If you are mashing the grains? Roasted grains aren't going to contribute to the sugars much and the enzymes in them have been denatured. So they won't contribute anything but flavors. So they will need to be added on top or to the grains you intend to mash to get the sugars from.
Now the flaked grain will contributed starch that can me converted to sugars in the mashing process. But a malt that has active enzymes will need to be used with it to supply the enzymes to convert the starches to sugars. Or powdered enzymes could be used.
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