I could immediately tell it was supposed to be corrugated iron, if there is a way to make it look maybe half as thick, I think it would do you a lot of good, as long as it can be placed on either edge or the centre of the tile it's on (So you'd sort of be treating each tile like 3*4 and it would either be placed on the back 6, front 6, or middle 6.
It would be less complicated mechanically if you could make it 1/3 of the width and it would look better, but it might (I don't do sprite work) make the art harder to make.
That's an idea. I would require quite a bit more sprite work. Right now I've gotten away with only 2 different sprites for walls. My new ground tiles are 47 sprites(!). I think it would make walls look better though. I'll put it on my art to-do list