The two grains are not even remotely close to one another.
Flaked barley is unmalted barley that has been pressed and rolled under heat to gelatinize the starches. It's used, most commonly, in stouts for body and head retention. It really should be mashed with a base malt to truly utilize it, as steeping will only lend a pretty unappetizing floury taste and haze.
Roasted barley is unmalted barley as well, but as the name suggests, it is roasted in a manner similar to coffee beans. It's extremely dark (around 300L or even darker depending on the maltster and variety) and has a dry and sharp flavor--though I happen to like that flavor a lot. It also is a key component to stouts, but for a whole different reason than the flaked barley. Unlike flaked barley, roasted barley can be steeped to extract most of its goodness--RB has a pretty low impact on gravity, and is used more for flavor and color.
Whatever your recipe is, I can just about 100% guarantee you that you won't be happy with a substitution like you're suggesting. Perhaps post your recipe, and the board members can give you alternatives.
HTH...