Language usage changes with time, but that does not make the old usage incorrect (nor necessarily the new usage correct). I am with your mother on this one though I will generally use 'roofs' in writing to avoid argument, coward that I am. There is a similar issue with 'dwarves' and 'dwarfs', the latter now being common usage thanks to JRR Tolkein who, as an OED editor, knew better and said in his foreword to his books that the 'v' form was correct but he was using 'f' when he pluralised it for some reason I can't remember now, but as a result everyone now assumes that 'dwarfs' is correct.

Language usage changes with time, but that does not make the old usage incorrect (nor necessarily the new usage correct). I am with your mother on this one though I will generally use 'roofs' in writing to avoid argument, coward that I am. There is a similar issue with 'dwarves' and 'dwarfs', the latter now being common usage thanks to JRR Tolkein who, as an OED editor, knew better and said in his foreword to his books that the 'v' form was correct but he was using 'f' when he pluralised it for some reason I can't remember now, but as a result everyone now assumes that 'dwarfs' is correct.