Not to be pronounced ruff because only someone from the south with low levels of education would say something that asinine.
Roof or ruff.
The degree of diphthongization of the vowel varies regionally but ruff for roof i ve never heard.
Roof that thing above your head as you sleep that protects you from the weather and flying monkeys.
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Hey go up on the ruff what the fuck is a ruff you ignorant yankee.
That s just how it s pronounced people learn it from their parents and from other people around them.
It is phonetically pronounced as both roof and ruff depending on colloquial variation.
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Mostly the distinction is between the two different double o pronunciations the long oo in food or the short oo in book.
Roof as ruff is rare although i don t doubt that there are some who say it.
As for why well as in with many things in language that s not really an answerable question.
Northern us accents tend toward the long foodish one southern accents tend toward the bookish one.