When you need a noun meaning (1) the act or process of rolling over, (2) an accident in which a motor vehicle overturns, or (3) the instance of rolling over funds, use the one-word rollover. The word works as an adjective in phrases like rollover funds and rollover accident. It is never a verb. For all verb senses, use the two-word phrasal verb roll over. So, for instance, one might roll over his or her rollover funds.
Because rollover is an accepted an adjective, there is technically no use for the hyphenated roll-over. Yet the hyphenated form is often used, especially in British publications, in place of the one-word form.
Examples
For example, these writers use the one-word rollover correctly:
The ECB is considering a rollover of bonds as an alternative means of easing Greece’s funding squeeze … [Irish Times]
A medical episode may be to blame for a rollover crash Thursday morning. [CTV]
The contract is structured with an automatic rollover each year … [USA Today]
And these writers correctly use the two-word roll over as a verb:
EU officials may try to persuade investors to roll over maturing debt by offering them preferred status … [Sydney Morning Herald]
But one politician out there would roll over this hapless group like a tractor through a cornfield … [Politico]
The Taliban could roll over the country as the Vietcong did in Vietnam. [Guardian]
