or from Hobby Holidays. It's only slightly thicker than aluminium kitchen foil though, so it's quite vulnerable where it overhangs eaves and gutters. I had a chat with the chap at Severn Models, whose etched huts feature a really nice representation, with a view to persuading him to produce some sheets. He felt the cost might put most people off, but having wasted quite a bit of the Ambis product I for one would think it a worthwhile investment. He said I wasn't the only one to suggest it so perhaps if enough of us ask......