My husband works construction and is paid per diem. He will be on this job one year in November. Will his per diem become taxable then? If so can they go back and tax from day one or will it just go forward from one year?

Here is the info you need - as you will see, there is not a straight yes or no answer.

http://www.irs.gov/pub/irs-regs/perdiemfaq%26a.prn.pdf

Generally, after one year has passed, the per diem becomes taxable, because the jpb is NO LONGER considered to be temporary in nature.