How fermentable are dark grains?

I did my second stout which is now 2.5 weeks in primary. It has been at 1.026 for 1 week despite raising the temp and rousing the yeast on more than one occasion. So I think it's done. I'm going to leave it in primary for a couple weeks longer but it has me wondering...

My recipe included 0.75 lb of Carafa III Special and 1.0 lb of Roasted Barley combined with US 2-row, crystal and flaked barley (for diastatic power >30), mashed at 153 and OG of 1.082. I use my own calculator and counted the dark malts as fermentables, which, using an AA of 80% with US05 would reach about 1.016. So I was a little suprised by the FG.

So I think the issues lies in a slightly higher mash temp than my norm and the use of the darker grains (which I rarely use). So, I know that dark malts are substantially less fermentable but does anyone know to what extent? If I assume they are 0% fermentable, that would raise my expected FG to 1.021-1.022. The rest of the difference could be from the mash temp.

Any opinions on this?

Dark malts are largely unfermentable. If you look up each of the grains you used in our wiki, you'll get the best information.

david_42 said:

Dark malts are largely unfermentable. If you look up each of the grains you used in our wiki, you'll get the best information.

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I have information from various sites (including wiki) and books about typical extract potentials, color, diastatic power etc. So, I know how much various ingredients contribute to gravity. And, it also sounds as though at least some dark malts, like roasted barley, can undergo conversion if the diastatic power of the mash is adequate.

When I was formulating my recipe, I searched all over and I found that black patent malt is largely unfermentable. And I think Carafa III Special is essentially debittered black malt. So it sounds like this is largely unfermentable.

But what about roasted barley?

I have seen attempts to quantify crystal malt fermentability (https://www.homebrewtalk.com/f128/testing-fermentability-crystal-malt-208361/) but I haven't seen that for dark malts. Maybe I just didn't search well enough?

At any rate, until this batch I have accepted that I can't always exactly predict the FG despite my best attempts. And really it had seemed like the main limiting variable for predicting FG for a given brewer on a given set-up was mash temp. But after this batch, it looks to me more like predicting FG gets more complicated for beers with darker malts.

Anyone tackle this issue?