thank you Yooper for your notesHow confident are you in your pH meter? If it's a freshly calibrated accurate pH meter, then we'd like to hear about it- it'd be pretty hard to get a pH reading under 5.0 even with distilled water and roasted malts unless you were using some malts that had a much higher DI pH than the ones in our catalog and using a lot of them. The pH will drop during the boil and the post boil pH will be .1-.2 lower.
Looks like your water your using is soft. Do you know your water hardness? You can blend RO into hard water to soften hard water, but it's really tough to add true hardness to water without using baking soda as Hawk Box suggested. Sometimes that works really well for porters and stouts because it adds some sodium which brings out the malt. Be careful not to add too much, or it will come out as salty.
thank you Yooper for your notes
im not sure my PH meter is 100% accurate, however, i usually get the PH between 5.2 and 5.6, i'm using the BF app and add some salts according to the recommendations - works all the time. this time it dropped below 5 for the first time - maybe it is the Carafa (0.3lb for 2.5G batch). i decided to do nothing and see what happens, so far the OG was on the spot and it looks as it should. i will calibrate my PH meter tonight to see if it is off and will update.