rule 3. Rule the roast (roost). The OED gives no countenance to roost, & does not even recognise that the phrase ever takes that form; but most unliterary persons say roost and not roast; I have just enquired of three such, & been informed that they never heard of rule the roast, & that the reference is to a cock keeping his hens in order. Against this popular piece of popular etymology the OED offers us nothing more succulent than 'None of the early examples throw any light on the precise origin of the expression'. In seven out of the eight pre-18th-c. examples quoted the spelling is not roast but roost or roste, but the OED philologists would doubtless tell us that rost(e) could represent Old French rost (roast), & could not represent Old English hrost (roost). Writers should take warning, at any rate, that rule the roast is the orthodox spelling, & that when they have written it, the compositor must be watched.
Fowler's Modern English Usage,
(1926 Edition)