I’m asked on a regular basis what the dark lines are in the ceilings of the homes I inspect. My response to them is Ill know more when I get up into the attic. Once in the attic, I will find either too little insulation and or no insulation covering the top of the ceiling joists. My recommendation is to: add additional insulation and then lay insulation over the top of the ceiling joists to prevent thermal bridging. This has not only helped in the black outlines of the ceiling joists but decreases heat loss in the home.