For a distribution from a qualified Roth 401(k), the Form 1099-R provided by the payer should have $0 in box 2a as the taxable amount and box 2b Taxable amount not determined unmarked. (It's the payer's responsibility to properly report this.) TurboTax will treat this as nontaxable, including the entire amount on Form 1040 line 5a but excluding it all from line 5b.
