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Royal icing, but mix in a good squeeze of lemon juice (about one quarter of a lemon for two egg whites & 100g icing sugar). Hold the relevant pieces together for about 30 seconds or so (sometimes this can take two goes) and, hey presto, sugar cement!
Mind you, on my gingerbread house thread, rips advised just using superglue...
Thanks so much everyone for some great and unusual tips on making gingerbread houses stick together. I can sympathise with the superglue, haven't tried that one yet!! Presuming the lemon juice in Royal Icing is just to keep it white?