Two words used as one expression may be written as one word, as a hyphenated word, or as two separate words. Which form to use often depends upon the use or position in the sentence.
We arrived at the football field at halftime. (noun)
This ad says it is a half-time position. (adjective before noun)
He lives in the first-floor apartment. (adjective before noun)
His apartment is on the first floor. (follows noun)
When in doubt about whether to hyphenate, check the dictionary. Also, see Chicago Manual of Style, section 7.77.