Never trust a skinny chef,
Be careful approaching scotsmen in funny hats
Never trust a man called julie
don't take some of the tosh on this board too seriously.
ah, but that's not how to read the advice. It means that you shouldn't drink in a pub if your companion is in fact a flat roof. Probably because the flat roof won't fit in the door, or perhaps flat roofs are notoriously heavy drinkers, or (most likely) cos they won't stand their round.