You know, you have to hand it to Duolingo for some of these sentences. The fact that they are mildly nonsensical and a tad funny actually means that you are understanding the vocabulary, grammar, and context of the translations, as opposed to just inserting the most appropriate sounding common phrase. Pretty slick!
I think "The roof is of sugar" should be accepted. It isn't the way we would say it in the United States, but maybe it is British, or maybe old-fashioned, because I have read that kind of sentence construction in books. Where I live, we would say, "The roof is made of sugar," but I will think of "The roof is of sugar" to help me remember the way to say it in German.